On guard for insulation testing!

1 September 2008

Electrical Tester

Paul Swinerd - Product manager

When properly carried out, high-voltage insulation testing can provide invaluable information about the condition of electrical equipment. However, testing insulation at 5 kV involves a few extra considerations compared with testing at lower voltages, as Paul Swinerd explains.

Detailed studies have confirmed that, as might be expected, testing the insulation of medium voltage electrical equipment at voltages close to its normal working voltage yields information that is far more reliable and useful than that produced by testing its insulation at lower voltages. The same studies have, however, also confirmed that using test voltages in excess of 10 kV usually adds little in the way of extra information.

The results of high voltage testing are of particular value when they are trended over a period of time. For example, if a test on a circuit breaker shows its insulation resistance to be, say, 2,000 MO, but a subsequent test shows that this has fallen to 500 MO, clearly the reason for the change needs to be investigated.

This is, in effect, predictive maintenance as incipient faults can be detected before they develop into full-blown failures. Note especially that had the insulation tests mentioned been carried out at a lower voltage – say 1 kV – the instrument would almost certainly have read infinity in both cases, giving no indication whatsoever of an impending problem.

For these reasons, the use of 5 kV and 10 kV insulation testers is becoming more and more widespread. Those more used to working with lower voltage testers may, however, be surprised to find that good quality high voltage insulation test sets have not two but three test terminals – usually labelled positive, negative and guard.

To understand the use of the guard terminal, it’s necessary to think a little about the insulation resistance testing process. In effect, the test set applies a known high voltage to the item being tested and measures the current flowing. It then uses Ohm’s law to calculate the insulation resistance.

That’s very straightforward, but now let’s think a little more closely about, say, a transformer bushing. If it’s been in service for a while, it may well be dirty.

If we were only interested in confirming, for example, that the bushing had an overall insulation resistance of more than few megohms or more, that probably wouldn’t matter. But if we’re trying to accurately measure the insulation resistance, which we expect to be in the order of hundreds if not thousands of megohms, matter it most certainly does.

It’s not hard to see why. The current produced by the insulation test voltage applied to a dirty bushing will flow not only through the body of the bushing, but also through the dirt on its surface – so called surface leakage. If no further steps are taken, the instrument will display a result equal to the surface leakage resistance and the insulation resistance in parallel with each other.

However, to determine the condition of the bushing, we are interested only in the resistance of its body – the surface leakage merely distorts the measurement. And that distortion can be enormous, as it’s not at all uncommon for the surface leakage current to be ten times of that flowing through the bushing itself.

While this example has discussed a transformer bushing, it’s important to note that surface leakage affects almost all high voltage insulation test results including those relating to cables. It is highly desirable, therefore, to find some way of minimising the effect of surface leakage, and this is the role of the third terminal – the guard terminal – on the high-voltage insulation test set.

In use, the guard terminal is usually connected to a bare wire wound tightly round the surface of the item under test. This wire effectively intercepts the surface leakage currents flowing across that item from both the negative and positive terminals of the test set. The surface leakage currents can, therefore, be shunted away from the main resistance measuring circuit, leaving the instrument free to measure the real insulation resistance of the item.

In passing, it’s also worth noting that performing a test with and without the guard terminal connected gives a good indication of the amount of surface contamination present and, therefore, whether the item under test simply needs cleaning.

Clearly, the guard terminal is a very useful feature, but a few words of caution are necessary. The presence of the guard terminal alone does not guarantee that an insulation test set will give accurate results in the presence of high levels of surface leakage.

In particular, it is difficult to maintain the performance of the guard terminal if the instrument is also to offer a CAT IV 600 V safety rating. Megger has, however, achieved this rating in its latest high-voltage insulation testers without in any way compromising the guard terminal performance.

There can be many reasons why some instruments achieve poor guard terminal performance, but one of the most obvious is that, with a guard terminal, the instrument not only has to supply the current needed for the actual insulation test, but also the diverted current that flows via the guard terminal.

If the voltage generator in the test set has insufficient capacity – effectively having a high internal resistance – the result will be that the test voltage falls, giving inaccurate results. This is a very important consideration because, as we’ve already implied, the current in the guard terminal circuit can be ten or more times greater than that in the test circuit itself.

The stability of the test set also has an effect on the accuracy of the results obtained when the guard terminal is in use, as does leakage on the surface of the test leads used. Megger has carried out an investigation into the influence that these effects have in practice, and has found that there are instruments currently available that can give results that are in error by as much 80% when the guard terminal is in use.

Such huge errors, of course, nullify the benefits of the guard terminal. In fact, they do worse than this because, by delivering spurious results, they may mask real problems. So what can purchasers of high-voltage insulation test sets do to avoid problems of this type?

Fortunately, the answer is straightforward. All that’s necessary is to ask the instrument manufacturer, before making a purchase, to confirm the accuracy that the instrument will deliver when the guard terminal is used. Any reluctance to provide this information will enable the obvious conclusions to be drawn, and the appropriate purchasing decisions made!

High-voltage insulation testing is invaluable both in fault diagnosis and in condition monitoring. The quality of the results obtained, however, depends on the quality of the test equipment used. Three terminal test sets, which incorporate a guard terminal, are invariably a little more expensive than their two terminal equivalent.

As we’ve seen, however, the small extra cost is money well spent. Provided, that is, that using the guard terminal doesn’t destroy the instrument’s accuracy – so don’t forget to ask for those accuracy figures before showing the supplier the colour of your money!

How this fits with other specifications:

IEC1010-1:2001