Electrical Hazards and Protecting Persons

Electrical Hazards and Protecting Persons

Electrical hazards and protecting persons 06 POWER GUIDE 2009 / BOOK 06 ELECTRICAL HAZARDS AND PROTECTING PERSONS The increasing quality of equipment, changes to standards and regulations, and the expertise of specialists have all made electricity the safest type of energy. However, it is still essential to take account of the risks in all projects. Of course, INTRO expertise, common sense, organisation and behaviour will always be the mainstays of safety, but the areas of knowledge required have become so specific and so numerous that the assistance of specialists is often needed. Total protection is never possible and the best safety involves finding reasonable and well thought-out compromises in which priority is given to safeguarding people. The safety of people in relation to the risks identified must be a priority consideration at every step of any project. During the design phase: By complying with installation calculation rules based on the applicable regulations and on each project’s particular features. During the installation phase: By choosing reputable and safe materials and ensuring work is performed correctly. During the operating phase: by defining precise instructions for handling and emergency work, drafting a maintenance plan, and training staff in the tasks they may have to perform (qualifications and authorisations). Risks to people Risk of electric shock � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 02 1� Physiological aspect � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 02 2� Protection against direct contact � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 05 3� Protection against indirect contact � � � � � � � � � � � � � � � � � � � � � � � � � � � � 08 Risk of burns � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 18 1� Contact with hot surfaces � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 18 2� Electric arcs � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 18 Exposure to electromagnetic fields � � � � � � � � � � � � � � � � � � � � � � � � � � 19 1� Low frequency magnetic fields (< 10 MHz) � � � � � � � � � � � � � � � � � � � � � � 19 2� Low frequency electric fields (< 1 MHz) � � � � � � � � � � � � � � � � � � � � � � � � � 20 3� High frequency electromagnetic fields (> 1 MHz) � � � � � � � � � � � � � � � � � 20 Protection by residual current devices Composition of residual current devices � � � � � � � � � � � � � � � � � � � � � � 22 1� Magnetic core � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 22 2� Current-sensing relay � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 22 Residual current devices � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 24 1� Determining the trip threshold � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 24 2� Choice of protection device � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 24 3� Specific conditions of use � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 27 Discrimination between residual current devices � � � � � � � � � � � � � � � 30 Requirement for class II � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 30 Protection provided by assemblies Class I assemblies � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 32 1� Connection of protective conductor � � � � � � � � � � � � � � � � � � � � � � � � � � � � 33 2� Equipotentiality of exposed conductive parts � � � � � � � � � � � � � � � � � � � � 33 3� Using the exposed conductive parts as protective conductor � � � � � � � 34 4� Checking the continuity of the exposed conductive parts � � � � � � � � � � 37 5� Checking resistance to overcurrents � � � � � � � � � � � � � � � � � � � � � � � � � � 37 Class II assemblies � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 38 1� Class II A enclosures � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 39 2� Class II B enclosures � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 40 3� Protective conductors (PE) � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 40 4� Checking the dielectric properties � � � � � � � � � � � � � � � � � � � � � � � � � � � � 41 Choice of products RCDs � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 42 RCBOs � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 43 Add-on modules � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 44 01 ELECTRICAL HAZARDS AND PROTECTING PERSONS Electric current is dangerous because it is Risks invisible. Its effects on humans are sufficiently well-known nowadays for us to be able to protect ourselves