CMI-2000
9
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 CMI-2000 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
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
AC Bandwidth
AC bandwidth is the range of frequencies over which AC measurements can be made within the specified
accuracy. It is the frequency response of the AC functions—not of the frequency measurement functions. A
meter cannot accurately measure the AC value with frequency spectrums outside its bandwidth. Complex
waveforms, noise, and distorted waveforms contain frequency components that are much higher than the
fundamental; for example, high frequency noise on a 50/60 Hz power line.
