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Preface,
Our long experience enables us to offer our customers integrated high-tech devices, from in- house development, manufactured in recognised industrial conditions and reaching the top quality for installers and end users. With standalone and modular concepts we are able to offer maximum flexibility for field applications. These innovative products are developed and manufactured to the highest level achieving an excellent level of quality.
The following guide lines which include real world examples as illustrations aim to benefit planners, installers and commissioning engineers, the theme is to provide an approach to diagnostics and troubleshooting of devices housed within and around a technical cabinet.
Modern devices are based on advanced electronics technologies, with this the accuracy is higher and the measurement faster than in previous generations. This greater precision and accuracy requires an environment with the same attention to detail. In the field more often these devices are mounted together with Frequency Converters (FC), Power switched mode power supply elements, commonly used for better and more accurate pumps management and ventilation systems, however due to the way these devices function they can affect other devices by propagating Electromagnetic distortion waves or electromagnetic interference (EMI) which disturbs many electronic systems, irrespective of the colour of the box or the label or mark stamped on the bottom.
This booklet is the fruit of the collected experience over recent years from many staff. It is the result of knowledge exchanged and transferred, applied and implemented, we hope it will bring support for field engineers, planners, programmers, project managers, technicians and all people getting in touch with this area.
It is not the intention that this document will replace local laws, standards, product data sheet, product information, mounting instructions and any other official document issued by this or any other manufacturer.
Special thanks to all NSO people who contributed and helped in the discourse, knowledge and experience transfer in order to give us the possibility to write this technical documentation
Through this guide you will find practical help, grounding will be highlighted, EMC will be explained and you will be armed in how to identify and overcome grounding and EMI issues.
SAUTER Technical Support
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Table of Contents
1. Grounding (Fundamentals) ...... 8 1.1. Grounding (equipotential relations) ...... 9 1.2. Grounding resistance ...... 9 1.3. Grounding and its position in electrical theory...... 10 1.4. Grounding connections ...... 10 1.5. Grounding in Buildings ...... 11 1.6. Grounding and transformers or DC Power supply ...... 11 1.7. Power supply ...... 12 2. ESD - EMI – EMC- Transients ...... 14 2.1. ESD ...... 14 2.2. EMI ...... 16 2.3. EMC ...... 18 2.4. Transients (or fast transients) ...... 20 3. The electrical cabinet ...... 22 3.1. General purpose ...... 22 3.2. Cabinet structure ...... 25 3.3. Grounding in cabinets ...... 27 3.4. Cabinets and projects ...... 31 3.5. Cabinet setup (According section 3.2.2) ...... 32 4. Diagnostics ...... 34 4.1. Step by step methodology ...... 35 4.2. Technical diagnostic ...... 38 4.3. Commissioning ...... 39 5. Real world examples of grounding issues ...... 40 5.1. Freezing of a modular based PLC system...... 40 5.2. Pending PLC inputs ...... 42 5.3. Conclusion ...... 46 6. Grounding First Aid Guidance ...... 48 6.1. EMC / EMI in the field ...... 48 6.2. Tips and tricks ...... 51 6.3. Cabling...... 53 6.4. Main ground ...... 53 6.5. Shielding issues ...... 54 7. Checklist for grounding ...... 56 7.1. Grounding (See chapter 1) ...... 56 7.2. Shielding (see section 3.3) ...... 56 7.3. Cabinet (see section 3) ...... 56 7.4. Cabling (see section 3.2 end 3.3) ...... 56 7.5. Mandatory (see section 3.4) ...... 56 8. With reference to SAUTER products ...... 58 8.1. SAUTER technical standardisation ...... 58 8.2. Modulo 5 ...... 58 8.3. Tips and tricks ...... 62 Personal notes ...... 64 Resume ...... 66
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Symbols and definitions used in this document
Danger
Forbidden
Recommended – Approved good practice
Caution
Abbreviations
N Neutral L Live Ph Phase PE Protected Earth PEN Protected Neutral Earth (TNC mode) (-) Negative pole (+) Positive pole ESD Electrostatic discharge EMI Electromagnetic interference TT Earth/Neutral regime according TT schema TN Earth/Neutral regime according TN schema IT Earth/Neutral regime according IT schema AC Alternative Current DC Direct current NB: Nota Bene GFCI Ground Fault circuit interrupter FC Frequency converter PCB printed circuit board UPS Uninterruptible Power Supply HMI human–machine interface BMS Building management system PPE Personal protective equipment PDS Product data sheet FI Fitting instruction EMF Electromagnetic field PI Product information CCC China Compulsion Certification
Note: Images and figures contained in this document are for illustration purposes only.
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1. Grounding (Fundamentals)
What’s grounding?
Grounding is fundamental in the theme of electrical current; the terminology is used to define the relationship between active lines and earth. In general any material able to conduct an electrical current must be connected to the Earth (“Earthed”). This connection must be strong and should never been switched or isolated.
NB: The main connection to the earth could be realised in different ways (Ground rod, grids in foundations, building surrounding earth wire, grounding plates) In the case of multiple buildings the main grounding points of each location should be interconnected, especially when other electrical lines are linking the differing buildings for example Ethernet, telephone lines, fire alarms, etc…
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1.1. Grounding (equipotential relations)
1.1.1. The main grounding
The main Grounding point is an interconnection point where all grounding points are linked to the earth by a single point. Generally this is done at the main electrical distribution point or a special dedicated “earthing point” of a building or a plant. The electrical resistance of this link must be as low as possible in order to guarantee the earth potential to be as low as possible (see chap. 1.2.1 and 1.2.2). This connection to the earth provides a basis for the provision of good electrical safety throughout the site.
1.1.2. Delocalised grounding
Usually the power supply cable contains an earth wire, as indicated by a “green/yellow” shield or sheath, which is connected directly to the main grounding point which goes to a rigid disconnector (see above) on one end and to the cabinet ground- distribution point on the other. This line ensures the ground is distributed throughout the whole electrical network and buildings.
1.2. Grounding resistance
1.2.1. Ground fault current In most countries a specified limit of 100 ohms is defined, (this may be different in your country). However this must be known, as the general GFCI (Ground Fault circuit interrupter) accepts 500mA. Also, 50Vac has been defined as the extra low voltage limit under dry conditions. Consequently the ohm law is applicable R x l = 50 V, differential 0.5 A ⇒ 50/0.5 = 100 Ω. NB: In other conditions (e.g. wet environments such as bath- or rest- rooms this limit is lower; 30mA or 10mA by 25Vac are common)
1.2.2. Ground line resistance A lot of derivations and connections are present in a grounding network, but one point often overlooked during planning and installation is the line resistance (or coupling resistance) of the grounding wires and interconnections. In fact the line resistance between two main grounding points must be lower than 2 ohms to be considered as functional and strong enough to absorb leakage current, the short circuit current in case of fault and to drain any EMI (Electromagnetic interference) disturbances present in the system.
1.2.3. Grounding voltage and current A misunderstanding of grounding often leads to the statement “the grounding voltage is zero volts”. (The current of course would then also be zero Ampere). In reality the grounding tension and current never reaches a zero state. In fact, the EMI, especially in cases where incorrectly filtered FC’s are present in the installation, residual radiated energy will be drained trough the grounding lines. Consequently some potential differences will appear in the grounding circuits.
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1.3. Grounding and its position in electrical theory.
The ground or “earth” connection has two different functions:
A) The major function of the grounding is the safety of an electrical circuit and the equipment. As all metallic parts are linked to the ground; it functions as drain for electrical leakage. The GFCI is designed to automatically switch off the circuit in case of any malfunction to help prevent a danger to life.
B) The second implicit function is that the grounding establishes the electrical reference point for all parts of the circuit. The grounding point is (must be) the only common point of all circuits. As it has a very low resistance, it will be used as the reference for all measurements, all detectors (GFCI) including any EMI Filters.
There are some exceptions, for example, totally independent isolated electrical circuits (ships, submarines, aircraft, cars or other installations with a self-contained power supply)
1.4. Grounding connections
The earth/neutral regimes: three different types of earth/neutral lines are commonly defined for the low voltage side. (Secondary of the high tension transformer or substation)
a) The TT regime: The first “T” indicates the neutral of the system is grounded on the generator side and the second “T” indicates that the masses (metal part or casing) are connected to ground (earth)
b) The TN regime: The first letter "T" indicates that the neutral of the system is grounded on the generator side and "N" indicates the interconnection of the earth and neutral line. There are three possible interconnection modes; TN-C, TN-S, TN-C-S TN-C combines PE and N from transformer to the main dispatcher. (Distribution and equipment) TN-S separates PE and N from transformer to the consuming devices, which are not connected together at any point after the building distribution point. TN-C-S combined PE and N (PEN) conductor from transformer substation to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.
c) The IT regime: The first letter "I" indicates that the neutral of the system is insulated from ground (i.e. no connection or high impedance connection) on the generator side and the “T” indicates that the equipment masses (metal part or casing) are grounded
Typical examples:
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1.5. Grounding in Buildings
In standard installations the power supply is dispatched through a high tension transformer from a 3 phase 20Kv primary to 230Vac 3~ (3 phases) on the secondary side. From this point the neutral line (PEN) is present. This stage of the circuit is still a TN-C system. In the next step (after main dispatcher) we are in the TN-S system which can be easily recognised by the separation of the PEN into PE and N
It is not permitted to convert a TN-S System back to a TN-C. In the TN-S system the neutral line (N) is an active line and in most of the cases it is switched through a Tetra-polar (4 poles) circuit breaker compared to tri-polar switching (only the 3~) in the TN-C system.
