Download How DMMs measure current

Basic electrical
measurements
Using handheld
electronic test tools
©2009 Fluke Corporation
Basic electrical measurements
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
©2009 Fluke Corporation
Basic Electrical Measurements
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Digital multimeter basics
Agenda
• Chapter 1:
• Chapter 2:
• Chapter 3:
• Chapter 4:
•
•
•
•
•
•
•
A first look at the DMM
Multimeter safety
Multimeter specifications
Multimeter measurements
Ohm’s Law: basic volts, amps, ohms measurement
Special functions: Min/Max, Peak only for 87V or 289
Voltage: understanding high input impedance
Current: using current clamps
Resistance: DMM source voltage and multiple sources
Testing components: diodes, caps, resistors
Measuring temperature
• Chapter 5: Non-linear loads
• True-rms vs. average-sensing
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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
• HOLD button
Hold function
©2009 Fluke Corporation
Basic Electrical Measurements
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Check out the back...
Look at the back of the
meter:
• Safety warning
• Fuse ratings - How are
fuses specified?
• Certifications
• Battery access
©2009 Fluke Corporation
Basic Electrical Measurements
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Front Panel Symbols
Symbol
Meaning
V
V dc
V
V ac
mV
Millivolts (.001 V or 1/1000 V)
A
Amps
mA
Milliamps (.001 A or 1/1000 A)
µA
MicroA (.000001 A or 1/1000000 A)
Ω
Resistance (Ohms)
kΩ, MΩ
Kilohms, megohms
))
))
©2009 Fluke Corporation
Continuity beeper
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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
Automatic Touch Hold /
Shift (second function)
Hold updates automatically.
©2009 Fluke Corporation
Basic Electrical Measurements
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First look at the DMM
Summary
What we learned:
• Meaning of front panel symbols
• Back panel safety warning and other information
• TouchHold & AutoHOLD functions - How they
work
©2009 Fluke Corporation
Basic Electrical Measurements
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Chapter 2
Multimeter safety
• Test leads & probes
• Fuses
• Overload protection
• IEC 61010 standard
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
Caution!
Don’t leave the
leads in mA or A
input jacks and
then take voltage
measurements.
©2009 Fluke Corporation
A, mA/uA inputs
Ammeter circuit inside DMM
COM
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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?
©2009 Fluke Corporation
Basic Electrical Measurements
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Safety inspection
High impedance on V/ 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
What about
protection
for ohms
measurement?
©2009 Fluke Corporation
Basic Electrical Measurements
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Safety inspection
Overload protection on volts inputs
With leads in V/ and COM inputs:
Step 1: Select V and put probes
in a live outlet.
Will you damage the meter if you...
Step 2: Select mV?
Step 3: Select ?
Step 4: Select A?
Overload protection is only to the DMM’s rated voltage.
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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Chapter 3
DMM specifications
• Display
• Accuracy
• Range and
resolution
Electrical
Electronics
©2009 Fluke Corporation
Basic Electrical Measurements
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Understanding DMM display
specifications
Display is specified as digits or as count
• Digits: 3 1/2, 4 1/2, etc.
• Example: 3 ½ - starting from the least significant digit,
3 “full” digits from 0-9, 1 “half” digit at less
5000 count
than 9. Example: 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
©2009 Fluke Corporation
Basic Electrical Measurements
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Understanding DMM accuracy
specifications
Accuracy is specified in percentage
• Closeness with which an instrument reading approaches the
true value being measured; largest allowable error
• Percentage of reading (DMMs) 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
©2009 Fluke Corporation
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Understanding DMM specifications
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
1V
(= 1000 mV)
(To exit Manual Range, hold Range button for two seconds)
• For maximum resolution, choose the lowest
possible range.
©2009 Fluke Corporation
Basic Electrical Measurements
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ABCs of DMM specifications
Summary
• What we learned:
• Display specifications - digits or counts
• Accuracy specifications - percent of range or
percent of reading
• Range and resolution specifications –
Low range, high resolution (e.g., 400.0 mV)
High range, low resolution (e.g., 400.0 V)
©2009 Fluke Corporation
Basic Electrical Measurements
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Chapter 4
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
©2009 Fluke Corporation
Basic Electrical Measurements
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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 =
©2009 Fluke Corporation
Basic Electrical Measurements
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Special functions
DMM as recorder: Min/Max/Avg
• Capture sags: (>100 ms)
MINMAX
• Fluke 179 - Push MIN MAX button.
(Meter beeps with each new MIN or MAX.)
• Scroll through Max, Min and Avg screens
by pushing MIN MAX button.
• Record voltage sag as motor is turned on.
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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:
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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) =
Reverse the procedure.
V OUT (METER 1) =
• Now connect both meters in 
mode across a known resistor. Both meters are sourcing
voltage. What is the  reading?
