Greenlee DM-860 Instruction Manual page 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-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 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 DM-860 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*
( )
* The crest factor is the ratio of the peak value to the RMS value; it is represented by the
Greek letter .
DC + AC True RMS
DC + AC 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
10
100 100
90 100
1.414
100
87
1 1.73
100
64
2