The practical application of the TN-S system based on 5 lines (3 phases +1 Neutral+ 1 Ground) gives the best results in terms of grounding stability, EMC/EMI in industrial and domestic building areas.
1.6. Grounding and transformers or DC Power supply
1.6.1. A transformer builds a tangible galvanic separation between the primary side (230Vac) to the secondary side (24Vac). It is the magnetic field which transfers the energy from one side to the other and this is realised in the secondary circuit as the defined potential difference (tension), once the circuit is closed and a current flows.
The secondary coil of the transformer should be grounded
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1.6.2. A DC power supply source has also to respect grounding rules.
A ground needs to be placed on the secondary, not of the transformer but on the negative pole of the DC source as shown below.
1.6.3. General rule
Remark: When using a DC supply, based on a power switch mode, it is necessary to check first if a galvanic barrier or separation is present before the negative pole (-) is connected as this must also be grounded. Generally if there is a direct link between primary side and secondary side (for example the (N) and the (-) are linked) than it is forbidden to connect this point to the ground. If connected there would be a ground loop, at this point and the 250Vac voltage would then be present at both sides of the circuit and the system is no longer safe. This could potentially destroy all DC equipment present as a load current from the DC equipment will flow through this point.
1.7. Power supply
1.7.1. Under-voltage, voltage breakdown, over-voltage
Under-voltage, voltage breakdown, over-voltage are the three normal states which appear in electric circuitry. Power supply voltages will fluctuate when switching high power devices, the inductive effect occurs due to perturbations in high current loads. This phenomenon is amplified by current disphasing when switching inductive or capacitive loads.
1.7.2. UPS (Uninterruptible power supply)
The role of the UPS is to supply a constant power; this is achieved in a smoothing process, together with a secondary power source, for example a generator for larger applications or battery (cell) for smaller ones. Spikes and troughs are dissipated through the device as needed, with any supply phasing issues addressed with active circuitry resulting in a smooth continuous power source.
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1.7.3. Presence of a FC (Frequency converter)
FC are often implemented for their ability to reduce vibrations in motors by applying a tuned power delivery, this also has the advantage of improving energy efficiency. Due to their internal filtering and switching mode, they require special attention during engineering, implementation and commissioning.
1.7.4. Transformers
The characteristic of a transformer is defined by its magnetic field structure. The permeability, transfer capability, defines the gain (Primary/secondary) and mainly the EMI filter capability. E.g. a ferrite ring kernel is considered as the optimal material on the market.
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2. ESD - EMI – EMC- Transients
Don’t confuse ESD, EMI and EMC
ESD refers to “Electro-Static-Discharge” and it is related to the ability of an element (electrically conductive or not) to become charged or loaded with potential energy through friction, movement or inductive effects.
EMI refers to “Electro-Magnetic-Interference” a naturally occurring phenomenon when the electromagnetic field of one device disrupts, impedes or degrades the electromagnetic field of another device by coming into proximity with it. Devices are susceptible to EMI because electromagnetic fields are a byproduct of passing electricity through a wire. Data lines that have not been properly shielded are susceptible to data corruption by EMI.
EMC refers to “Electro-Magnetic-Compatibility” and refers to the electronics equipment standards and behaviour in process environment. EMC aims to ensure that equipment items or systems will not interfere with or prevent each other's correct operation through spurious emission and absorption of EMI.
Transients are the result of power switching. Mainly in cases of inductive loads they can reach higher tension levels destroying electronics components.
2.1. ESD
2.1.1. What is ESD?
ESD is generated by high potential differences. The discharge is commonly seen as a spark. Within a fraction of a second the potential differences of two points are equalised and the energy transfer is in the KV level, although it has nominal current. This high energy pulse can damage and break down electronic components and is technically referred to as “dielectric breakdown”
2.1.2. ESD Indicator symbols
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2.1.3. ESD preventive actions
In tackling ESD the goal is to mitigate the formation of load potentials and maintain an active continuous current draining. This can be achieved by creating equipotential links drainage through RC (resistance-capacitor) filters connected to ground. There is a whole industry producing material which is dedicated to ESD draining, and also to the prevention of load potentials.
2.1.3.1. In Electronics manufacturing plants
SAUTER manufactures according to international ESD standards and recommendations. The whole process starting with component storage through to implementing, welding, assembling, continuing through the test phase, is in accordance with the appropriate ESD recommended practices. This is continued with the packaging and storage material. The production areas are access controlled, lineated, kept at an appropriate relative humidity together with continuous training for personnel.
Typical equipment in use at SAUTER
ESD flour treatment Shoes and clothes ESD bracelet ESD bags and cases
2.1.3.2. In the field In the field rules must be respected, the use of equipment such as grounding bracelets, ESD footwear help to prevent ESD during work with electrostatically sensitive materials. It is important to note that ESD bracelets, for example, must be connected to the metallic chassis of the cabinet – the common ground.
For safety reasons before connecting an ESD bracelet the electrical equipment must be switched off and disconnected from all supply circuits.
TIP: Please always use the original ESD packaging together with the ESD bracelet. This also helps to prevent any oxidation of sensitive components minimise the handling of any electronics.
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2.2. EMI
2.2.1. What is EMI?
Globaly EMI is based on frequencies related to the field through the electrical lines and hardware. Originating from inductive equipment such as motors, FC, relays and transformers. The electromagnetic interference transmits energy emanating from circuit interruptions, power switching, radio guided interferences and degrades or obstructs the effective performance of other devices. This energy can influence measurement by disturbing or disrupting communication lines signals.
2.2.2. What does EMI look like
.
2.2.3. How is it coupled
2.2.3.1. Galvanic coupling arises at common impedances of current loops. This may happen in common components or line sections of circuits, e.g. flow over the transient currents that induce tension of the common line section. For PCBs, an impedance coupling could possibly occur with an inadequately sized ground tray and / or decoupling capacitors.
2.2.3.2. Capacitive coupling refers to the influence of an electric field (Tension) radiating to parallel conductors in a cable, a cable channel or guided in parallel tray on a printed circuit board. This effect will occur between high impedance parallel guided lines at a frequency range up to 30 MHz.
2.2.3.3. Inductive coupling refers to the transfer of electromagnetic interference or noise. The Inductive coupling is caused by magnetic field coupling, usually in conductor loops, e.g. between parallel-guided conductive loops, each having low termination impedances. The frequency range is generally up to 30 MHz.
2.2.3.4. Radiated coupling occurs when an electromagnetic field is applied directly to a circuit. Unconnected electric conductors or wires on a PCB will act as an antenna and receive radio signals these will appear as interference on the conductor. The frequency range begins at 30MHz up to 3 GHz.
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2.2.4. How to measure it ?
Visulisation of these types of signal is normally achieved with an Osciloscope or Spectrum analysier, these can also be used as acquisition devices for recording.
Caution: A simple multimeter can NOT be used in such a case. The measurement equipment needs to have a wide frequency band width.
2.2.5. Real world examples
Here we can identify two different signals based on the same 20Hz shape.
In the left image a highly disturbed signal from a modulo5 NI1000 input is observed.
In the right image the same signal after re-enforcing the grounding lines and connections by increasing the cross section of the grounding cable and short cutting the ground loops. The small bursts are residuals of a 50Hz component generated by an FC placed on another circuit in another cabinet.
