10
AC Measurement
AC measurements are usually displayed as RMS (root mean square) values. The RMS
value is equal to the value of a DC waveform, which would deliver the same power if it
replaced the time-varying waveform. Two AC measurement methods are average-re-
sponding RMS calibrated and true RMS-reading.
The average-responding RMS calibrated method takes the average value of the input
signal after full wave rectification, multiplies it by 1.11, and displays the result. This
method is accurate if the input signal is a pure sine wave.
The true RMS-reading method uses internal circuitry to read the true RMS value. This
method is accurate, within the specified crest factor limitations, whether the input signal
is a pure sine wave, square wave, triangle wave, half wave, or signal with harmonics.
The ability to read true RMS provides much more measurement versatility. The Greenlee
CM-660, CM-860, CM-960, CMH-600, and CMI-600 are true RMS meters.
The Waveforms and Crest Factors table shows some typical AC signals and their RMS
values.
Waveforms and Crest Factors
Waveform
RMS Value
100
100
100
100
Average
Value
90
100
87
64
Crest
Factor*
(
x
)
1.414
1
1.73
2
* The crest factor is the ratio of the peak value to the RMS value; it is represented by
the Greek letter
x
.
AC + DC True RMS
AC + DC true RMS calculates both of the AC and DC components given by the
expression
when making measurements and responds accurately to the total effective RMS value
regardless of the waveform. Distorted waveforms with the presence of DC components
and harmonics may cause:
• Transformers, generators, and motors to overheat
• Circuit breakers to trip prematurely
• Fuses to blow
• Neutrals to overheat due to the triplen harmonics present on the neutral
• Bus bars and electrical panels to vibrate
CM-960 and CMI-600 can be set to display AC true RMS or AC + DC true RMS.
Summary of Contents for Greenlee CM-660
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