Electrical Tester Contractor edition
February - March 2017
Insulation testing: making the right choice

Insulation testing: making the right choice

29 March 2017

If you’re a maintenance engineer, you probably already have a handheld insulation tester and it’s likely that you think it does everything you need. If you're tester is new, you may be right, but if you’ve had it for several years it’s probably time to see what the latest models have to offer. There’s a new generation of handheld instruments out there and they deliver valuable extra benefits.

Take, for example, the test voltage. It’s easy to assume that if you’ve set your tester for a 1,000 V test, it will actually test at 1,000 V. Unfortunately, that’s not necessarily so. The IET Wiring Regulations allow an overvoltage of up to 20%, which means that the tester could be operating at 1,200 V and still be within the permitted tolerance. Often, this level of overvoltage is undesirable and could damage the equipment or circuit under test.

The best instruments now solve this problem by employing feedback stabilisation for the test voltage. The output voltage is automatically monitored and adjusted so that it can never be more than 2% above the nominal value. This applies on all test voltages provided by the instrument so, for example, the maximum overvoltage on a 500 V test is just 10 V and on a 1,000 V test just 20 V.

Most insulation testers provide a choice of test voltages, typically 250 V, 500 V and 1,000 V. These choices are adequate for users with basic requirements, but the wider choice offered by many of the latest instruments can be a big benefit. A 100 V option, for example, is invaluable for pre-testing circuits when there’s doubt about their condition, before moving on to a higher test voltage. It’s also the right choice for testing cables that have a normal working voltage between 24 V and 48 V, and for testing many types of telecomms and data cables.

A variable test voltage facility can be even more useful, as some manufacturers of electronic equipment specify specific non-standard insulation test voltages. For these applications, insulation testers are now available that allow the user to adjust the test voltage in 1 V steps over a very wide range.

Another point to watch out for when choosing an insulation tester is battery life. Few things are more inconvenient than having the tester batteries run out while you’re in the middle of a series of tests or trying to deal with a breakdown, so the extended battery life provided by the latest instruments is a very useful feature. Possibly even more useful is the option of using high capacity fast charging rechargeable cells. Not only are these a very convenient power source, over time they also save a lot of money compared with the cost of buying ordinary disposable batteries.

Traditionally, handheld insulation testers have offered a maximum test voltage of 1,000 V and, for many users, this is entirely adequate. In industrial applications, however, higher test voltages are often needed, especially when working on equipment that operates at 1,000 V or more. In fact, practical experience has shown that testing the insulation of such equipment at 1,000 V is likely to miss incipient problems that would easily be detected by testing at a higher voltage.

Until recently, the only solution was to purchase one of the many high-voltage test sets that are currently available. These are very capable instruments, but they’re certainly not handheld devices and, while they offer excellent value in applications where their many features are needed, they’re unnecessarily costly for everyday insulation testing.

Fortunately, a new handheld tester has recently been introduced that operates at up to 2,500 V. This is the same size as a typical 1,000 V handheld tester and is just as easy to use. It also incorporates feedback-stabilised test voltage, which is especially valuable when working at higher test voltages.

Another key feature is provision of a guard terminal. This helps to eliminate the effect of surface leakage over items under test, which would otherwise seriously degrade the accuracy of the results. Like the best of the new 1,000 V testers, this versatile instrument even offers automatic polarisation index (PI) and dielectric absorption ratio (DAR) tests. These tests take a little longer to perform than standard insulation resistance measurements, but they can provide a more accurate indication of insulation condition.

We set out to decide what makes a “good” insulation test set for industrial maintenance applications. In this short article it’s not been possible to provide a complete answer but, as we’ve seen, some key points to look out for are stabilised test voltage, a choice of test voltages that will suit all of the applications in which the instrument will be used, and long battery life. If there’s any prospect of testing equipment that works at 1,000 V or more, it will also be worthwhile choosing an instrument that tests at up 2,500 V.

For more information on the 1,000 V MIT400/2 series testers click here

For more information on the 2.5 kV MIT2500 tester click here