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2.3. EMC
2.3.1. What is EMC?
EMC is the branch of electrical sciences which studies and regulates through standards the unintentional generation; propagation and receipt of electromagnetic energy with reference to the unwanted induced EMI effects. It aims to ensure that equipment or systems will not interfere with each other during specified operation. EMC standards are separated into groups based on the field environment and application, such as “Industrial environment”; “Residential - commercial environment” and “Medical branch”. EMC also defines the two main methods of transfer: Emission and Immunity. The applicable standards also define performance classes (A, B, C), providing the permissible function degradation during the immunity test for specific category of devices according the appropriate product family standard (Industrial or Domestic). More literature and further reading is available at the following link: http://www.cenelec.eu
For example
Coupling way Generic Standards topic Immunity EN 61000-6-1:2007 Immunity for residential, business and commercial environments and small businesses Immunity EN 61000-6-2:2005 / AC: 2005 Immunity for industrial environments Emission EN 61000-6-3:2007 + A1: 2011 Emission standard for residential, business and commercial environments and small businesses Emission EN 61000-6-4:2007 + A1: 2011 Emission standard for industrial environments
NB: Not all products are implemented in the same way in the same application. (Some are in electrical cabinets, others not. Accordingly they follow their dedicated EN as described in the product PDS. Consequently not all SAUTER products refer to the same EN standards. This document uses Modulo5 as an example. In order to apply the correct product to the correct role the PDS must be read and understood. If there is anything unclear this needs to be addressed first in order to ensure the correct product for the correct environment and application. The technical requirements of the application and its positioning within the installation needs to fulfil the appropriate EMC, EMI protection and filtering rules.
2.3.2. EMC world wide
Around the world different standards apply; they are all based on the same physical laws and technical understanding. However, from a legal aspect, the basic rule is as follows: The country which exports a product to another is required to prove that it has correctly achieved the rules and terms valid in the country it wishes to export to. A direct consequence of this is; if a manufacturer wants to produce overseas, he needs to demonstrate that the product has been manufactured with the same process as if it had been produced in the country where the device is to be imported to. All of the relevant national regulations must be observed.
2.3.2.1. In Europe
The CE mark on electric devices is the stamp which denotes the rules and standards relevant to that product, starting from component purchasing, storing, through production and assembly to the ex-works position have been applied.
2.3.2.2. In USA
FCC Part 15, MIL-STD461, etc…
2.3.2.3. In China
The CCC rules are applicable (China Compulsion Certification)
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2.3.3. Frequency Diagram
2.3.4. How is it coupled?
2.3.4.1. “Conducted” or “line bounded”
2.3.4.2. “Radiated”
NB: As it is coupled through EMI, please refer to paragraph 2.2.3. Above for further details
2.3.5. How to measure it?
As per definition EMC is radiated or line coupled, the measurements are specific for each type. Sonic-equipment (ultra-sensitive microphones) and Antennas are required for the radiated coupling. For the line coupled frequencies the use of filtered frequency range probes connected to the spectrum analysers are commonly in use. In order to perform rightness and accuracy the test sample is placed in a faraday cage or cabinet. For the measurement of the radiated component, acoustic chambers as shown below are used.
In both cases the rules are similar. The test sample is placed into the appropriate chamber and will be tested for emitted and received signals according the product defined standards.
NB: Field tests for complete building are not possible for evident technical reasons. Consequently handheld equipment exists for the measurement of a specific area or zone.
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2.4. Transients (or fast transients)
2.4.1. How are they generated?
Physically long interconnects, such as power lines and cables between equipment, may be subject to very large voltages and current transients due to a wide variety of phenomena such as line bounded inductive switching. Unlike ordinary EMI and/or lightning which normally cannot cause permanent damage to equipment. Transients are fast events of high magnitude and could significantly harm electrical and electronic apparatus.
2.4.2. What do they look like?
Single spikes from inductive switching results in high amplitude, high tension (some kilovolts) but less energy
Transients can also occur in multiples, caused by rebounding switching which results in a combination of spikes called “Bursts” and these commonly also contain harmonics. When the tension is also high then naturally they will transfer a higher energy rate.
2.4.3. How are transients measured?
As previously stated, transients are fast spikes, consequently to make those visible an appropriate measuring and recording apparatus is required. See section 2.2.4 for further details
2.4.4. How to avoid Transients?
Electronic equipment is provided with resistors their function is to drain and unload the energy. But not all equipment has such integrated protection. Implementation of dedicated filters (transient diode combined with RC filters or varistor) may be required.
Further details see in chapter 6.1.2.2
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3. The electrical cabinet
3.1. General purpose
An electrical cabinet is a plastic or metal box dedicated to housing electrical connections and equipment. The dimensions are dependent upon application and differing functions, for example IP protection, electrical dispatcher, automation cabinets, Ethernet switching, telecommunication distribution, etc… As a general rule, cabinets are externally sourced and the guidelines for their production originate during the planning phase of the project and are engineered to the specific requirements of the application. The cabinet maker, the design engineer and purchasing department should be fully aware of the technical, electrical and physical limitations of the application. This is a highly technical function requiring expertise in and understanding of the aforementioned issues.
3.1.1. Basically an electrical cabinet has the following roles:
It protects the equipment inside (IPxx) from physical environmental effects It protects the equipment from environment such as EMI or mechanical damage It protects personnel from accidental electrical shock It permits the centralisation of similar equipment, functions and operations
3.1.2. Electrical cabinets need to follow good practices like :
Never mix the AC and DC Never mix circuits of differing equipment types Never mix different types of loads (e.g. inductive, capacitive)
Avoid mixing electrical devices with non-electrical ones (Wet, hot, cold, radioactive, etc…) Avoid parallel connections on one terminal
Always consider grounding as a special circuit which requires special attention Always use dispatchers or additional terminal blocs for multiplying connections Always use the correct wire/cable cross sections Always use the correct fuse or disrupter Always check the tightness of the connections
3.1.3. Input / output
Electrical cabinets must have different entrances and exits. Usually they are as follow:
Power cable entrance Dispatched or switched power exit Low voltage circuits Ethernet or communication lines Grounding HMI such as switches, signalling: visual or audible, touch panels
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3.1.4. Cabling
Cabling is one of the most important steps in the construction of an electrical cabinet, because it requires the careful preparation and correct realisation.
Prepare and define
Choose the correct implementation of the devices, positioning and placement Choose the appropriate positioning for cable management, routing or trays Choose the appropriate cabling types with appropriate cross sections and lengths Choose the correct and appropriate dispatchers and terminal blocs Choose the correct colours to differentiate active and passive cabling
Implement to realise
Step by step working (fit the physical then the electrical) First wire the power cables and then the command signal cables Respect the correct order of wires and cables (don’t mix them) Respect the schema point by point (No interpretation of common lines) Don’t forget the testing phase at each important step
3.1.5. Prerequisites for accessing electrical equipment within an electrical cabinet
Accessing electrical equipment within an electrical cabinet for maintenance or other functions should only be carried out by qualified personnel. It is a normal requirement that a technician must gain authorisation from a responsible person before they can access for the purpose of maintenance or to gain access to electrical cabinets, equipment, devices. It is country dependent if a third party is required to monitor for safety.
3.1.6. Rules of thumb
3.1.6.1. Reserve enough “free space” in the cabinet for easy placement, cabling and future modifications
A 30% rate is usual Place it at the right level (dispatcher, disrupters, relay, terminal blocs, etc.) Consider the right time for cabling. Everybody in this branch knows that cabling/identifying/testing are the most time consuming operations for the cabinet builder: A rate about 4/5 is common Don’t underestimate the testing operation
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3.1.6.2. Power lines
Place the incoming power as close as possible to the main terminal by using the shortest lines. If you standardise cabinets, place the power cable outside in a dedicated cable tray along the cabinet instead of going through the whole cabinet length. (Case of the upside down orientation)
3.1.6.3. Grounding (see chapter1)
Always use the adequate cable section. The section linking the main grounding reference point to the main earth point must be consistent and have the correct dimensions. Use adapted dispatcher Connect the wires one by one. No serial or relayed connections
3.1.6.4. EMI in the cabinet
Don’t mix electrical grounding and EMI shielding/grounding
Use at least a five to ten times thicker wire for EMI protection cables (6 to 10mm2 is a minimum for active cable of 1.5mm2)
The cabinet chassis should never be used for relaying EMI cables or potentials, Prefer direct wiring by using stranded wired cables and interweaved wires for equipotential links
Don’t forget to link and connect the shielding/grounding of the remote equipment
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3.2. Cabinet structure
For safety reasons and achieving local, national and international regulatory compliance establishing an electrical cabinet may differ from region to region, however basic electrical rules have to be respected. (See paragraph 3.1)
Nevertheless different types of devices and potentials have to be present in the same area, which can give rise to certain problems.
3.2.1. The normative part
Depending on the environment in which they will be placed in, electrical cabinets must comply with certain standards as for example:
2004/108/CE EMC standards 2006/95/CE Electrical equipment design for use within certain voltage limits EN 60204-1 stands for the cross section of the PE/Grounding lines EN 50262 stands for the connection screw dimensioning EN 60617 defines the symbolists for schematics and identification EN 61346-1 defines the labelling for devices (e.g.: “G” stands for “Generator”) EN 50274: Low-voltage switchgear and control-gear assemblies EN 61439: Low-voltage switchgear and control-gear assemblies EN 60947: Low-voltage switchgear and control-gear assemblies
NB.: Electrical schemas are part of the cabinet
They must be present in the cabinet They must be up to date All modifications must be implemented and documented
3.2.2. Equipment placement
1. Identify the different types of equipment, devices according their voltage and consumption. 2. Assign an optimal location for each group of devices in order to facilitate the cabling and avoid mixing of differing signals. 3. Harmonise the global placement in order to permit separated and dedicated grounding for each group. There will be one electrical ground as all connected to one point (See chapter 1) 4. Cost effective evaluation before starting the operative purchasing of the required equipment.
