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The waveforms on today’s AC power lines are anything but clean.  Electronic equipment such as office computers, with their switching 
power  supplies,  produce  harmonics  that  distort  power-line  waveforms.    These  distortions  make  measuring  AC  voltage  inaccurate 
when you use an averaging DMM.

USE TRUE RMS WHEN MEASURING 

AC WAVEFORMS

 

®

Average voltage measurements work fine when the signal you’re measuring is a pure sine wave, but errors mount as the waveform 
distorts.    By  using  true  RMS  measurements,  however,  you  can  measure  the  equivalent  heating  effect  that  a  voltage  produces, 
including the heating effects of harmonics.  

Table 1

 shows the difference between measurements taken on averaging DMMs & those 

taken on true RMS DMMs.  In each case, the measured signal’s peak-to-peak value is 2V.  Therefore, the peak value is 1V.

For a 1-V peak sine wave, the average & RMS values are both 0.707V.  But when the input signal is no longer a sine wave, differences 
between the RMS values & the average readig values occur.  Those errors are most prominent when you are measuring square waves 
& pulse waveforms, which are rich in harmonics.

Table 1.  Average versus true RMS comparison of typical waveforms.

Waveform

Actual
Pk-Pk

True RMS
Reading

Average
Reading

Reading 

Error

Sine Wave

Triangle Wave

Pulse (25% duty Cycle)

Pulse (6.25% duty Cycle)

Square Wave

Pulse (12.5% duty Cycle)

2.000

0.707

0.707

0%

2.000

0.577

0.555

-3.8%

2.000

1.000

1.111

+11.1%

2.000

0.433

0.416

-3.8%

2.000

0.331

0.243

-26.5%

2.000

0.242

0.130

-46.2%

One limitation to making true RMS measurements is crest factor, and you should consider crest factor when making AC measurements.  
Crest factor is the ratio of a waveform’s peak (”crest”) voltage to its RMS voltage. 

 Table 2

 shows the crest factors for ideal waveforms.

Table 2.  Crest factors of typical waveforms.

Waveform

Crest Factor

DC

Sine Wave

Pulse (12.5% duty Cycle)

1.000
1.000

1.414
1.732

Square Wave

Triangle Wave
Pulse (25% duty Cycle)

Pulse (6.25% duty Cycle)

1.732

2.646

3.873

A DMM’s specifications should tell you the maximum crest factor that the meter can handle while maintaining its measurement 
accuracy.  True RMS meters can handle higher crest factors when a waveform’s RMS voltage is in the middle of the meter’s range 
setting.  Typically, a DMM may tolerate a crest factor of 3 near the top of its scale but it might handle a crest factor of 5 that’s in the 
middle of the range.  Therefore, if you’re measuring waveforms with high crest factors  (greater than 3), you should adjust the DMM 
so the measured voltage is closest to the center of the measurement range.

Another limitation of true RMS is speed.  If you’re measuring relatively clean sine waves, then you can save time & money by using as 
averaging DMM.  True RMS meters cost more than averaging meters and can take longer to produce measurements, especially when 
measuring millivolt-level AC signals.  At those low levels, true RMS meters can take several seconds to stabilize a reading.  Averaging 
meters won’t leave you waiting.

D:/Chhaya/Coreldraw files/True RMS when measuring AC waveforms.cdr

Summary of Contents for KM 877

Page 1: ... 0 15 x specified accuracy C 10ºC 18ºC or 28ºC 40ºC or otherwise specified Low Battery approx 4 6V APO Timing Idle for 20 minutes APO Consumption 50m A typical Dimension 208 L X 103 W X 64 5 H mm with holster Weight 635 gm with holster TRUE RMS DIGITAL INSULATION MULTIMETER MODEL KM 877 Preliminary Data D Chhaya My Documents Chhaya backup catlogs NEW CAT 2013 2014 KM 877 cdr An ISO 9001 2008 Compa...

