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4
True RMS
RMS (Root-Mean-Square) is a term used to describe the effective or equivalent DC value of
an AC signal. True RMS is the term which identifies a DMM that responds accurately to the
effective RMS value regardless of the waveforms such as: square, sawtooth, triangle, pulse
trains, spikes, as well as distorted waveforms with the presence of harmonics. Harmonics
may cause :
1) Overheated transformers, generators and motors to burn out faster than normal
2) Circuit breakers to trip prematurely
3) Fuses to blow
4) Neutrals to overheat due to the triplen harmonics present on the neutral
5) Bus bars and electrical panels to vibrate
Crest Factor
Crest Factor is the ratio of the Crest (instantaneous peak) value to the True RMS value, and
is commonly used to define the dynamic range of a True RMS DMM. A pure sinusoidal
waveform has a Crest Factor of 1.414. A badly distorted sinusoidal waveform normally has a
much higher Crest Factor.
4) OPERATION
CAUTION
Before and after hazardous voltage measurements, test the voltage function on a known
source such as line voltage to determine proper meter functioning.
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