Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6.4 Voltage and current considerations
6.4.1 True-rms readings
When ac voltage or current measurements are to be made on non-sinusoidal or distorted wave forms, a DVM
or digital ammeter using true-rms conversion techniques should be used. There are three true-rms conversion
techniques in use today. These are best described as thermal, analog, and digital. The thermal true rms con-
version is based on the heating of a resistive load with the input signal. The amount of heat generated by this
load is directly proportional to the rms value of the signal. A thermocouple is placed adjacent to the load in
an evacuated chamber. The dc voltage output of the thermocouple is proportional to the generated heat. The
output of the thermocouple is then routed to the meter where the rms value is read. A feedback circuit con-
taining a second thermocouple may be included to account for the nonlinearity of the primary thermocouple.
The analog-based true-rms converters use circuits that measure the input voltage, and then calculate its
square, mean of its squares, and the square root of the mean. The time constant of the conversion circuits is
set by an averaging circuit that may be adjusted from a few milliseconds to several hundred milliseconds.
The longer time constant yields less ßuctuation on the output signal but may lead to a poorer response when
dealing with rapidly oscillating loads. In some cases, this conversion technique may not be able to deal with
high slew rates, rapidly-changing signals, or large crest factors.

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
31
True-rms converters based on a digital approach sample the input signals at approximately 100 times the
anticipated signal frequency and convert the samples to digital values. A mathematical processor squares
each of the values, sums the squares along with some previously squared samples, and then calculates the
square root of the sum. This technique will yield a true rms value on any arbitrary waveform.
Some meters will measure and display peak values as well as true-rms values. This feature can be beneÞcial
if diagnosing potential harmonic overheating concerns.