<p> Module 2 Basic electrical measurements Using handheld electronic test tools Goals of this presentation • Understand safety specifications and how to operate handheld electronic testers in a safe manner • Understand how handheld electronic testers and accessories perform basic measurements • Learn how to set a digital multimeter (DMM) to the correct function and range for a given measurement • Learn how to measure a variety of electrical parameters and test electrical components • Determine the proper measurement tool for safe and accurate measurements • Understand the differences between average responding and true-rms measurement on non-linear loads Digital multimeter basics Agenda • Chapter 1: A first look at the DMM • Chapter 2: Multimeter safety • Chapter 3: Multimeter specifications • Chapter 4: Multimeter measurements • Ohm’s Law: basic volts, amps, ohms measurement • Special functions: Min/Max, Peak Hold • Voltage: understanding high input impedance • Current: using current clamps • Resistance: DMM source voltage and multiple sources • Testing components: diodes, caps • Measuring temperature • Chapter 5: Non-linear loads • True-rms vs. average-sensing A first look at the digital multimeter • Visual inspection • Front panel symbols • Hands-on safety inspection: • Test leads and probes • Amps inputs: fuses • Volts/Ω inputs: overload protection Front panel features: • Volts / Ω / inputs How is this input protected? • Amps, mA, mA inputs How is this input protected? • CAT IV - safety rating • Range: select manual ranging • button Second function • button Hold function Check out the back... Look at the back of the meter: • Safety warning • Fuse ratings How are fuses specified? • Certifications • Battery access Front Panel Symbols Symbol Meaning V V dc V V ac mV Millivolts (.001 V or 1/1,000 V) A Amps mA Milliamps (.001 A or 1/1000 A) µA MicroA (.000001 A or 1/1,000,000 A) Ω Resistance (Ohms) k Ω, M Ω Kilo-ohms, megohms ) )) ) Continuity beeper Front panel symbols Symbol Meaning Capacitance (uF: microfarads, (nF: nanofarads) Diode test Hz Hertz (cycles/sec) dB Decibels Range Manual measurement ranging Hold TouchHold/AutoHOLD - last stable reading MIN MAX Highest, lowest recorded readings Dangerous voltage levels Caution: see manual TouchHold Displays last stable reading • Turn dial to Vdc • Press Hold • Take measurement • Remove probes • Press hold a second time and you are in Autohold • Turn dial to Ω Press Hold Measure resistor Remove probes Measure second resistor First look at the DMM Summary • What we learned: • Meaning of front panel symbols • Back panel safety warning and other info • TouchHold & Autohold functions -- how they work Multimeter safety • Test leads & probes • Fuses • Overload protection • IEC 61010 standard Safety inspection Test leads and probes Check test lead resistance: Step 1: Insert leads in V/  and COM inputs Step 2: Select  , touch probe tips Good leads are 0.1 - 0.5  How do you check a single test lead? Visually check for: • New category rating (CAT III-1000 V or 600 V CAT IV recommended) • Double insulation • Shrouded connectors, finger guards • Insulation not melted, cut, cracked, etc. • Connectors not damaged: no insulation pulled away from end connectors • Probe tips: not loose or broken off Safety inspection Amps inputs need fuses • In a power circuit, use current clamp accessory or stand alone clamp meter • In low energy ckt, 10 A or less, open the circuit: • Measure in series (current is the same in a series circuit). The amps circuit resistance must be small to have a minimal effect on the current. This low impedance input requires fuse protection. Safety inspection Checking meter fuses on most meters Step 1: Plug test lead in V/  input. Select  Step 2: Insert probe tip into mA input and read value Step 3: Insert probe tip into A input and read value Is the fuse okay? What would an open fuse read? Safety inspection High impedance on Volts/ inputs • Volts measurements need high impedance circuit • Voltage measurements are in parallel Voltage is the same across each parallel branch • Parallel circuits divide current: High impedance branch = less current Low impedance branch = more current Safety inspection Overload protection on volts inputs</p><p>With leads in V/  and COM inputs:</p><p>Step 1: Select V and put probes in a live outlet. Will you damage the meter if you...