Owners Manual

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.