Insulation testing: a word of caution!
Modern insulation testers are carefully designed to ensure that, as far as possible, the output they deliver is non-lethal. No one should, however, be tempted to verify this statement by “testing” either themselves or someone else: people vary enormously in their susceptibility to electric shock, so non-lethal operation can never be absolutely guaranteed.
It is also essential to understand that, in a real-world test situation, things are rather different. During an insulation test the tester charges up the insulation of the device or cable under test. The output from the tester alone may be insufficient to cause serious injury or death, but the stored charge is an entirely different matter and, in many cases, it will be easily capable of delivering a fatal electric shock.
This issue is all the more insidious because the device or cable under test may hold the charge for a considerable time after the test has been completed and the test set disconnected. In fact, the better the insulation, the longer it will retain the charge and the greater the shock hazard. In these situations, with the test object out of service and disconnected from the test set, there is absolutely no visual or other indication that it is carrying a dangerous charge.
The solution is to discharge the object under test immediately after the insulation test has been completed. This is necessary, in fact, not only from a safety point of view, but also because a test performed on a device that is already charged can give misleading results, as this extract from the IEEE43 standard makes clear:
Before any testing is conducted, the winding insulation must be discharged. It is not safe to begin testing before the discharge current is zero and there is no discernible return voltage (less than approximately 20 V) after the ground is removed. After completion of the test, the winding should be discharged through a suitable resistor, sized to limit the instantaneous current to 1 A. A minimum discharge time, which is equal to four times the voltage application duration, is recommended.
This standard refers specifically to transformer testing, but the same general comments apply to all forms of insulation testing.
Many modern insulation testers have an automatic discharge feature that discharges the insulation at the end of the test, while simultaneously showing the residual voltage on the instrument display. This is a very useful feature, but nevertheless there are some practical points that must be considered. The first is that automatic discharge can only work if the test leads remain connected for a suitable period at the end of the insulation test proper and, as we have seen, that “suitable period” should be a minimum of four times the duration of the insulation test.
The next point to bear in mind is that even if the instrument shows that the residual voltage has fallen to a very low value, this is not a totally reliable indication that discharge is complete. This is because if the discharge circuit – which in this case is built into the tester – is disconnected too soon, reabsorption effects may mean that the voltage reappears.
Further, it should be noted that all insulation testers have a limited discharge capacity. In most cases, this will be more than adequate but, for test objects that store a lot of charge, the resulting high discharge current over a comparatively long time could damage the instrument. The maximum capacitance that can be safely discharged will be specified by the manufacturer of the insulation test set, and should not be exceeded.
Since failure to discharge the device or cable under test after an insulation test can give rise to serious safety issues as well as disrupting further testing, some engineers and technicians prefer to use an external discharge device to supplement that automatic discharge function of the insulation test set. This is good practice provided, of course, that the discharge device is safely constructed and has an adequate rating.
Before concluding, it’s worth noting that stored charge after an insulation test isn’t always a nuisance. A dielectric discharge test monitors the current while the insulation is being discharged and can provide very useful information about the condition of the insulation.
Insulation testing is one of the most commonly employed techniques for assessing the condition of cables and electrical equipment of all types. Correctly performed, it’s a testing technique that’s both useful and safe. However, as we’ve seen, it’s important never to forget the potential hazards and, in particular, to be aware of the dangers associated with stored charge.