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One of the choices you have when selecting a multimeter is whether to select a true-RMS or non-RMS multimeter. To understand the difference we first need to understand what RMS represents. The Root-Mean-Square (RMS) value of any a/c voltage or current is its effective or power-producing value. An ac waveform is in a constant state of change. It is constantly changing its value from zero, to a positive-peak value, back to zero, to a negative-peak value, and back to zero.
On a 60 hertz power supply, this happens 60 times per second. So, for every 1/60th of a second, a standard 120 V ac waveform will go from zero, to 170 V, back to zero, then to -170 V, and then back to zero. However, its RMS value, or effective value, is 120 V.
Non-RMS meters calculate the RMS value of an ac waveform using a shortcut method. They are designed to be average-responding RMS-indicating meters. These meters capture the rectified average of an ac waveform and scale the number by 1.1 to calculate the RMS value. It is not the true RMS value, but rather a calculated value based on an assumption about the waveform. As long as the waveform of an ac signal is sinusoidal (not distorted) these meters will yield accurate results.
They work well with linear loads, such as induction motors, resistance heaters, incandescent lights, and similar linear loads. However, when an ac waveform becomes distorted (as is the case with nonlinear loads, such as adjustable speed drives, or any circuits which contain semiconductors, rectifiers, SCRs, and similar devices), these meters cannot yield accurate results. A true-RMS meter must be used to accurately measure the RMS value of the ac signal. These meters have a specialized circuit within the meter to read the true RMS value of a nonlinear (distorted) ac signal.
So, when does a technician need to use a true-RMS meter? Basically any time the ac supply being measured is distorted by nonlinear loads (such as adjustable speed drives) or any circuits which contain semiconductors, rectifiers, SCRs, and similar devices. Using a non-RMS meter on these loads can cause a lower-than-true value — up to 40-percent lower in some cases.
Therefore, the next time you need to purchase a multimeter, spend some time looking at your options and select the right tool for the job.