</p><p>Step 2: Select mV Step 3: Select  Step 4: Select A. Overload protection is only to the DMM’s rated voltage. Common DMM / tester hazards • Arc from transients (lightning, load switching) Protection: Independent certification to meet CAT III-1000 V or CAT IV 600 V • Voltage contact while in continuity or resistance Protection: Overload protection in ohms up to the meter’s volt rating • Measuring voltage with test leads in current jacks Protection: High energy fuses rated to the meter’s voltage rating Use meters / testers without current jacks • Shock from accidental contact with live components Protection: Test Leads double insulated, recessed / shrouded, finger guards, CAT III – 1000 V. Replace when damaged • Using meter or tester above rated voltage Protection: Good karma Multimeter safety Summary • What we learned: • How to check for good test leads • Why amps inputs need fuse protection • Low input impedance circuit • How to check for open fuses in the meter • Function of overload protection on V/ inputs DMM specifications • Display • Accuracy • Range and resolution Understanding DMM display specs Display is specified as digits or as count • Digits: 3 1/2, 4 1/2, etc. • Example: 3 1/2: starting from the least significant digit, 3 “full” digits from 0-9, 1 “half” digit at less than 9. Ex: 1999 • Can be confusing: how do you specify 3999? • Count: 6000 5000 4000 3200 etc • 4000 count display reads from 0-3999 • 3200 count display reads from 0-3199 • Hands-on: 6000 count display • Select V, measure battery Understanding DMM accuracy specs Accuracy is specified in percentage • Closeness with which an instrument reading approaches the true value being measured; largest allowable error • Percentage of reading (digital multimeters) vs. percentage of scale or range (analog meters): Example: 1 % scale vs. 1 % reading % scale: If scale or range is 1000 V, an accuracy of 1 % is equal to +/- 10 V. 120 V reading could = 110-130 V % reading: 1 % accuracy with 120 V reading = 118.8-121.2 V • Least significant digit unstable: Example: Accuracy spec = +/-(1 % +2) Reading of 200.0 mV= 197.8 - 202.2 mV </p><p>Understanding DMM specs Range and resolution • Resolution is the smallest change in measured value to which the instrument will respond • As the range increases, the resolution decreases: Turn Fluke 179 to Vac and hit Range button (Auto disappears): Range: Resolution: 600.0 mV .1 mV (=1/10 mV) 6.000 V .001 V (=1 mV) 60.00 V .01 V (=10 mV) 600.0 V 0.1 V (=100 mV) 1000 V 1 V (=1000 mV) (To exit Manual Range, hold Range button for 2 secs) • For maximum resolution, choose the lowest possible range ABCs of DMM specs Summary • What we learned: • Display specifications: Digits or counts • Accuracy specifications: Percent of range or percent of reading • Range and resolution specs: Low range, high resolution (e.g.: 400.0 mV) High range, low resolution (e.g.: 400.0 V) DMM measurements Basic measurements: Ohm’s Law Special functions: Min/Max How DMMs measure voltage: Understanding high input impedance How DMMs measure resistance: No other voltage please How DMMs measure current: Using clamp-on accessories Testing components: Capacitors, diodes, LEDs How DMMs measure temperature Ohm’s Law (V=IR) Can you prove it, Mr.  • Battery voltage: V = • Resistor: R = • Calculate current: I CALCULATED = V / R = • Measure current: create series circuit with resistor and battery and measure current (use mA inputs): I MEASURED =</p><p>Special functions DMM as recorder: Min/Max/Avg • Capture sags: (>100 ms) • Fluke 179: push MIN MAX button. (Meter beeps with each new MIN or MAX) • Scroll through Max, Min and Average screens by pushing MIN MAX button. • Record voltage sag as motor is turned on. How DMMs measure voltage Measuring volt / input impedance Step 1: Meter 1 (179): Select ohms Meter 2: Select Vdc Use meter 1 to measure input impedance of meter 2. Meter 2 input Z = ______Ω Step 2: Reverse procedure Meter 1 select Vdc, meter 2 select ohms: Meter 1 input Z = ______Ω How DMMs measure voltage Advantages of High Input Z • Exercise: Gum wrapper battery • Step 1: Construct battery with foil, wet card and penny (don’t overlap penny onto foil) • Step 2: Select mV-dc and measure battery voltage How DMMs measure voltage Demonstrating “ghost” voltages • Turn meter to Hz. Lay leads parallel to power lines. What does the display read? • Voltage from hot to capacitively coupled ground: • Effect of floating ground: How DMMs measure resistance • The meter supplies voltage to the circuit • Presence of external voltage in circuit being measured causes meaningless readings and can damage a meter without overload protection • How it works: Measured V1 across a precision R1 is compared to measured V2 across an unknown Rx</p><p>How DMMs measure resistance Open circuit voltage • First measure “open circuit voltage” of meter when in ohms mode Meter 1: V (dc) mode Meter 2:  mode V OUT (METER 2) =</p><p>Reverse the procedure. V OUT (METER 1) =</p><p>• Now connect both meters in  mode across a known resistor. Both meters are sourcing voltage. What is the  reading? </p><p>How DMMs measure current Current clamp accessories • In power circuits, clamps are used to measure amps • Two types of clamps: ac or ac/dc (Scope clamps have BNC connectors: AC or AC/DC both output mV ) How DMMs measure current AC current clamp accessories • Current transformer (CT) style preferred for ac: • CT clamps have good noise immunity: recommended for ac variable speed drives and other noisy environments • How to use: use A inputs • They are CTs with 1:1000 turns ratio: 1 A on primary (circuit being measured) = 1 mA on secondary (input signal to DMM) • Connect probe to amps jacks of DMM • Select mA function on the Fluke 179 • True-rms measurements require a true-rms meter. How DMMs measure current AC/DC current clamp accessories • AC/DC clamps: use V inputs of DMM • Use Hall-effect technology: require batteries in clamp • 1 mV per amp • Select Vdc or mVdc to measure dc current • Select Vac to measure ac current • To measure ac+dc, use the following formula: • V total = Vac ² + Vdc² • Example: Vac = 5 V, Vdc = 5 V, but V total = 10 V</p><p>V total = 5² + 5² = 25 + 25 = 50 = 7.07 V • True-rms measurement (of ac current) requires a true-rms meter How DMMs measure current Measuring load current and inrush Plug the ac current clamp accessory into the meter: Fluke 179: use mA inputs Remember: 1 mA = 1 A • Select mA function • Select auto range and connect to mA input and common • Measure motor inrush current: • Select MIN MAX How DMMs measure current Single phase measurements • Measuring load current: measure hot conductor • Checking for shared neutrals: • Measure with load on and off: current in neutral with load off indicates shared N • If neutral current > hot current, indicates shared N • Ground current: • Measure hot and neutral separately. Difference is leakage current. Assumes non-shared neutral. How DMMs measure current Three phase measurements • Current imbalance on motor loads: % current imbalance = Deviation from average Average Max (of three phases) Example: A = 50 A, B = 30 A, C = 40 A Average = (50 + 30 + 40) / 3 = 40 A Max deviation = 10 A (10 / 40) x 100 = 25 % imbalance Motors should not exceed 15 % - 25 % current imbalance • Neutral currents at panelboard: • Fundamental N current caused by uneven distribution of single-phase loads among the three phases • 3rd harmonic N current from non-linear single-phase loads 3rd harmonic is additive in neutral How DMMs measure temperature Temperature accessories • Integrated temperature function • Use type K thermocouple probes (requires no adapter) • Non-contact: Infrared probe • Non-contact can measure electrically live or moving parts • 1 mV dc per ºF or ºC • 4:1 distance-to-target ratio: 4” away reads 1” circle • Internal 9 V battery (10 min. auto shut-off saves battery) • Contact: Thermocouple module • Uses mV dc function (requires input Z of 10 M  ) • Adapter for type-K thermocouple probes Comes with a general purpose bead probe • Switch selectable for ºF or ºC • Internal 9 V battery How DMMs measure temperature Temperature accessories</p><p>• Type-K thermocouple temperature probes • Mini-connectors plug into adapter • Different probes are specialized to measure: • Liquids and gels • Air and gases • Food • Surfaces including hot rollers and plates • Pipes (probe designed to clamp onto pipe) How DMMs measure temperature Some DMMs have integrated temperature measurement functions • Adapter accepts type K thermocouple probes. Remove for voltage measurement. MIN MAX temperature. • Select TEMP (C/F) Select MIN MAX. • Measure hot (Max) and cold (Min). Testing components Capacitors • Capacitors store electrical charge • Caution ! • Before measuring a cap, disconnect circuit power and make sure it’s discharged. Use Vdc to test if cap is discharged (= 0 V). • The 179 will display “disc” while discharging cap. • How it works: • The meter charges the cap