3.2.3. Cable tray
Enough cable/wire trays must be positioned for each cabling group, (Power, Data …) in order to permit the correct separation avoiding interference and crosstalk
NB: Cable trays use a lot of space in a cabinet, consequently it is required to check their dimensions and integrate in the space calculation of the cabinet. See section 3.1.6.1
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3.2.4. Identification
The ability to identify each cable is a prerequisite of cabling a site or cabinet. The identification of each wire must be appropriate and comply with the electrical schema. Each wire must be numbered with the same number appearing on each end. The grounding cables have mandated “yellow/green” bicolour Shielding should also be clearly identified (which shield comes from which cable)
3.2.5. Cabling (see chapter 3.1)
The way cables and wires cross each other is relevant to the EMI. Crossing must be perpendicular to avoid a long coupling of radiated emissions The bending angle of the cable tray and wires must be adapted to the wire cross section to avoid cracks and stretching due to bending
3.2.6. Connections
The following points must be respected for realising correct connections
Use correct wire terminals Use appropriate tooling Use correct terminal blocs and bridges
Always tighten the terminal bloc screws even if they are not connected Always check for the correct physical tensioning on the wires Never connect more than one wire in the same terminal bloc, parallel or dispatching is not permitted; with the only exception for common potential dispatching 0V,(+),(-),GND and only when using the appropriate adapted terminal bridge (They are calibrated by the manufacturer for a specified cross section)
NB: Wrong or incorrect connections will introduce resistance and false contact, flickers, voltage spikes and EMI
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3.3. Grounding in cabinets
As already described in the previous chapters, grounding requires special attention, due to its complexity. Three grounding types are commonly identified each have all their own sets of rules
3.3.1. Electrical grounding
Standard electrical safety rules required for the protection for personnel, public and products. (See chapter 1.2.1 and 1.2.2)
3.3.2. Shielding/grounding
This is covered by normal EMI rules which provide long-term, efficient and reliable function
3.3.3. EMI rule
Equipment such as FC or inductive devices engenders induction which is coupled through the lines (see sections 2.2.3.3 and 2.2.3.4). Consequently such devices should never be placed in the same electrical cabinet without additional preventive measures.
As defined by Fleming’s rule, whenever a current carrying conductor comes under the influence of a magnetic field, there will be a force acting on the conductor. This force will induce a current. For this phenomenon there is a relation between magnetic field, electric current and force
EMI filters, a separate shielded dedicated cabinet area, separate cabling tray and terminals should be used. Nevertheless, the use of dedicated cabinet for such devices is highly recommended. Such devices should never be placed close to signal and/or communication lines or sensitive low power cables, such as NI1000 temperature measurement
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The capacitive effect of cables (see section 2.2.3.2), especially in the cable tray, must be taken into account. The capacitance of cables increases with their length and proximity to each other.
Separate dedicated cable tray is required to avoid common mode, or any radiated influence. A minimum distance of 200mm between the cable trays is required.
If cabling is crossed it must be at 90° (section 3.2.5)
Shielding is a fundamental part of the EMI rules, by linking electromagnetic disturbances to the ground the disturbance is dissipated.
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Grounding of the cabinet consists not only of the correct connection of the yellow/green wires, but also into interconnecting all metallic parts of the cabinet. (Front and rear plates, doors, mounting plates, barriers, etc…)
NB.: The different cabinet layers must also be interconnected and grounded
The same rules are valid for mounting the devices and cabling into cabinets or on electrical chassis
These rules apply to the cable trays. Using the correct interconnects and tools
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Long cables, communication cables, sensitive signal cables and motor cables must be shielded on both ends as soon as they can be connected to the same grounding potential.
If this is physically not possible, because of long distance and/or different building they must be grounded on the cabinet side at least.
NB: Single side shielding (cabinet side) is better than no shielding, but in most of the cases this will not be enough for creating a stable ground capable of draining (or suppressing) the EMI efficiently. In order to reinforce the stability it is recommended to connect than the grounding cables on both ends to the local ground.
NB: As ground is supposed to have a very low resistance, this type of interconnection helps the stability and draining the EMI disturbance to the ground by the shortest way.
Data lines are often very long (>100m). The shielding of the second end of the cable is grounded through capacitors to facilitate filtering of high EMI frequency. It is also recommended to use double shielded cable where each end is connected locally to the ground.
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3.4. Cabinets and projects
Electrical cabinets can be considered as the “heart” of an installation. In order to manage it professionally, it is mandatory that projects and cabinet building are checked for compliance to the above mentioned rules prior to construction, on completion and after delivery. Basically two main points and additional rules are in the focus
3.4.1. New projects
The cabinet management follows a strict chronology and canvas.
Cabinet builder and project leader must agree on technical terms according to these guide lines. This phase consists in a dialogue and common meeting resulting in a written agreement which includes all technical specifications, standard compliance and the concrete test phases of the cabinet and their results. Only after those phases a cabinet should be delivered and installed.
3.4.2. Renovation projects
In renovation projects the time spent in this phase is more critical as additional steps have to be done. The actual electrical situation of the site should be determined and recorded and a study of suitability for a retrofit of the existing installation is viable as regulations may have changed since the original implementation. Nevertheless section 3.4.1 must be included in this assessment.
Of course the fact of adding/removing cables requires also a special attention, especially for cable trays, ground connections, because they are based on the existing and possibly outdated regulations and understanding. Consequently, the ground resistance and reliability must be checked as well.
3.4.3. Global cabling rules
All equipment placed or used outside an electrical cabinet or related to, follows this same basic guide lines and rules. For instance cable tray placement requires respect of certain distances between the different lines and signal groups. Cable cross section is mandatory for good function of the installation.
3.4.4. Respect of the rules
As written above, mandatory means it binds partners, to respect the defined and agreed terms. Consequently if a delivered cabinet or installation is not conforming the agreed terms, it must be re-built from the beginning before it can be used in the field
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3.5. Cabinet setup (According section 3.2.2)
FC’s are commonly used in the field. It is also evident they play a significant role in the appearance of EMI based issues and thus they require special attention as described in section 3.3.3. Consequently two hypotheses are possible for cabinets.
3.5.1. FC in separate cabinet
Cabinet (A) is “clean zone” Cabinet (B) is “dirty zone”
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3.5.2. FC in the main cabinet
Although it is recommended to place the FC in separate cabinet (see previous chapter) based on commercial decisions it is not unusual to find small FC’s placed in the main cabinet.
“Such a case requires special attention and improvement!”
As shown in the above figure a clear separation “------“identifies a “dirty” zone from a “clean” zone. It shows the placement of different device types according their technical characteristics. Cable tray and metallic barriers are used to separate physically and electrically the two main zones. The shielding connection terminals are placed close to the “sensitive equipment” in the clean zone. Using the shortest cabling avoids radiative and capacitive coupling of the cables.
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4. Diagnostics
Overall definition and philosophy
The following chapter aims to provide help and a methodology to facilitate diagnosis of issues surrounding grounding by providing a trouble-shooting framework. The philosophy behind this is a continual learning model, helping the user to gain in aptitude into solving problems based on experience. With this step by step strategy progress towards to the solution by hypothesising possible causes first and eliminating them as potential culprits. Trouble-shooting requires a smooth progression as per its definition it is success oriented based on a logical approach at each step backed up by concrete information. By never eliminating any possibilities which have not been checked, it requires open minded thinking. Preconceived ideas have no place in this process; they act as unintentional brake and destroy the positive efforts. In such a case it is fundamental not to put the focus and all effort in one direction or on one product only. The view must be large and open in order to acquire the maximum information and inputs which will be treated systematically.
“Step back and acquire the global perspective”
Once a hypothesis has been proposed it is essential to follow it to its logical conclusion. In most cases the effects could have several root causes building a real chronological chain of accumulated faults.
“Trouble shooting is the rolling back of this fault chain”.
Of course fault finding and trouble isolation is time consuming and requires rigor and pragmatism. It is therefore better to plan the project correctly in the first instance.
Einstein is quoted as having said: “If I had one hour to save the world I would spend fifty-five minutes defining the problem and only five for finding the solution”
“Occam's razor” principles could also be related to fundaments of a troubleshooting:
"If two theories both explain the observed facts, then you should use the simplest one until more evidence comes along"
"If you have two likely equal solutions to a problem, choose the simplest."
During the diagnostics process one may or may not have the experience to find a solution, but with every investigation the depth of knowledge is deepened and this expanded knowledge will help in the next investigation. The following sections have been conceived to assist in finding a solution when none seems possible.
