What is True RMS?

The normal alternating current mains power supply comprises a sinusoidal waveform that has symmetrical positive and negative half cycles. The RMS (Root Mean Square) value of an alternating current is measured in terms of the direct (dc) current that produces the same heating effect in the same resistance.

The FORM FACTOR of a waveform: = RMS value / Average value

The PEAK or CREST FACTOR of a waveform: = peak or max.value / RMS value

For a sinusoidal wave:

  • FORM FACTOR= 1.11
  • CREST FACTOR= 1.414

Problems can arise when the mains waveform becomes distorted because of the effects of certain types of loads. Thyristor and triac drives, switched mode power supplies, etc, create harmonics of the main waveform because they take ‘gulps’ of current during a cycle. This results in a non-sinusoidal waveform.

Low cost measuring instruments assume that ac waveforms are sinusoidal and can thus give false readings. Typically in a waveform with a high CREST FACTOR i.e. a high peak current, a low cost instrument will only measure the average current and thus give a low reading. This can result in cables, fuses and breakers being sized too low. When measuring electricity consumption it will also result in a low reading.

When measuring power consumption both voltage and current need to be measured. Some instruments take the RMS current and the RMS voltage for each half cycle to calculate the average power over the half cycle.

The Hawk3000 and Hawk5000 go to the ultimate in accuracy. They sample the current and voltage waveforms 10,000 times per second and calculate the instantaneous power at each point rather than assume an average.