effectively. The risks that may to people affect the installation itself and the surrounding property must form the subject of an analysis leading to the adoption of the most appropriate protection solutions. RISK OF ELECTRIC SHOCK The effects of electric current on the human body depend on two factors: 1 PHYSIOLOGICAL ASPECT - The duration of the current flow through the body When subjected to voltage, the human body reacts - The intensity and frequency of the current like a conventional receiver that has a given internal These two factors are independent of one another, but resistance� Electric current flows through it, with the level of risk will be more or less high, according three serious risks: to the value of each factor� the intensity of the current - Tetanisation: the current keeps the muscles through that is dangerous for humans will depend on the volt- which it passes contracted, and if this involves the rib age and the tolerance of the human body� In practice, cage, this may lead to respiratory arrest the intensity of the current is defined based on a limit - Ventricular fibrillation: this is a total disruption voltage UL taken as being 50 V� this voltage takes of the cardiac rhythm account of the maximum current that a human being - Thermal effects causing varying degrees of tissue with minimum internal electrical resistance can injury, even deep burns in the case of high currents withstand, under specific conditions� It also takes The table below shows that, for a touch voltage account of the maximum admissible duration of the of 220 V, a current of 147 mA will pass through current flow through the body, with no dangerous the human body� this current must therefore be physiopathological effects (cardiac fibrillation)� broken in less than 0�18 seconds to avoid any risk� Maximum breaking time according to prospective touch voltage Prospective touch Electrical impedance Current flowing through Maximum duration voltage of the human body the human body of flow UC (V) Z (Ω) IΔ (mA) t (s) ≤ 25 (1075) (23) 50 1 725 (925) 29 (54) (0,48) 75 1 625 (825) 46 (91) 0,60 (0,30) 100 1 600 (800) 62 (125) 0,40 (0,22) 125 1 562 (762) 80 (164) 0,33 (0,17) 220 1 500 (700) 147 (314) 0,18 (0,05) 300 1 460 (575) 205 (521) 0,12 (0,025) 400 1 425 280 0,07 500 1 400 350 0,04 Values in brackets are for double contact, both hands, both feet (UTE C 15-413) RISK OF ELECTRIC SHOCK 02 In view of the two parameters to be taken into account In some installations or under particular for evaluating the risk, the standards define the time/ conditions (damp areas, moist skin, low current limit curves� resistance flooring, etc.) a lower impedance value can be taken into account (values in These curves, taken from IEC 60479-1, give the brackets in the table on the previous page). various limits of the effects of 50 Hz alternating current on humans and define 4 main risk zones� Time/current curves showing the effects of 15 to 100 Hz alternating current on humans 10000 (1) abc1 c2 c3 5000 AC-4.1 2000 AC-4.2 1000 500 AC-1 AC-2 AC-3 AC-4.3 200 ent flow t in ms rr 100 50 (1) If the current flows for less than 10 ms, the limit of the current passing 20 through the body for line b Duration of cu 10 remains constant at 200 mA 0,1 0,5 2 10 50 200 500 2000 10000 0,2 1 5 20 100 1000 5000 Body current i in mA Zone Physiological effects AC-1 Generally no reaction AC-2 Generally no dangerous physiological effect Generally no organic damage� Probability of muscle contractions and breathing difficulties if current flows for longer than 2 s� AC-3 Reversible interference in the formation of cardiac impulse propagation without ventricular fibrillation, K increasing with the intensity of the current and the flow period� SHOC Increasing with the current intensity and duration, pathophysiological effects such as cardiac arrest, respiratory arrest and severe burns can occur in addition to the effects of zone AC-3� RIC AC-4 AC-4�1 : Probability of ventricular fibrillation up to approximately 5% T AC-4�2 : Probability of ventricular fibrillation up to approximately 50% AC-4�3 : Probability of ventricular fibrillation greater than 50% F ELEC O ISK R 03 ELECT RICAL H AZARDS AND PROTECT ING PE RSONS Risks to people (continued) Conventional time/current zones of effects of d.c. currents on persons for a longitudinal upward current path (hand to feet) 10000 abc1 c2 c3 5000 2000 DC-4.1 1000 DC-4.2 500 DC-1 DC-2 DC-3 DC-4.3 200 ent flow t in ms rr 100 50 20 Duration of cu 10 0,1 0,5 2 10 50 200 500 2000 10000 0,2 1 5 20 100 1000 5000 Body current

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