Look to the current
(VA & W vs. A)

When it comes to nuisance tripping and other current related power quality issues, it is amazing how many still want to know the power involved. To recap; Power factor is the angular relationship between the voltage and current waveforms (in plain English, where the peak of the current occurs on the voltage waveform).

Most parts of a distribution system, this includes the wiring and items such as circuit breakers, could not care what voltage is present on them (ok, within reason). This means these items are also not interested in the power factor of the load, but are sure concerned with the current flowing through them! Unless you are trying to conserve energy owing to a large power factor issue, this attribute is doing nothing more than clouding the main issue.

VA comes a little closer to solving the problem, and is a handy means for deriving the required figure - amps. And it is amps we are interested in with regard the wiring, fuses, and circuit breakers.

A word of warning about VA on some items. The VA specified is sometimes what the device can deliver, not what is being drawn by the device (and usually excludes the magnetism current, etc.).

There is one factor that is important with regards circuit breakers and that is whether the circuit breaker is thermally or magnetically operated. If thermal, then we are interested in RMS current only. If magnetic, then 'crest factor' comes into play. Although a complex problem was shown in the section "nuisance tripping", the issue can exist at extremely simple levels too.

On a pure sinewave, the crest factor is 1.414, but hi-tech loads are more in the region of 3 to 4 with some being as high as 6. This means if you have a 10Arms load, the peak is 60A. If a plug circuit is fed through a 30A breaker, the maximum peak allowed is 43A - and the breaker starts complaining!

The crazy part is a 'sparky' comes along with his "true RMS" meter and only measures 10A, he leaves even more bewildered as to why the new breaker he has just installed is doing the same thing - of course, you are left with the bill !

So, when interpreting a current related problem, always look at both RMS and crest factor (or peak current if that's what your instrumentation can do).

Voltage-Current Relationships  >>

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© 02.04.04