Explain RMS (Root Mean Squared)

I’ve noticed that the common term ‘RMS’ causes confusion, fear and doubt in some superyacht engineers, when talking about electrical measurements

Don’t worry! There’s a hard way and an easy way to explain why electrical engineers refer to ‘RMS current’, and ‘RMS voltage’. The letters stand for Root Mean Squared and applies to AC currents and voltages.

All you really need to know is that the AC RMS value gives an equivalent heating effect in a resistance as exactly the same DC current or voltage.

If we look at the two AC waveforms below, then you can see why the shape of the waveform makes a difference to the heat (Watts) from a resistor. The square wave has a bigger average in each half cycle than the triangle wave even though they both have the same ‘peak’ (maximum) voltage, in this case 10V.

So we use the RMS ‘effective’ value calculation so can we compare AC waveforms of different shape to know if they will develop more or less power in a given resistance. Same RMS value, same power, regardless of shape.

So why should I care? Well, most of the time you don’t have to, because the AC voltages we measure on board are largely sinusoidal (from generators, inverters, or the dock supply) and most multimeters, when set to their AC range, assume that we’re measuring sinusoidal voltages, at least. But if we have a distorted AC waveform, especially and more likely a distorted current waveform then we’ll have an increasingly inaccurate measurement of current (or voltage) the less like a sine waveform it is.

This is where we need to check if our multimeter has ‘True RMS’ stamped on the front, or in the instructions, and all is well. You can still just use good old OHMs law to calculate the heat (Watts) from those corrected AC measurements.

[(*) For much more in-depth information and training on electrical measurements and the practical application of Ohm’s law and so much more, see module 3 and others in our course: Electrical Control Systems for superyacht engineers.]

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