Page 2: ...ified measuring range is 0 020MW 25 0GW for percentage operating uncertainly B 30 per IEC EN61557 2 requirements INSULATION RESISTANCE 3 000M 30 00M 55 0M W W W 3 000M 30 00M 110 0M W W W 3 000M 30 00M 275 0M W W W 3 000M 30 00M 300 0M W W W 50 V 100 V 1 Test Voltage 250 V 500 V Range Test Current 1mA 50kW 1mA 100kW 1mA 250kW 1mA 500kW Accuracy 1 5 rdg 5dgts 3 000M 30 00M 300 0M W W W 1000 V 3000M...

Page 3: ...000 0 433 0 416 3 8 2 000 0 331 0 243 26 5 2 000 0 242 0 130 46 2 One limitation to making true RMS measurements is crest factor and you should consider crest factor when making AC measurements Crest factor is the ratio of a waveform s peak crest voltage to its RMS voltage Table 2 shows the crest factors for ideal waveforms Table 2 Crest factors of typical waveforms Waveform Crest Factor DC Sine W...

Page 4: ...er Distance Meter Digital Lux Meter Network Cable Tester Power Factor Regulator Earth Resistance Tester Digital Panel Meters DC Power Supplies High Voltage Detector Calibrators Gas Analysers Frequency Counter Function Generator Phasing Sticks Battery Tester Waterproof Pen Testers Solar Power Meter EMF Detector Wood Paper Grain Moisture Meter Transistorised Electronic Analog Digital Insulation Resi...

Page 5: ... TRMS DIGITAL INSULATION MULTIMETER MODEL KM 877 ...

Page 6: ...t Category Measurement Category IV is applicable to test measuring circuits connected at the source of the building s low voltage MAINS installation Examples are measurements on devices installed before the main fuse or circuit breaker in the building installation Measurement Category III is applicable to test measuring circuits connected to the distribution part of the building s low voltage MAIN...

Page 7: ...able rating changes The meter protection rating against the users is double insulation per IEC UL EN61010 1 Ed 3 0 IEC EN61010 2 030 Ed 1 0 IEC EN61010 2 033 Ed 1 0 IEC UL EN61010 031 Ed 1 1 and CAN CSA C22 2 No 61010 1 12 Ed 3 0 to measurement Category III 1KV Category IV 600V AC DC All Terminals are also rated to such measurement categories requirements The meter also meets the relevant parts of...

Page 8: ...BRP21S2 C Remote probe Optional Accessories BMH 01 magnetic hanger 05 SAFETY Safety Double insulation per IEC UL EN61010 1 Ed 3 0 IEC EN61010 2 030 Ed 1 0 IEC EN61010 2 033 Ed 1 0 IEC UL EN61010 031 Ed 1 1 and CAN CSA C22 2 No 61010 1 12 Ed 3 0 Measurement Category CAT III 1000 V AC DC and CAT IV 600V AC DC Compliance to IEC EN61557 Per CE requirements not certified by UL or ETL IEC EN61557 1 IEC ...

Page 9: ...put voltage 170mA 1000V output voltage 440mA Tester can perform at least 950 insulation tests with new alkaline batteries at room temperature These are standard tests of 1000 V into 1 MW with a duty cycle of 5 seconds on and 25 seconds off 07 Input Impedance 10MW 110pF nominal 6 000 V 60 00 V 600 0 V 1000 V 2 rdg 3dgts 1 mV 10 mV 100 mV 1 V 1kHz 3kHz 6 000 V 60 00 V 600 0 V 1000 V 4 rdg 5dgts 1 mV...

Page 10: ...itialization of test 1 Specified measuring range at percentage operating uncertainty B 30 per IEC EN61557 4 requirements Accuracy Resolution Range 2 000 W 20 00 W 1 5 rdg 3dgts 200 mA 90 mA Measuring Range1 0 015W 2 199W 0 15W 21 99W Diode Tester Test Current Typical 0 5mA Open Circuit Voltage 2 8VDC 2 000 V 1 5 rdg 4dgts Accuracy Resolution Range 1 mV 09 Resistance Open Circuit Voltage 1 5VDC typ...

