5 kV, 10 kV and 15 kV insulation resistance testers

As the insulation value increases, the test current decreases and becomes harder to measure with the same level of accuracy.

The current rating is important, as an underpowered instrument will take a very long time to charge high capacitive test objects, such as long cables; it may also be unable to maintain the required test voltage when high levels of surface leakage are present. It is, however, necessary to be careful when comparing different instrument current ratings. An instrument with a 3 mA short-circuit capability that incorporates power regulation technology to ensure maximum power transfer into all load types will, for example, almost always be faster and more convenient to use than a 5 mA rated instrument that doesn’t use this technology.

The answer, at least in part, is in the question! An insulation resistance tester is designed to be used only on dead circuits, but that’s no guarantee that it won’t ever be accidentally connected to a live circuit. And if it is, an appropriate CAT rating is essential, especially as the environments in which HV insulation testers are most frequently used often have high supply transients. We recommend a CAT IV 600 V rating, and it’s imperative to be sure that this rating applies to all of the instrument’s terminals, including the guard terminal.

The answer to this question depends on the test set you are using. It is certainly challenging for instrument manufacturers to produce test sets that deliver good performance when the guard terminal is in use, not least because the guard terminal diverts a lot of current away from the measuring circuits. It is by no means unknown, for example, to have a surface leakage resistance of the order of 0.5 MΩ in a test sample with an insulation resistance of 100 MΩ. In other words, the guard terminal current is around 200 times greater than the current in the measuring circuit. This high level of guarded current can cause many problems in a poorly designed instrument, including greatly impaired accuracy. If you’ve got such an instrument, there’s not much you can do about it. However, if you’re buying a new instrument, the answer is simple. Insist that the manufacturer gives you meaningful data about measurement accuracy when the guard terminal is in use. The latest Megger units, for example, have a maximum error of 2 % when guarding 0.5 MΩ leakage with a 100 MΩ load.

There are several reasons to select a test set with a high output current. Possibly the most important is that a high output current means that the item under test will be charged more quickly, which means that the test can be completed in a shorter time and also that there’s less risk that the readings will be taken before the test voltage has had time to stabilise properly. And, if you’re using the instrument’s guard terminal, don’t forget that 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 that the output voltage will collapse, and the test results will not be valid.

That depends upon the size, complexity, and criticality of your equipment. Even identical units can differ in the required check periods; experience is your best guide. In general, however, working apparatus – such as motors and generators – are more likely to develop insulation weaknesses than wiring, insulators, and the like. A test schedule for working equipment should be established, varying from every 6 to 12 months, depending on the size of the equipment and the severity of the surrounding atmospheric conditions. For wiring and the like, tests once a year are generally sufficient unless the installation conditions are unusually severe.

These facilities are useful in a wide range of applications. For example, when testing a large item such as a power transformer, the instrument can be positioned on top of the asset near its terminals so that the test leads are kept short and operated from a much more convenient – and much safer – location, using the remote control option. Additionally, it’s sometimes necessary to carry out tests in hazardous areas, such as inside an energised substation. In these cases, once it has been connected, you can operate the test set and access your results outside the hazardous area, significantly increasing the operator’s safety. Finally, in production line test applications, it’s often desirable to control the test unit and retrieve the test results automatically. The remote control and remote downloading facilities offer a convenient way of achieving this and providing any safety interlocks that may be needed.

In cases of this type, the source of trouble is almost always induced noise in the measuring circuit. You can reduce noise pick-up on the test leads by keeping them as short as possible and using screened test leads. With screened leads, the screen is connected to the insulation test set guard terminal to divert the noise currents from the measuring circuits. However, if the noise is being picked up by the item under test rather than the test leads, these measures can’t help. In such cases, the only effective solution is to use an insulation test set with high noise immunity and effective filtering. The S1 has noise immunity of 8 mA, which ensures reliable operation in the harshest 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 with enhanced noise rejection when working in the most challenging environments.