©2009 Fluke Corporation
Basic Electrical Measurements
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How DMMs measure current
Current clamp accessories
• In power circuits, clamps are used to measure amps.
• Two types of clamps: ac or ac/dc
AC
AC/DC
Output signal
Current
Voltage
Scale factor
1 milliAmp
per Amp
1 milliVolt
per Amp
Sensor
Current
transformer
Hall effect
Battery
No
Yes
(Scope clamps have BNC connectors: ac or ac/dc, both output mV )
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
©2009 Fluke Corporation
Basic Electrical Measurements
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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
True-rms measurement (of ac current)
requires a true-rms meter.
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
• Inline current measurements (meter in series):
• Measure current through the DMM using a battery and resistor.
©2009 Fluke Corporation
Basic Electrical Measurements
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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 °F or °C
• 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 °F or °C
• Internal 9 V battery
©2009 Fluke Corporation
Basic Electrical Measurements
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How DMMs measure temperature
Temperature accessories
• 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)
©2009 Fluke Corporation
Basic Electrical Measurements
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How DMMs measure temperature
Some DMMs have integrated temperature
measurement functions. Temp
function
• Adapter accepts type K
thermocouple probes. Remove
for voltage measurement.
Temp
°C/°F
MIN MAX temperature
• Select TEMP (C/F).
Select MIN MAX.
• Measure hot (Max)
and cold (Min).
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
©2009 Fluke Corporation
Basic Electrical Measurements
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Testing components
Capacitors
• Fluke 179:
• Turn dial to Capacitance
• Press yellow button to select
• With probes in voltage jacks,
measure cap
• Measurement note:
• 1.0 µF (microfarads) = 1000 nF
(nanofarads)
• 0.1 µF = 100 nF
©2009 Fluke Corporation
Basic Electrical Measurements
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Testing components
Diodes
Diodes turn ac to dc.
• A good silicon
diode will have a
voltage drop of
approximately
0.5 V to 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.
©2009 Fluke Corporation
Basic Electrical Measurements
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Testing components
Diodes
• Forward bias = ____ V
Red lead
Black lead
anode
cathode
• Reverse bias = ____ V
Red lead
Black lead
©2009 Fluke Corporation
anode
cathode
• Shorted:
0 in both directions
• Open:
OL in both directions
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Testing components
Diodes
• 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)
©2009 Fluke Corporation
Basic Electrical Measurements
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DMM measurements
Summary
What we learned:
• It’s the law: Mr.  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.
©2009 Fluke Corporation
Basic Electrical Measurements
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Chapter 5
Measurement issues with non-linear
loads
• True-rms vs. average-sensing
• Crest factor
©2009 Fluke Corporation
Basic Electrical Measurements
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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?
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
Power consumed in R1 is the same for both ac and dc
source if the Vacrms equals Vdc.
©2009 Fluke Corporation
Basic Electrical Measurements
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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
©2009 Fluke Corporation
Basic Electrical Measurements
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True-rms vs. average-sensing
What if the waveform is non-sinusoidal?
• 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.
©2009 Fluke Corporation
Basic Electrical Measurements
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True-rms vs. average-sensing
What causes non-sinusoidal 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
The diode -capacitor input
circuit draws short pulses
of line current during the
peak of the line voltage
©2009 Fluke Corporation
Basic Electrical Measurements
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True-rms vs. average-sensing
What if the waveform is non-sinusoidal?
• Average-sensing meters typically measure rms high for
voltage and low for current where there is waveform
distortion.
• True-rms meters or clamps accurately measure both
distorted waveforms and sine waves.
Multimeter type
©2009 Fluke Corporation
Average
True-rms
Response to
sine wave
Correct
Correct
Response to
square wave
10 % High
Correct
Response to single
phase diode rectifier
40 % low
Correct
Response to threephase diode rectifier
5 % to 30 % low
Correct
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True-rms vs. average-sensing
What if the waveform is non-linear?
Current measurement exercise:
• Measure these loads with true-rms and
avg-sense clamp, noting differences:
• Linear load (hair dryer/drill, incandescent
light bulb)
• Non-linear load (TV, monitor, PC, dimmer,
• CFL light bulb)
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?
©2009 Fluke Corporation
Basic Electrical Measurements
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True-rms vs. average-sensing
What is crest factor?
• Crest factor = Peak / rms
• For ideal sinewave, CF = 1.414
©2009 Fluke Corporation
Basic Electrical Measurements
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True-rms vs. average-sensing
What is crest factor?
• For this current waveform, crest factor = 2.9.
©2009 Fluke Corporation
Basic Electrical Measurements
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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.
C.F. = 2.39
C.F. = 4.68
C.F. = 1.43
©2009 Fluke Corporation
Basic Electrical Measurements
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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 minimum rating
• Distribution level: power distribution equipment.
©2009 Fluke Corporation
Specifications subject to change without notice. 9/2009 2427834B AO-EN-N
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