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Insulation testing is not trivial

Simon Wood, UK Wholesale and Distribution Sales Manager at Megger Ltd., explains insulation
testers and techniques for working safely and obtaining dependable results.

The essential starting point for safe, efficient and dependable testing is to choose the right test equipment. For insulation testing, there are two main options; standalone insulation testers and multifunction installation testers with an insulation test function. Both types of instrument are equally satisfactory, providing that they meet a number of key requirements.

Safety is always the top priority, so the first thing to consider is the CAT (safety category) rating of the instrument, as defined in EN 61010. The absolute minimum acceptable rating for general use on electrical installations is CAT III 300 V, but insulation testers with this rating must not be used close to the source of supply; that is, where the supply enters the building or on supply circuits outside the building. It is therefore far better to choose an instrument with a CAT IV 300 V rating or better, since this is safe to use on any part of the installation.

It may seem strange to worry about the CAT rating of an insulation tester as this rating relates to supply transients and insulation testing is a dead circuit test.

However, it is by no means unknown for insulation testers to be inadvertently connected to live circuits, so the CAT rating is definitely relevant and important.

Accidental connection to live circuits is in fact a major issue with insulation tester safety.

The best instruments will instantly inhibit the insulation test if they detect a voltage above a certain threshold, typically about 25 V, and generate an alarm to warn the user of the problem. The very best models go one stage further by providing protection for the instrument and the user even if the circuit becomes live after the insulation test has started.

An issue of safety
It is bad practice to disconnect an insulation tester while a test is in progress, particularly if it is connected to a long cable, as the charge stored in the cable may well be sufficient to deliver a shock. Always terminate the test before disconnecting the test leads, as this will ensure that the circuit under test is properly and safely discharged.

Turning now to the actual performance of the insulation tester, reliable results will only be obtained if the instrument operates on the ratiometric principle. This means that it measures current and voltage during the test and calculates the insulation resistance from these two measured values. The alternative, which is to measure only the current and assume that the voltage is correct, is not satisfactory as many factors can influence the test voltage and produce misleading results.

Using the right test voltage for insulation tests is essential. If the voltage is too high, there is a risk of damage to the circuit or equipment being tested and if it is too low, it’s possible that weaknesses in the insulation that would have been detected at the correct test voltage will be missed.

The insulation test voltages prescribed for installation testing are 250 V for SELV and PELV circuits, 500 V for all other circuits with a nominal working voltage up to and including 500 V, and 1,000 V for circuits with a nominal working voltage above 500 V. Almost every contractor will therefore need an instrument that can test at 250 V and 500 V, but the 1,000 V test option is less likely to be required.

If an instrument with the 1,000 V option is used, special care should be taken that this range is not accidentally selected as equipment that would easily withstand a 500 V test may be damaged or destroyed by the higher voltage.

Having an instrument that supports insulation testing at a particular voltage is a good start, but how can the user tell that the test really is being carried out at the correct voltage? There are many things that can affect the voltage delivered by an insulation tester. For example, weak batteries may reduce the test voltage in some instruments, or the instrument may have developed a fault.

To address these issues, some of the latest insulation testers provide a dual display. As well as showing the insulation resistance, they simultaneously show the actual voltage being applied to the circuit under test. This is a big benefit as any problems are seen instantly rather than being discovered later when it may be impossible to decide how many circuits have been incorrectly tested.

Insulation testing does not have to be complicated, but it’s certainly not trivial, especially as reliable insulation test results are the key to ensuring that electrical equipment and installations are safe. It is worth spending time on choosing the right tester and working out how to get the best from it.

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