Root Mean Square. A method of averaging, most often used in the theatre when discussing DMMs in measuring voltage and current. See Learn Root Mean Square(RMS)/Quadratic Mean(QM) tutorial, definition, example, formula.

RMS is most applicable when describing or measuring the power contained in an AC waveform that is not a pure sine wave. A common place in the theatre where this occurs is on the output of a phase-control dimmer. Likewise, the current drawn by a phase-control dimmer is non-sinusoidal, or nonlinear. A true-RMS meter must be used for accurately measuring both voltage and current on a phase-control dimmer.

"Crest factor" is a measure of just how non-sinusoidal a particular waveform is.

STEVETERRY;74834 said:
1. Any non-true-RMS meter will give you an inaccurate reading on a non-linear current waveform. The clamp-on current transformer has almost nothing to do with the inaccuracy.

2. Almost all field current measurement on AC line circuits are accomplished with clamp-on or toroidal current transformers, not by breaking the circuit and measuring across a resistance. That is just not practical in most cases.

3. Good true RMS clamp-on meters are capable of quite high accuracies even when presented with high-harmonic content waveforms. For example, the Fluke 345 clamp-on meter is capable of +/- 3% of reading on crest factors between 1.1 and 3, and +/- 5% of reading on crest factors between 3 and 5. This is pretty darned accurate.

4. Typical instruments have frequency response of 15 Hz to 1kHz for the meter and clamp-on transformer combination. The accuracies above are measured across that frequency range. This takes care of the instrument response to harmonic frequencies in no-sinusoidal waveforms like a phase-control dimmer.

The bottom line: magnetically coupling the meter to the conductor under measurement does not introduce gross inaccuracies, even when the waveform is harmonic-rich. Gross inaccuracies come from using a non-true-RMS instrument on a non-sinusoidal current waveform. ...

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