The terms RMS (root mean square) and TRMS (true root mean square) need a bit of highlighting, particularly since nice sinewave voltage and currents are pretty much of the past, writes Loc Duong from Power Parameters.

When an instrument is described as ‘RMS’, it is usually the case that it is actually ‘RMS-indicating’, implying that the measurements are only correct, making allowances for basic accuracy, for sinewave currents and voltages.

We learn in trade school that the RMS value of a sinewave current or voltage is the peak value divided by Ö2 (approx. 1.41), thus representing the heating equivalent of a DC current or voltage of the same value. These days there are not so many electrical loads that draw nice sinewave currents, even if the applied voltage is a sinewave and RMS-indicating instruments relying on the 1/Ö2 relationship (peak to DC equivalent) will not give an accurate indication of the RMS value.

This is where TRMS comes into play.

An instrument labelled as ‘TRMS’ is capable of extracting the RMS value even if the current or voltage is ‘distorted’, i.e. no longer a smooth sinewave.

In the days of analogue instruments, usually equipped with d’Arsonval movements (a spring-loaded current-carrying coil enclosed by a permanent magnet), there was no option but to calibrate their scales on the basis of RMS indicating a non-linear scale. Digital instruments have done away with this, but even so they are still available as RMS-indicating.

There is a cost differential with TRMS instruments costing more but it is really money for jam.

When purchasing a TRMS meter, a specification to look out for is the crest value, which is the ratio of the peak voltage or current to their RMS values. The higher, the better because the crest value shows the level of distortion the instrument can measure accurately.