It could be argued with considerable justification that insulation testers regularly save lives by revealing potentially dangerous insulation faults and failures. But this story is a little different. It begins with a Megger MIT515 5 kV insulation tester that was returned to the factory for repair under warranty.
MIT515 as it comes from our Dover factory
The user reported that he’d been using the instrument for phase to earth tests on a 5 kV transformer when it failed, and now it wouldn’t do anything except display an error code. He also commented that immediately before it failed, the instrument had emitted a few wisps of smoke. Given the widely reported problems with mobile phone batteries, he suspected that the root of the trouble was the instrument’s rechargeable battery.
Like all Megger test equipment, the MIT515 is designed from the ground up with safety in mind and for the MIT515 that means, among other things that, within its outer casing, it has a tough fire- and shock-resistant internal enclosure that houses all of the components that are exposed to high voltages.
The faulty instrument was initially sent in the usual way to the repair department for attention but, on removing the outer casing, it was immediately apparent that there was significant damage to the inner HV enclosure. Since such damage is virtually unknown, the instrument was passed to the engineering department for detailed examination.
MIT515 with its tough outer case opened to reveal the fire resistant inner case
A first and very easy step was to check the rechargeable battery, which is in any cases mounted outside the HV enclosure. As expected, this was in excellent condition with no sign of any fault or failure.
Next the HV enclosure was opened and the report from the engineering department notes, with a degree of understatement, that when this was done there was “evidence of significant burning”. In fact, there was very extensive damage including resistors that had unsoldered themselves from printed circuit boards, broken ferrite cores and integrated circuits with holes blown in them. There was also evidence that arcing had occurred between connections and components.
The Megger engineers analysed the damage forensically and came to the conclusion that there were only two ways in which the damage could have been caused. The first was that the instrument had accidentally been connected to a live 5 kV system and left connected for long enough for the heating and burning to occur in the HV compartment. The second was that the instrument was connected to a dead system and, before it was disconnected, the system was accidentally energised.
Both of these situations are exceedingly hazardous and, if the construction of the instrument had not been sufficiently strong and sufficiently fire resistant to contain the effects, the user might well have been severely injured or even killed.
In short, it seems fair to say that in this case, the robust double-enclosure construction used in Megger high-voltage insulation testers was nothing less than a life saver.
To find out more about this remarkable tester click here