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4.1. Step by step methodology
The idea is that if two people start from differing points they should arrive at the same conclusion. A linear approach enables clear step both forwards and backwards when needed. This approach defines a structure and function for each process so that it could, if needed, be repeated, and the same result achieved, this reduces risk, costs and demonstrates a professional approach to the customer.
4.1.1. Defining the rules
This step is mandatory and has two relevant aspects: structural and operative. It requires also surrounding and defining the problem itself (see also section 4.1.8). Afterward, clear rules are required to define the role, and position, of each person. This will assist in avoiding further conflicts and fulfilling the rule defined in section 3.1.5. It will also characterise the responsibility structure of each process or stage - preparation, processing and finalising of operations. As a prerequisite for work in or around a cabinet, it is also mandatory to check the ability and qualification of the personnel involved. (See section 3.4, 3.1.5)
4.1.2. Safety assessment
As a direct consequence of previous section, the safety assessment is required to guarantee the success of the diagnostic and trouble shooting in a safe way for personnel. It includes of course all the required safety procedures and authorisations. It must be in a written form and signed up by all involved people.
4.1.3. Definition of the terms of operation
Interested parties should agree on the current status and then agree on a plan. This phase consists in a dialogue with all partners and involved personnel
Further steps must be discussed, planed and scheduled
4.1.4. Definition of technical responsibilities
This step is relevant especially if different companies are involved and have to engage their support and responsibility with others
A summary agreement should be used as a reference if an issue over responsibility arises
4.1.5. Definition of operative zoning
Defining local access rights, both physical access and timed access, ensures optimal usage of the available Personnel, “dove-tailing”
The end user or their representative must acknowledge the terms of planned actions for each sector
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4.1.6. Defining additional special clauses
This step takes place only if required from one of the participants.
Generally it describes the technical, legal, operational restrictions caused, for example, by on-going uninterruptible processes
4.1.7. Define the field
Normally a pre-study should have defined the different aspects of any operative implication in a process. Consequently, the troubled field should be known and defined.
A special clause can avoid limitations if necessary and permit the enlarging of the operative field
4.1.8. Problem Identification
To enable a solution and getting an issue resolved, two steps are required
a) Customer description This is an important and fundamental phase, during which the direct contact helps and facilitates the information flow and exchange
b) Collecting of evidence Issue root causes are deduced by the effects which could be hidden or not visible in first place. That makes this step crucial for the further diagnostic. Remember Einstein saying above
Good practice is to write down in a summary the technical points in short and factual frame document for expanding an elimination of probable causes
4.1.9. Technical diagnostics
This part is a cornerstone for which we dedicated a whole chapter. It consists in a succession of chronological actions related to the expected results. See section 4.2
4.1.10. Applying the chosen solution
This operation comes directly after the diagnostic phase (see section 4.2.4) as the fruit of the different diagnostic tests carried out and their consequences. Application of the agreed solution
4.1.11. Efficiency testing of the applied solution
This step is very helpful to acquire confidence that the solution is valid. It is an important stage in problem solving. It will technically validate the applied solution and set new bases or fundaments for long term solutions.
4.1.12. Results
This operative part acts as conclusion of the technical part. It summarises all the results on the different actions and previous steps and is preparing the technical documentation which is the direct next step of the process
4.1.13. Update and consolidate the information with all the interested parties
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4.1.14. Documentation
Problem solving can only be efficient if it is documented in detail. At this stage the documentation is a “one to one” report of the previous steps supported by records, descriptions, collecting of evidence, pictures, video, etc…
4.1.15. Feedback capitalisation of knowledge
By giving a feedback we make statements based on our experience and skill level. It helps other people gain better understanding of the phenomena and is part of the teaching activities for the new comers, trainees and experts alike. Feedback will also help to improve products and their dedicated documentation from a technical prospective. Feedback is also a major part of the technical support process not only in cases where the solution requires help from partners, technical support engineers or product support groups
CAUTION: Trouble shooting requires a step by step process and mainly one step at a time to avoid confusion and unexpected results.
4.1.16. External Expert
In some cases it could be mandatory to order an external expert for investigating at the installation. In such a case, please ensure the qualification and experience of the expert is in accordance to the requirements and the personnel have a valid accreditation.
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4.2. Technical diagnostic
Diagnostic consists in monitoring an action and interpreting the observed result. The action may be present or may require manipulation. It is also the accumulation of the symptoms and effects resulting from previous actions. Where is no place for doubts or approximation in this part and the result is mathematical. It is also the step in which you need to make “things” visible by using the appropriate equipment and apparatus (*); Only if “things” are visible, they can be quantified, recorded, appreciated and remedied.
(*) Intrusive or not intrusive
In order to perform measures and test it is fundamental to understand this “nuance”. Test results are dependent upon the choice of the test equipment. Consequently good preparation is required for success; here we identify two types of test equipment.
Intrusive equipment. This provides direct values and measures from an internal perspective of the system.
For example a recorded data file by a BMS system will provide information about the system. A web access will also give an instant value of the live system.
The positive point for such information is that it comes quickly and accurate from inside of the system.
The negative point of this type of information is, if the system is corrupted there can be inaccuracies in the information recorded leading to false and unusable information.
Not intrusive equipment provides an instant record of the status through an instrument placed outside of the system. It generates data and values very accurately without influencing the system or the sample under investigation.
The positive points of such equipment are high accuracy, stable without influencing the samples and the process, permitting discrete sample acquisition
The negative point is, it needs additional equipment, knowledge, training and availability
4.2.1. Summary
The initial step is to know where you are before starting manipulating and generating further actions are listed as follow:
a) Collect all available information and evidence b) Summarise and cross check all the received information c) Check how precise the information is d) Crosscheck the results with your experience and knowledge pool in order to eliminate incorrect or inaccurate information
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4.2.2. Action/reaction
Before proceeding to the next stage one should have some indications in which technical direction you want to orientate the diagnostics. (EMI, electrical, software, product misunderstanding or misuse)
a) Establish new probabilities and rules b) Provide new measurements and evidence in direct relation with the previous result c) Check the correctness (robustness of your diagnostic method) d) Crosscheck the result with your knowledge base e) Continue and loop steps b, c, d, as long as the results differs from expectations
4.2.3. Result and contra measures
If the obtained result in 4.2.2 (e) is the direct answer to the early fault statement than continue with following steps:
a) Build up your last workout as a potential solution b) Generate a procedure for reproducing the issue c) Reproduce it and document the results
4.2.4. Efficiency test
The idea of this step is to find out if the reproduced solution in 4.2.3 (c) is reliable or if it fits only to the particular situation you are faced with.
- If the reliability test confirms the solution and is applicable in same conditions everywhere, it means you found the origin of the trouble and you can eradicate it
- If in contrary the reliability test proves that your solution works only in that particular situation, then you found a work around and not the root cause of the trouble Conclusion
Two main possibilities are offered in 4.2.4. The customer may accept the workaround. However the actual cause should be identified. The professional way, and good practice, leads you to re-start the technical diagnostic process from the beginning. Of course later on you will have much more reliable evidence provided by this new situation which should result in arriving at solution more quickly.
The use of identical devices for testing provide results by comparison and could be a good and simple way to rapidly arrive at the solution. But it doesn’t state of the exactitude of the test (case of a serial trouble of the used device)
Don’t forget to record everything at every time as it is very helpful and will guide you to review a step of the process.
4.3. Commissioning
Diagnostics could be supported by using all the collected and constituted documentation and records during the commissioning phases of on installation.
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5. Real world examples of grounding issues
5.1. Freezing of a modular based PLC system.
We have here the case that EMI is disturbing the PLC internal communication bus causing the “freezing and desynchronisation” of the communication with the base station.
5.1.1. Customer trouble description
“There is the problem that after a few hours, after power reset some modules of the PLC ceased to work. The 0-10V output is frozen, although it is still commanded to refresh the values.”
5.1.2. Analysis and diagnostic
The analysis demonstrated that the site was disturbed by EMI generated by the FCs. The following basic test clearly shows this.
The following measurements were been taken directly on the PLC module input connection of the NI1000 sensor.
The following measurement was taken in the same conditions on the base station
This is the measurement taken on the module again after having switched off the FCs
This next picture shows the cabling of the cabinet surrounding the PLC. As shown here the signal lines, power lines and switched outputs are more or less separate.
However, the connected tensions are mixed between base station and the modules.
For this reason it is essential to clearly define the location of the different module types within the station.
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This picture shows how the cables were placed in the cable trays supplying the cabinet under investigation
Please refer to section 3.2 for the cabling information
5.1.3. Results
As shown in the pictures and with measurements the disturbance appears only when the FCs are switched ON. The cabinet cabling is also not optimal. The cables, trunking and routing is not optimised for a problem reduction scenario.
