A true RMS meter calculates effective value directly, by sampling or analog computation, so it reads correctly on distorted waveforms. Cheaper average-responding meters rectify the signal and multiply by 1.11, the form factor of a sine wave, which can under-read the pulsed current of a drive or computer load by 30 to 40 percent. Meters also state a crest factor limit, the peak-to-RMS ratio they can handle.
Average-reading clamp says 18 amps on the neutral, the true RMS meter says 31, those office PCs are loading it with third harmonic.
Also heard as
- TRMS
Related terms
RMS
Root mean square: the effective value of an alternating quantity, giving the same heating as that DC value.
Clamp meter
Handheld meter whose jaws close around a conductor to measure current without breaking the circuit.
Harmonics
Voltage or current components at whole multiples of the supply frequency, produced mostly by nonlinear loads.
THD
Total harmonic distortion: the combined size of all harmonics relative to the fundamental, as a percentage.