Back to Basics – True RMS

There used to be a time whereby the price between a non-True RMS (aka Average Responding) and a True RMS meter/clamp is pretty significant; so much so that if one is just to measure voltage or current for checking if the circuit is ‘Live’ or not, one will go for the cheaper Non-True RMS device.

Recently, price difference have narrowed down quite a bit and in my opinion, one should just get a True RMS meter/clamp straight away.

Loads today are pretty much almost non-linear these days anyway.

For a pure sinusoidal wave;

true rms vs average responding
true rms vs average responding

Hence for an average responding meter, it will scale the rectified average of the ac waveform by 1.11.
This holds true only for pure sinusoidal waveform; which do not exist in the practical world.

The difference in reading can vary from between 5 and 40%, depending on what type of waveform that is being measured.
An example below shows a difference of about 10%. The load being measured was a combination of a couple of CFL and LED bulbs.

It is funny to me to see some contractors out there who uses a non True RMS meter/clamp to verify the readings obtained from their expensive PQ meters.


True RMS vs Average Rectified Clamp
True RMS vs Average Responding Clamp

*Besides True-RMS capability, one should also check for its Safety CAT category. This is an important safety consideration that should not be disregard.


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