with a known current for a known period of time, measures the resulting voltage (up to 1.2 V) and calculates the farads. Testing components Capacitors • Fluke 179: • Turn dial to Capacitance • Press yellow button to select • With probes in voltage jacks, measure cap</p><p>• Measurement note: • 1.0 µF (microfarads) = 1000 nF (nanofarads) • 0.1 µF = 100 nF Testing components Diodes Diodes turn ac to dc. </p><p>• A good silicon diode will have a voltage drop of approximately 0.5-0.7 V when it is forward biased (conducting). It will be open when it is reverse biased. • To test a diode, the DMM forces a test current through the diode in the forward bias direction and measures voltage drop across the diode.</p><p>Testing components • Forward bias = ____ V Red lead anode Black lead cathode • Reverse bias = ____ V Red lead anode Black lead cathode • Shorted: 0 in both directions • Open: OL in both directions Testing components • Diode forward bias = ____V (Red lead) + ---- P / N ---- - (Black lead) • Diode reverse bias = ____V (Black) - ---- P / N ---- + (Red) • LED forward bias = ____V (Red) + ----- P/N/P/N/P/N ------(Black) • Transistor: finding the base lead (Black) ------N / P / N ------(Black) + (Red) DMM measurements Summary What we learned • It’s the law: Mr. Ohm was right. • MIN MAX and other recorder functions • Voltage measurements: The ups and downs of high impedance inputs • Resistance: DMM is the only voltage source • Current: Capturing inrush current • Use of temperature accessories • Components: Capacitor and diode checks Measurement Issues with non-linear loads • True-rms vs. average-sensing • Crest factor True-rms vs. average-sensing How accurate is your meter? • When can you use an average-sensing meter and when do you need a true-rms meter? • Are you measuring a sine wave or something less ideal than a sine wave?</p><p>True-rms vs. average-sensing What does “rms” mean • Rms is the root mean square or effective heating value of any ac voltage or current waveform. • Rms is the equivalent dc heating value of an ac waveform. True-rms vs. average-sensing Average-sensing works for a perfect sinewave • An average-sensing meter assumes a non-distorted sinewave and does the following calculation: Rms value = 1.11 X average value True-rms vs. average-sensing What if the waveform is nonsinusoidal? • For this current waveform, the effective or true-rms value = 1.85 x average value • An average-sensing meter’s reading (1.11 x average) would be 40 % too low True-rms vs. average-sensing What causes nonsinusoidal waveforms? • Waveform distortion is caused by non-linear loads, which includes virtually all electronic loads: • Switching-mode power supplies (PC, office equipment) • Light switch dimmers and electronic ballast • Variable speed drives</p><p>True-rms vs. average-sensing What if the waveform is nonsinusoidal? • Average-sensing meters typically measure rms high for voltage and low for current where there is waveform distortion • True-rms meter or clamp accurately measures both distorted waveforms and sine waves</p><p>True-rms vs. average-sensing What if the waveform is nonlinear? Current measurement exercise: • Measure these loads with true-rms and avg-sense clamp, noting differences: • Linear load (hair dryer/drill) • Non-linear load (TV, monitor, PC) Voltage measurement: • Measure voltage using true-rms and average sensing meters while someone makes adjustments at the source. • When are the readings closest and when do they differ? True-rms vs. average-sensing What is crest factor? • Crest factor = Peak / rms • For ideal sinewave, CF = 1.414 True-rms vs. average-sensing What is crest factor? • For this current waveform, crest factor = 2.9 True-rms vs. average-sensing Crest factor is an indication of harmonics • For current or voltage measurements, the higher the CF, the greater the waveform distortion. • CF spec is important for accurate measurements. It is only specified for true-rms products. It is more critical for current measurements since harmonic distortion typically is higher for current than for voltage. True-rms vs. average-sensing Summary Minimum specifications for measurements on electrical power systems: • True-rms • Accurate for both linear and non-linear loads • Crest factor = 3 • Accurate for current waveforms with CF not exceeding 3 • CF = 3 at max range; CF = 6 at half-range • IEC 61010-1 CAT III-600 V • Distribution level: power distribution equipment.</p>