Marc's Technical Pages: Power Quality Symptoms and Solutions

MEASUREMENT TECHNIQUES:
A Matter of Timing

"Packing two weeks worth of clothes
for an overnight stay
is nothing short of daft!"

It's all a matter of timing. It sounds daft to set a recorder to record for two weeks when all that one wants to capture is a motor start. Oh yes, it still happens! All would be well if all recordings were done at the same resolution, but they're not - and that is the start of the problem.

The importance of resolution:
Setting a recording interval of too long leaves one open to having high minimum and maximum readings which could leave you bewildered. Setting it too short could leave you with not capturing the fault and never getting to the root of the problem.

There is one very dangerous trap that is now being fallen into by almost all UK PQ investigators. There is this belief that because the regulator states that "... an investigation will commence within 7 days, and that a 6 day recording must be taken of the client's supply, that this means the recording equipment must be set for a minimum of a 7 day recording period.

The first part can be understood as this simply ensures the fault will be trapped before it mysteriously disappears. The second part is a guideline and not to be taken literally. It is not the intention of the regulator to dictate how the analysing equipment is set up (how can they without knowing the fault). The intention is to ensure the supply is analysed over a period to ensure the fault is captured. If the fault can be captured and analysed in a matter of minutes then by all means do it. The whole point of the exercise is that the situation be resolved.

It is important to listen to the complaint first so as to ascertain what length of recording is actually necessary. If, for example, flicker is permanent, or happens many times a day, then a one day recording is all that is necessary. In fact a few hours should do the trick. If it's a long term voltage complaint then a long term recording is obvious.

There is a simple rule:

"The shorter the recording period, the better.

But why? The answer lies in the resolution of the recordings. Longer recording intervals are likely to mask the real fault. The impact of long recording intervals is covered in the section 5110.

Long term averages vs. event captures:
Don't confuse these two! It has been noticed that many recorders are set for sag and swell limits at the accepted minimum and maximum (UK = 230V nominal -6%..+10% which translates into 216V and 253V). The problem with setting these limits at this level is should the voltage reach and hover at either of the limits this will trigger the event capture at almost each cycle. If the event capture buffer is a rotating type (captures the last e.g. 200 events, and overwrites the oldest with the latest) then genuine events that need investigation could be missed.

Again, it is imperative to listen to the complaint and make judgements based on the complaint, not on limits set by regulatory authorities or industry. For those who are concerned about voltage limits, the Avg, Min, & Max graphs are used for this purpose and are covered in the section 5110.

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