Test meters used to just provide the basics: voltage, current, and resistance. Thanks to solid-state circuitry, though, today's instruments are packed with several features the typical electrician may never use, and in some cases, may not even recognize. But as Editorial Director John DeDad explains, a feature doesn't have to be well-known to be useful.
Q. I've heard and read about a multimeter capability called “peak capture” and a measurement called “crest factor.” I don't quite understand their importance or how they're used in a power quality site analysis. Can you explain?
DeDad's answer: As you know, electric utilities generate various voltages of electricity at 60 Hz that results in a sine wave waveform. The height, or amplitude, of this sine wave is called its “peak value.” Instead of using the peak value, however, we describe the amplitude of a sine wave as its effective, or root-mean-square (rms), value. As shown in the Figure above, the rms value of a pure, undistorted sine wave is about 71% (0.707) of its peak value.
The term “crest factor” describes the ratio of the peak value of a measured waveform to its root mean square. So a pure, undistorted sine wave's crest factor would be 1 divided by 0.707, or 1.414.
This crest factor value will vary from 1.414 if the waveform is distorted, which results in a peak value different than that of a pure, undistorted sine wave. In other words, a good voltage sine wave will have a peak value that's close to 1.414 times its true rms value.
This is where the “peak capture” capability of a multimeter comes into play. Basically, you would compare the theoretical peak value with what you measured using the peak capture feature. For example, let's suppose you're measuring voltage on a 120V circuit. Theoretically, the peak value of this voltage waveform would be 120V times 1.414, or 169.7V. A measured voltage peak, using the peak capture feature, that's significantly higher than this is an indication of the presence of harmonics.
Be careful here. Many nonlinear loads will cause these peaks to be reduced or clipped, especially if the source impedance is high. Use the following three-step test to verify voltage clipping due to harmonics:
Measure the true rms value of the voltage, and then multiply this value by 1.414 to get the theoretical peak value.
Measure the actual peak value using the peak-capture feature.
Compare the actual value to the theoretical value. If they're significantly different, the waveform is distorted and contains harmonics.
For voltage harmonics, the typical crest factor is below 1.414, as in “flat top” waveforms, for example. For current harmonics, the typical crest factor is a great deal above 1.414.
Typical true rms handheld digital multimeters have a crest factor of 3.0 at full scale, which is more than adequate for most power distribution measurements. At half scale, the crest factor is double, or 6.0. For example, some meters will have a crest factor specification of 3.0 when measuring 300VAC and 6.0 when measuring 150VAC.
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