5.1.4. Action / Reaction
Improving locally the grounding of the cabinet by increasing cross sections of the grounding cables helped to reduce the EMI influence (see right picture in section 2.2.5) Rewiring partially the grounding in the cabinet brought also good results by reducing the crossing of cables Connecting shields of signal cables were also part of the improvement, which helped to stabilise the signals and enforce draining the disturbances to the ground Reorganising the position of some modules within the station helps in avoiding the crossing of high and low energy cables which stabilised the whole station environment. The customer was advised to reposition the FC to another location far away from the PLC cabinet as mentioned in section 3.5.1
5.1.5. Conclusion
Evidence, proving the trouble origin has been identified and validated as the cause. The module freezing has been reproduced online and recorded. Implementation of countermeasures in accordance to best practice and guide lines has been performed. The site has been stabilised and the PLC is still running fine. Of course the disturbance induced by the FCs continues to operate on site (Even following repositioning) because the device needs additionally dedicated EMI filters in order to minimise their influence.
Nevertheless the end customer has followed the recommendations and applied them to his installation. The local technical staff acquired more experience and gained more confidence in dealing with such issues.
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5.2. Pending PLC inputs
Under this paragraph we discuss two scenarios the reporting of which were similar, however had different origins.
5.2.1. First example
5.2.1.1. Customers trouble description
Sometimes the universal PLC input displays between -50 °C and +150 °C (Min / Max of defined process value) and the digital input can’t see any status changes
5.2.1.2. Analysis and diagnostic
“The same effects give the same results”; Based on that we applied the same trouble shooting procedure as usual:
Following points have been analysed
- Investigation of grounding/shielding, Earth connection, Power supply, electrical cabinet, wiring and analysis of existing electrical schema - Check of the assembly of the PLC, modules topology, product index, respect of Best Practice and all product information. - Analysis of the input signals with an oscilloscope - Analysis of the signals and disturbances by oscilloscope - Analysis and measurements of the earth ground potential
For all plants, the following measures should be implemented when necessary
- Implementation of additional enhanced grounding connections, rework and measures - Control of the implemented contra measures - Implementation of a recording device for evaluation ( => if not present - In buildings BMS software generates historical data which can be utilised) - Optimization of PLC and regulating parameters if necessary - Firmware update of the PLC if required - Customer teaching and hand over recommendations and written report
5.2.1.3. Action / Reaction
Following this procedure, we found out immediately that on this site the disturbance level was continuous and very high.
This picture shows a very disturbed PLC input on which the inquired NI 1000 Temperature sensor is connected.
This following picture shows the same input after turning OFF some FCs present in the same area.
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Following this initial approach we started to improve the installation by deploying the same strategy as described above and which we used in previous examples. We found out first that FC’s were incompletely grounded. From electrical side the yellow/green earth wire was correctly setup with a cross section of 1.5 mm2. But the equipotential point (marked in red in the following pictures) was missing
Measuring the ground current and voltage we found high values such as 115 Vac, generated by high frequencies (about 15 KHz). Tracing the grounding lines we could find the main ground entrance point of the technical room.
As we found it, “things” became clearer. In fact on the left picture you can see that paint is covering the whole connection inside and outside. It results that the main grounding for the whole HVAC installation was isolated from ground and Earth. After clearance of this point (right picture) the same PLC input signal was now as shown in following oscilloscope record
As the PLC signals were still disturbed we continued to check line by line the inputs in order to find out the highest disturbance amplitude and we followed the cables through the cabinet and technical rooms.
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Of course this disturbance is still not acceptable for a PLC and espeically as it transfers a temperature signal other a NI1000 sensor based on pulsed signal and in a very low voltage level = very sensitive.
Consequently we decided to continue the investigations line by line in order to find out on which the disturbance amplitude is the highest. We found it and followed it once again through separate walls and rooms and finaly found a small integrated FC placed into the housing of the heat recovery exchanger.
The shielding of the 0-10Vdc command signal cable was connected only on the cabinet side.
Per chance this cable included shielding and grounding (yellow/green), thus we could connect the shielding to the local grouding point (HVAC housing) and the wire coming to the electrical cabinet. Aftwards we checked the signal on the PLC and it was as follows
5.2.1.4. Results
The FCs were radiating energy through cables and shielding thus creating disturbance and influencing the PLC measurement signals to such a level that they were swinging from minimum to maximum of the defined process value.
5.2.1.5. Conclusion
Using now the improved procedure (from site to site) and rolling out the described methodology (see above and section 4.1 and 4.2) implementing contra measures as described in section 5.1.3 and 5.2.1.2, the EMI could be dissipated to an acceptable level which does not disturb the installation and function of the PLC.
But it also demonstrates that in the major cases the troubles were coming from field and not from devices or products. As it is explained above, it is not possible to certify a whole site, plant or building for EMC. Consequently, it is the responsibility of each manufacturer to fulfil the appropriate product standards and it is under the end user full responsibility to follow the guide lines and best practice for installing, commissioning and operating.
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5.2.2. Second Example
5.2.2.1. Customer trouble description
In the BMS the temperature of one of the heating dispatchers shows unrealistic temperature values. These are manifested by positive temperature outliers varying between 90 and 150 °C.
5.2.2.2. Analysis and diagnostic
As a first step we applied the same analytical procedure as described in section 4.1 and 4.2. Due to the vague customer description a recoding tool (moduWeb Vision in this case) was positioned in order to acquire a cleared picture of the spikes and unrealistic values. Using an oscilloscope these were successfully traced as shown below.
It can be seen that the incoming temperature signal to the PLC is disturbed, also the normal square signal (pulsed PLC input) is absent. Consequently the PLC acquires sporadic average values which it relayed to BMS with the consequences of a very instable regulation loop.
5.2.2.3. Action /reaction
Checking cable by cable and connection by connection of the concerned sensor line, it was found out that one cable was insufficiently screwed on into the terminal bloc. Consequently, sporadic electrical contact was present on this line.
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After reconnecting the cable properly into the terminal bloc, the input signal recorded by the oscilloscope was displayed as shown in the follow picture.
NB: Typical square signal of the PLC pulsing NI1000 sensor input.
5.2.2.4. Results
In this case the simple re-connecting of the cable ended in winning back customer’s “lost function”. A later long term record proved the absence of spikes or sporadic unrealistic values.
5.2.2.5. Conclusion
Improper cabling is the root cause of the malfunctioning of the installation. Using the appropriate tools (measuring apparatus) and applying the right investigation strategy provided a good trouble shooting experience. From technical prospective, the false electrical contact can be masquerading as an EMI disturbance to the highly sensitive PLC input.
5.3. Conclusion
“EMC or not EMC!”
All those examples showed different origins causing the same apparent result. It can be simple to refer to these as “EMC trouble” However this is clearly not the case in most issues. Technically it provides the customer with a false image of the product, the technician and the manufacture name.
Given the experiences above and following back along the fault chain, using a logical approach and gaining all the facts, what appears to be EMC trouble has its origin linked to an EMI issue which is generated by the installation and not by the product.
Consequently, the so called “EMC trouble” is in fact EMI caused by grounding issues or poor or incorrectly applied cabling
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6. Grounding First Aid Guidance
6.1. EMC / EMI in the field
6.1.1. Differential mode // Common mode
In the EMC two modes of interference propagation are recognised and operate as follows.
6.1.1.1. Differential mode (symmetric)
The interference current (Idiff) propagation is glided by the existent active lines. For example if the perturbation is generated in (L) the current glides through the receiver and comes back to the start point by the return line (N). Interference potential (Udiff) will appear between (L) and (N)
6.1.1.2. Common mode (asymmetric)
The Common mode interference applies to all active line simultaneously in the same sense. The common mode current (Icom) propagates in the same way on the active lines (L,N) and feeds back through the capacitive grounding
6.1.2. Filters and their technology
In order to reduce disturbance and avoid dysfunctional systems, filters have been developed for damping EMI. Since electronic circuits are built on the above basic principles, the filters have to reduce the effects in both cases, common and differential mode. To create an optimal filter, capacitors will be placed in parallel between the interference and main reference point. In a second step inductances will be place in line to compensate for the current disphasing. Surge resistors could also be added for dissipating the discharged energy of the interfering spikes.
Two main effects are sought by filters.
Spike suppression, transient cut-off and energy dissipation Continuous damping of EMI frequencies
Independent of the filter type the fundamental rule is to place them as close as possible to the interference source.
Interference protection (immunity): filter directly at the network noise source Interference suppression (emission): Filter close to the disturbance source
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6.1.2.1. Different EMC filter levels
Level1 Level2 Level3
Cx = filters the differential mode; Cy = filters the common mode; L = are inductances
The level differentiation is generated by the frequencies which will be filtered out on each level. Level3 filter is basically the association of the three level 1 filters. Consequently each level acts differently and could sometimes interfere to another level. Thus level3 filters are not commonly used for filtering of main power lines, they are most commonly integrated into electronic devices.
