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Insulation testing—tips and troubleshooting

Common questions and answers for more consistent test results

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—Sponsored article by Megger

High-voltage insulation testing at 5 kV and 10 kV is a subject that comes up regularly in the questions received by our technical support group. Below are a few answers for some of the questions we’re most frequently asked in relation to this important and interesting topic:

Is there any good reason to choose an insulation test set that offers a high output current as preference to one with a lower output current?

There are actually several reasons to choose a test set with a high output current. Possibly the most important is a high output current will mean the item under test will be charged faster. The test can be completed in a shorter time, and there’s less risk of the readings being taken before the test voltage has had time to properly stabilize.

If you’re using the instrument’s guard terminal, don’t forget a lot of output current may well be diverted via the surface leakage of the item under test. Unless the instrument has a high output current capability, this could mean the output voltage will collapse and the test results won’t be valid.

Sometimes, it seems impossible to get consistent test results when performing insulation tests—the readings just won’t stabilize. This seems to happen a lot, for example, in substations. What’s the problem, and what can be done about it?

In cases of this type, the source of the trouble is almost always induced noise in the measuring circuit. Noise pick-up on the test leads can be reduced by keeping them as short as possible, and by using screened test leads.

With screened leads, the screen is connected to the insulation test set guard terminal, so the noise currents are diverted away from the measuring circuits. These measures can’t help, however, if the noise is being picked up by the item under test rather than the test leads. In such cases, the only effective solution is to use an insulation test set with high noise immunity and effective filtering.

Instruments are now available with noise immunity of 8 mA, which is enough to ensure reliable operation in the toughest conditions, such as EHV substations. They also have adjustable long-time constant filtering, which allows users to choose between faster operation, when noise levels are only moderate, and slower
operation but enhanced noise rejection when working in the most challenging environments.

Why do some of the latest insulation testers have facilities for remote control?

These facilities are useful in a wide range of applications. For example, when testing a large item such as a power transformer, the tester can be positioned on top of the item near to the terminals so the test leads are kept short, and operated from a much more convenient—and much safer—location, using the remote control option.

Also, it’s sometimes necessary to carry out tests in hazardous locations, such as inside an energized sub-station. In these cases, once it;s been connected, the test set can be operated from outside the hazardous area, greatly increasing the safety of the operator.

Finally, in production line test applications, it’s often desirable to control the tester automatically, and the remote control facility offers a convenient way of achieving this, and of providing any safety interlocks that may be needed.

Megger is a premier provider of electric test equipment and measuring instruments for electrical power applications. Contact Megger for more information or visit

For more troubleshooting tips, products and innovations, visit the Electrical Testing centre.

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