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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 wave-
form. Two AC measurement methods are average-responding 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 accu-
rate, 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 DM-860A is a true RMS meter.
The Waveforms and Crest Factors table shows some typical AC signals and their RMS values.
Waveforms and Crest Factors
Waveform
RMS Value
Average Value
Crest Factor*
(ξ)
* The crest factor is the ratio of the peak value to the RMS value; it is represented by the Greek
letter ξ.
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
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