Classical EMC filters are structured as explained in 6.1.2
Example of integrated solutions
In order to be efficient, filters need to be defined according the main voltage and the load current of the supplied equipment based on a 50/60Hz main frequency.
EMC Line filters EMC Plug filters
6.1.2.2. Other filter elements
Depending if the power supply is AC or DC, passive components such as varistor, transient suppressor diodes, ferrites or active dynamic microprocessor controlled filters are available on the market.
Transient suppressor Varistor Ferrite
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Example of use of ferrite kernels
6.1.2.3. Manufacturer integrated systems
Relay manufacturers provide integrated EMC Filters which are calibrated and dedicated to their devices. There are also filters which could be connected separately as shown below
+ Integrated solution Universal solution
6.1.2.4. Filtering of PLC inputs
As previously described, PLC cabling must follow the guide lines. It is sometimes necessary to add capacitors for damping differential current mode generated through long cables or capacitive coupling on the lines. The disturbance could sometimes be so high as it would trigger the input to logical “1” even though no actual signal is present and the input is on a logical “0”. In such a case an oscilloscope analysis would quantify the amount of disturbance.
Typical value is 100nF/100V connected directly in parallel to the PLC input as shown below
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6.2. Tips and tricks
6.2.1. How to measure
As described in section 2.2.4 appropriate measurement apparatus is required for identifying EMI.
Additionally some rules can help to present a professional image. Remembering, it is mandatory to check the safety rules and defined conditions (see section 4.1.1, 4.1.2) Make sure all required PPE are present and in good conditions
Before using measurement equipment, ensure it is functioning correctly and a self-check diagnostic has been run when available. Check it regularly with calibration checks to ensure the device is still working as expected. Sometimes there may be additional calibration required for specific environments and one should be aware of the limits of the test equipment.
6.2.1.1. Oscilloscope
An oscilloscope provides voltage measurement. It is recommended to reference the input channels to the ground for accurate and correct measurement as it is then linked to the only common reference point. It permits the ability to record spikes displaying magnitude variations.
CAUTION: to avoid short circuits during measuring, some oscilloscopes require a galvanic separation (isolated transformer) from their own power supply in order to uncouple the ground which is common with the probes. In order to avoid this technical problem, it is recommended to use a battery powered device. They are completely isolated from the field and do not shortcut the ground. Noise reduction filters for probes are required, especially if the measurement is based on high frequencies, or very small disturbance levels
6.2.1.2. Ground current
Ground current should be measured using a current clamp multimeter
6.2.1.3. Recording
As EMI is not constant in the field, it fluctuates according the root cause of the disturbance and the level of interference, this is dependent on the emitted EMF. It is then recommended to record the signals over an appropriate time period. This could be hours or days depending on the type of disturbance and the installation.
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Good practice is to record before the EMI investigation starts as it will give a baseline picture of the installation. It will also enable comparative diagnostic to demonstrate and quantify the progress of the investigation.
It is good practice is also to continue with the recording following application of contra measures as result of the investigation. It assists in the fine tuning of filters and gives conformation of a stable function of an installation. See section 5.2.2.4
6.2.2. How to detect EMI
First step is to identify clearly the origin of the EMI. For this the use of measurement equipment is essential in order to fulfil the diagnostic procedure and acquire records of the disturbance shapes.
The second step in good practice when applying a “risk methodology” and rules (*) is to turn off the probable causes. In cases such as section 5.1.2 the shutting down of the FC demonstrated immediately the origin of the disturbance. If a few such devices are present, it is recommended shunting down one by one with a certain time interval. At least 2-3 minutes is required for the disturbance signal to decrease before switching off the next unit.
(*) If a risk situation is identified, and the origin is known, then the best way to solve an issue is to cut- off the origin. By eliminating the risk at the source the situation is clean. See Occam’s razor in section 4, remember though there may be more than one source, one can mask another.
Good practice is also to the measure the ground current as it relates the interference energy flowing through it. Normally the ground current is extremely low and stable. Example: If the current rises higher than 500mA in a 230V domestic installation the GFCI will engage to disrupt the circuit due to exceeding the current limit.
6.2.3. How can the EMI effects be reduced
In fact EMI can’t be avoided totally because of the necessity of the equipment which is EMC proofed, but which give up some disturbance as soon as it is connected to an installation. This of course is not a fatal flaw: By applying the guide lines and good practice rules the root causes for EMI could be eliminated. If a there is a persistent disturbance in the installation, it can only be reduced by identifying it and using an adapted logical progression.
6.2.3.1. Action /reaction
Identify and isolate the disturbed line as a potential origin Check the grounding through the whole installation following the technical diagnostic procedure described under section 4.2. Most of EMI issues can be solved by reinforcing and improving the grounding If necessary apply in a second step adapted contra measure as described in sections 6.1 Caution: as mentioned in section 4.2.4, a solution is valid only if it could solve the problem globally and definitively Check the results. As EMI is not constant, it is recommended to continue the recording to confirm that the applied contra measure is a long term solution.
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6.3. Cabling.
The following table is none exhaustive. Please fill free to apply your own experiences
Identified trouble Remedy Insufficient connection tightening Unfasten the connection and rebuild it correctly Incorrect cabling – too many wires in the Place additional terminal blocs and use appropriate same terminal terminal bloc bridges Incorrect grounding - equipotential lines Build up additional grounding lines with appropriate missing cross sections and connections Not respecting grounding types Build up the grounding as defined in the schema. Never mix TNC-TNS Grounding using serial connections is forbidden Incorrect cable cross sections (to small) Change the cables for the correct cross section, appropriate terminals and connection blocks Incorrect cables placement in cabinets or Rebuild the cable positioning according section 3.2, cable trays 3.3, 3.4, 3.5 Use separate colours for different signal groups Group different signal groups by using separate cable tray Not respecting the standard cable colours Rebuild the cabling using the correct colour scheme according the country regulation. e.g.: Yellow/green is Ground / Earth Untightened screws and/or unconnected They must be tightened as if they were connected terminals Incorrect cable/terminal numbering Each end of a cable must have the same number in accordance with the electrical schema.
6.4. Main ground
The following table is none exhaustive. Please fill free to apply your own experiences
Identified trouble Remedy Incorrectly tightened connections Unfasten the connection and rebuild it correctly Too high ground resistance See section 1, 1.2.1, 1.2.2 and 5.2.1.3 Reposition the main ground Place additional earth points (see section end 3.3.3) Check ground cable for appropriate cross section Request a resistance measurement and value of the main ground connection point High EMI disturbance measured by Establish if the grounding line is correctly connected oscilloscope to the main ground/earth. Make sure all interconnections are sufficiently tightened. Establish where the disturbance is coming from: - By isolating line by line and following it to its origin - By switching ON/OFF Potential disturbance sources
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Identified trouble Remedy High tension is found on Grounding cables Check the cabling for direct power line mix Check the cable tray in order to find out radiated or inducted EMI FC in use FC has to follow special rules for grounding. See manufacturers PDS and apply Ground current is to high Check the exact origin of the induced current. Are the correct shielded cables in use? Are the grounding/shielding connections correctly tightened? Has the main ground reference point the required ground resistance? Check for grounding loops Check if the ground is switched or disconnected from the main grounding connection point GFCI is switching off the circuitry Check the cabling for direct power line mix Check for short circuits between grounding and active lines
6.5. Shielding issues
The following table is none exhaustive. Please fill free to apply your own experiences
Identified trouble Remedy Incorrectly tightened connections Unfasten the connection and rebuild it correctly Inappropriate cable is installed Change the cables for the appropriate cable with the correct shielding. (single or double shielding) See section 3.3.3 Inappropriate shielding placement in the Reattach the shielding to the correct location. See electrical cabinet section 3.3.3 Shielding and grounding For efficient shielding, it must be grounded with a separate ground cable to the main grounding point Cable shielding See section 3.3.3 Use of FC Connecting FC’s requires shielded cables. See section 3.5
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7. Checklist for grounding
The role of the following checklist is to guide the user through the whole process in order to ensure the installation has been properly designed and realised. It gives also the confidence that the commissioning complies too the fixed and educated SAUTER rules and standards. The list is based on the different sections of this document.
