What does TRMS in electrical measurements mean?

What is TRMS and when do I need it?

TRMS is the abbreviation of True Root Mean Square.  For alternating current or power, the RMS value of the variable measurand is equal to the value of a constant direct current that would produce the same power dissipation over an resistive load.

A test instrument using TRMS is needed when for example voltage or current do not have the shape of a sinus but are “disturbed”, for example by harmonics. This is because standard instruments work with the method of averaging (AVG) which does not reflect the electric power correctly when the supply is not “clean” but disturbed.

Such distrurbances are caused for example when equipment with non-linear current-voltage characteristics or with non-stationary operating behavior (switch-on and switch-off processes) is operated in a power system. Typical for such equipment are for example dimmers, switching power supplies, frequency regulators or rectifiers.