Page 11: ...ous voltage measurements test the voltage function on a known source such as line voltage to determine proper meter functioning VI OPERATION Hz Hz ACV VFD ACV Functions Press the SELECT button momentarily to toggle the subject functions Last selection will be saved as power up default for repeat Hz measurement convenience For ACV function press the RANGE button momentarily to select other ranges w...

Page 12: ...nob position PI DAR ZERO COMPARE W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 1 SELECT ZERO COMPARE PI DAR W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 0 4A 12 W Resistance Continuity Press the SELECT button momentarily to toggle the functions Last selection will be saved as power up default for repeat measurement convenience Continuity function is convenient for checking wir...

Page 13: ...e auto ranging select a range Press hold for 1 second or more to resume auto ranging 15 The is active as long as the TEST button is pressed and hold The TEST buttons on the meter on the Remote Probe work alike The Continuity Resistance readings are shown on the primary display The RANGE of measuring current is indicated on the secondary display as 200mA or 90mA The meter further gives a beep sound...

Page 14: ...V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 0 4A PI DAR ZERO COMPARE 1 TEST ZERO COMPARE PI DAR W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 0 4A 1 ZERO ZERO COMPARE PI DAR W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 0 4A 1 TEST INS WV W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA 0 4A PI DAR ZERO COMPARE 17 Insulation Resistance function The LCD icons us...

Page 15: ... INSULATION 50V 100V 250V 500V 1kV 200mA 1 0 4A W V HzV VFD OFF INSULATION 50V 100V 250V 500V 1kV 200mA ZERO COMPARE PI DAR 19 NOTE Maximum display reading of each Insulation Resistance range is subjected to the test voltage selected They are 55 0MW 110 0MW 275MW 550MW 25 0GW for 50V 100V 250V 500V 1000V respectively Over range is indicated as maximum display reading 1 TEST M SELECT RANGE HOLD H L...

Page 16: ...SED 0 25A HBC FUSED 0 4A INSULATION 50V 100V 250V 500V 1kV 200mA W V V Hz OFF VFD C M W 500V W M 50MW C 1 COMPARE 21 TEST R H HOLD REC INSULATION PI DAR SELECT RANGE COMPARE ZERO LOCK JACK INS COM INS WV 200mA MAX 1000V CAT III 600V CAT IV INSULATION HBC FUSED 0 25A HBC FUSED 0 4A INSULATION 50V 100V 250V 500V 1kV 200mA W V V Hz OFF VFD C M W 500 V PASS W M TEST C TEST flash PASS Press Hold TEST T...

Page 17: ...MPARE ZERO LOCK JACK INS COM INS WV 200mA MAX 1000V CAT III 600V CAT IV INSULATION HBC FUSED 0 25A HBC FUSED 0 4A INSULATION 50V 100V 250V 500V 1kV 200mA W V V Hz OFF VFD DAR 500V LOCK flash TEST LOCK 23 Backlighted display Press button momentarily to toggle the LCD backlight The backlight will also be turned off automatically after approximate 37 seconds to extend battery life Auto or Manual rang...

Page 18: ...ads etc and replace as necessary Double check operating procedure as described in this manual Set Beeper Off Press the RANGE button while turning the meter on to temporarily disable the Beeper feature Turn the rotary switch OFF and then back on to resume 25 Battery and Fuse replacement Loosen the screws from the access cover of the case bottom Lift the access cover Replace the batteries or fuse Re...

Page 19: ...CO authorized dealer This warranty does not apply for damaged Ic s fuses burnt PCB s disposable batteries carrying case test leads or to any product which in KUSAM MECO s opinion has been misused altered neglected contaminated or damaged by accident or abnormal conditions of operation or handling KUSAM MECO authorized dealer shall extend this warranty on new and unused products to end user custome...

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