INTERPRETING THE READINGS:
The Lies Told By
Portable Appliance Testers

Roll out the meter,
the good old multimeter I own.
This one has a needle,
it lies not so I'm not found to moan!

Now I could carry on, but am wary there may be a few who want to strangle me as they fall asleep singing this (modified) sailors' song in their heads (especially if they've read this in the morning and have been humming it all day long!).



PN + Fuse Continuity:

It's just that; A test to ensure there is a continuous electrical path from Phase (Live) to Neutral which should include a fuse. If all is well i.e. the power cord is ok, the fuse is intact, the switch is making contact (oh yes, the switch must be on!), and, finally, the physical circuitry is functional (able to draw current), then the reading should state "Pass". Obviously, should there be a break anywhere in the circuit then one would expect the PAT to claim "Fail".

These testers, especially the simpler ones and those that test at low voltage (around 7.5 to 12VAC), are all good and well for simple appliances such as kettles, toasters, and the like. They are also fine for devices that use tratransformers as the first component in the power supply (yip, there are some of us who grew up on big lumps of specially formed iron with copper wire wrapped around them!).

But when it comes to modern switch mode power supplies, these PATs can be more of a problem than a help. Not only is the modern PC fitted with such a power supply, but printers, scanners, and many other PC peripherals too. Funny thing though, the really tiny perpherals, such as modems, tend to still use 'AC adapeters' which utilize transformers! The problem is not restricted to the IT end of things, but 'entertainment' items too. Items such as video recorders, satellite receivers, Hi-Fi components, and even the long standing television set are all now employing the more 'efficient' switch mode power supply.

With these more modern power supplies, the first components the PAT will encounter (I am going to ignore the EMC filter here as the currents flowing in it are negligible) are the bridge rectifier and smoothing capacitor. The higher voltage of 12VAC represents 17V peak meaning a the capacitor will be charged to 15.8V (17 less the 1.2V drop across the bridge rectifier). This voltage is far too low for the power supply to even begin thinking of "jumping to life" i.e. drawing current. The PAT will therefore, as soon as it has charged the capacitor, no longer see continuity and define a "FAIL" condition. These tests can take 5 seconds or so, more than long enough to sufficiently charge the capacitor and therefore return an undesired reading.

The other issue here is if one has just unplugged the device with such a power supply, the capacitor is likely to still have some charge in it, and is likely to be far higher than the ±16V required to show a fail. The lowest I have seen switch mode power supplies "drop out" (stop supplying output) is about 40VAC, which means the capacitor could have as much as 55V (40 x 1.414) across it a few moments after the power is removed. The circuitry will draw some current, but it is infinitesimal as the main supply is not functioning at such a low voltage. This low current will take ages to discharge the main capacitor to a voltage low enough for the PAT to "see" continuity - but we only arrive back to the situation in the previous paragraph.

Now I am going to make a very controversial statement here; Did the device work when plugged in to the mains? If so, then the PN + Fuse Continuity check can be said to comply i.e. pass. All that is then required is to do the Earth Screen and Insulation tests.

Please do not do the Earth Bond tests (those ranging between about 8A to about 30A) on any electronic equipment unless you want to be the reason behind the purchase of a whole suite of new devices! Such tests can, and do, kill most electronics. I once landed up repairing a whole stack of expensive laboratory instruments after a "know-it-all" chap did the wrong test with a PAT!

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© 19.06.04