7.1. Grounding (See chapter 1)
The grounding type has been identified and understood ☐ The grounding resistance is known and conforms ☐ The grounding connections are in accordance with the guide lines ☐ The grounding connections have been tested ☐ The grounding connection terminals are tightened (including the unconnected ones) ☐ The grounding current is at the lower (acceptable) limit ☐ The grounding cables present the appropriately dimensioned ☐ The power supply, transformers are grounded according this guide lines ☐
7.2. Shielding (see section 3.3)
The shielding cables are clearly identified ☐ The shielding connection type is in accordance with the guide lines ☐ The shielding terminals are tightened (including the unconnected ones) ☐ The shielding connections are grounded properly ☐ The grounding cables relaying the shielding presents are appropriately dimensioned ☐
7.3. Cabinet (see section 3)
The cabinet contains enough “free space” ☐ Enough cable trays have been planned ☐ The chassis, doors, walls are grounded according this guide lines ☐ The material and equipment has been implemented according this guide lines ☐ The cabinet is grounded with appropriately dimensioned cabling ☐
7.4. Cabling (see section 3.2 end 3.3)
The different signal types are separated in different cable trays ☐ The different signal cables have a different colour for each signal type ☐ The different signal types do not cross each other or cross perpendicular ☐ The cable terminals are tightened (includes also the unconnected ones) ☐ The cable trays outside the cabinet have been implemented according these guide lines ☐ The cable trays are grounded sufficiently, with appropriately dimensioned cable ☐
7.5. Mandatory (see section 3.4)
The project has been approved and clearly identified ☐ The recommendations in this guide lines has been read, understood and applied ☐ All the implicated people are informed and have approved the project ☐ The drawings, diagrams and electrical schemata have been approved ☐
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8. With reference to SAUTER products
The following section is supplementary to other product specific documentation available from SAUTER, (PDS. FI etc.), as with the whole documentation the aim of this section is to inform and illustrate with real world scenarios
8.1. SAUTER technical standardisation
8.1.1. Development process
SAUTER also defined and managed rules for product development. Based on project management, the product development consists in several chronological phases according to the maturity of the project and the product. Each developmental phase is based on a review which sanctions the next phase.
8.1.2. Signal directive
With respect to the ISO rules, SAUTER defined technical requirements for electronic signals and their function in accordance with the product standards. A technical document relating different aspects and rules is consigned in the SGR Management system. The fundamentals of the documents guides the developer to good practice with respect to the standards and country specific regulations. It provides defined technical values for PLC, actuators, power supply.
8.2. Modulo 5
8.2.1. EY-AS525 modular automation station with BACnet/IP and web server.
As its name is self-explanatory, the concept is based on a modular system composed by a base station including high performance electronics. The system permits the addition of up to 8 modules which are communicate via an integrated serial bus relayed through each module by the interconnection pin connector (picture below)
8.2.2. Technical features
Every product has to fit to the application which it is dedicated to in accordance and respect to the standards and regulations. Therefore its technical features and characteristics must be adapted with the appropriate tolerances. All this information is included and detailed in the corresponding PDS. Consequently, all connected device have to fulfil the correspondent rules too. For example: A NI1000 temperature sensor must conform to DIN 43760 in order to be connected to the modulo 5, as the input of the system requires a defined impedance and sensor curve
NB: The PDS, handbook, FI, guidelines and best practice must be read, understood and applied.
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8.2.3. Relations between base station and modules
….
Order of the modules
Modules are based on different technologies. They are physically different to each other and as a consequence their placement must respect a certain defined logical order for their correct function. We can classify them into two priority categories as following:
The product dependent
For example the communication modules must be placed at the first position after the base station. This is required in order to avoid time delay, unstable impedance and corrupted data and communication between the base and the module.
The hardware conditioned
In this category we can identify the cabling and signal dependent devices. As described under section 3, the different input signals and the voltage levels define the way the cabling must be constructed. This automatically defines the placement order of the modules.
NB: With exception of the communication module, a predefined order cannot be established as the modularity is open to too many possibilities induced by the multitude of the end user applications. Nevertheless, the EMI and cabling rules are a prerequisite for their correct placement.
Conclusion
The communication modules should be positioned directly, against the base station. All the other following modules must be placed according their sensitivity and the level of the voltage they carry. Cabling must respect the information provided in this document in section 3 end 7
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8.2.4. Cabling
In first instance the cabling of the complete assembled modulo station (base + modules) must respect the PDS and Pi. It is also mandatory to apply the guidelines of the previous sections
Short summary of the basics:
Place the modules according to their features and power levels Assign a different colour for each signal type (NI1000, Tension, Current, Digital output, etc…) Separate the different type of signals by cabling Use correctly dimensioned cable trays Place enough cable trays surrounding the complete station Place the cabling in separate cable trays if signal crossing can not be avoided
As noted the following examples are provided for illustration purposes. Here describing the relationship between the base station and modules and the cabling
In this example the modules have been reordered to avoid the crossing of cables. The use of separate cable trays or barriers in the same cable tray is recommended. If cable crossing persists, it will require a perpendicular placement with a respectable distance in order to avoid radiated EMI
The placement of the module is dependent on the differing types of signals. Therefore it is mandatory to realise a pre-schema before the definitive module allocation and data points could be affected to each module.
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NB: If the station needs to be placed vertically (not recommended as the air flow is not optimised in this configuration). When not avoidable, it must be immobilised with isolated block terminals placed on each end. This requirement is mandatory in such cases, as it will relieve the mechanical tension of the connections between each module and the base station.
8.2.5. Signals
In order to enable a correct functioning of the station, the different signals need to be grouped in signal families, such as:
Sensitive
Low power supplied lines as for example RS-485, NI 1000, Pt1000, Ref Tension (1.2V), 4-20mA, potentiometer, and isolated input module.
NB: For these lines it is recommended to use shielded cable as much as possible
Normal
In this category the digital input together with 0-10V output lines or command lines powered by the internal 13.5V
Switching lines
Digital outputs such as relays, open collectors switching small inductive loads, coils, heating elements are the devices which are usually connected. As described in section 3.5.1 they could be identified as “dirty lines” from an EMI prospective
NB: Additionally the following rule must be observed. Therefore each signal type must have its own GND connection
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8.2.6. Open connections on the station
To avoid malfunction of the station, it is mandatory that unattributed hardware inputs and outputs must be disconnected from any wire or cable as the radiated EMI would disturb the proper function of the station
8.2.7. Interconnections between stations
In some installations an interconnection of stations could be induced by the application. This situation requires a special attention, especially if the electrical cabinets are positioned in different locations or if the electrical diagram or schema is split or merged. In fact the hardware needs to have the same grounding reference; otherwise floating potentials will appear and could disturb the proper working of the stations Always prefer the use of BACnet software possibilities to copy the data from one station into another, instead having hardware wiring between the both units.
8.3. Tips and tricks
8.3.1. In the case of a identified EMI trouble on one or more inputs it is recommended to apply the contra measures described in section 6.1.2.2 (ferrite kernel) and/or 6.1.2.4. A combination could be required, but only if there is a diagnostically evident EMI issue. Such contra measures should never be applied preventively or systematically without prior diagnostic and measurements.
8.3.2. Station grounding loop
In some rare and specific cases a grounding loop, connecting the of each module with base station connected to the main grounding reference point of the cabinet will help to reinforce the stability of the signals. Such contra measures should never be applied preventively or systematically without prior diagnostic and measurements.
8.3.3. Changes and additional modules
Retrofit, reorganisation, additional devices, implementation of new functions and hardware is common during the lifecycle of an installation. Consequently the addition of lines or modules to an automation station induces new setup of the whole system. (Hardware and software) It is then mandatory to check and commission all equipment affected by any rework. Please refer to section 4 for help on how to manage such changes
8.3.4. Current / consumption
The protection fuse for a station needs to be defined in accordance with the result of the calculation of all potential load currents which apply during normal working. The consumption of a station is defined by the average current which applies during normal function of the station and the system. Nevertheless, consumption spikes will appear during starting and high traffic in the system. The protective fuse should be capable of absorbing consumption spikes without disturbing the power supply. (A temporised fuse is recommended)
NB: don’t forget to include in your calculation the consumption for the display, additional control modules
8.3.5. Relays
The integrated relays of the station and modules are designed and conform to the applicable EMC product standards. Consequently, they don’t need additional EMI protections or filters Concerning external relays commonly used as coupling or dispatcher, need to fulfil the rules of section 6.1.2.3.
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8.3.6. Work in an electric cabinet
In some case it is necessary to rework or implement some changes after the cabinet has been delivered and commissioned. In such a case the use of isolated shields and protection is required to avoid dust and any unwanted material falling into the electric equipment, as they would generate malfunction or short-circuits.
This picture shows the presence of small metallic chips on the electronics of a basic station.
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Personal notes …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… ……………………………………….…………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… ………………………………………………...... …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… …………………………………………………………………… ……………………………………………….…………………… …………………………………………………………………… …………………………………………………………………… V1.0- 9.01.2015 Page 64 of 68
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Resume
The authors hope, having transcribed the collective experience in a realistic manner, a booklet which will benefit each reader.
This document should also encourage the reader to desire solving problems using simple but directed applicable solutions
As with every guide line, this document is a reference for good practice and professional approach for novice and experienced technicians who face EMI and grounding issues.
Many thanks to the SAUTER sales organisation for continuing to share their experiences and technical knowledge
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