TN-C-S Systems and Insulation Monitors

Does anyone have any experience in what happens if you use an Insulation Monitor (particularly a Bender or equivalent) normally used in an IT system on a standard, grounded neutral TN-C-S System?

Got an answer from the OEM, despite the confusing 3(N)AC designation in the datasheet, they don’t work at all on grounded systems, which I kind of suspected.

Won’t work. At least not the way it was intended to work.
That is the simple answer. And the reason is that the garden variety Bender measures insulation between N and PE - which are connected in a system with grounded neutral. All the Bender will tell you is that N and PE are connected - and you knew that already.

If you have a system where a GFI/RCD (like a five conductor system - in European parlance) should be used then, that is quite a different thing. But that is not what your question is about, I think.

I am right now involved in a large commuter train terminal and its 400 V TNC system. Or is it a TNC-S system? Perhaps even a TNS? (which we do not believe). No-one seems to know. And there is nothing said about it in the documentation.

There is an RCD monitor somewhere and the corresponding pictogram on the monitor screens. Flashing red. The hardware can’t be found anywhere but there are two conductor going to the “Utility’s Building” and they do not let us in to have a look.

There are also lots of 16 and 25 mm2 blank Cu ground connectors some of which have been cut and sometimes insulated. No-one knows why. Didn’t stop the flashing, though.

I finally got the guys engaged in the BIG RCD MONITOR HUNT and we will also try and find out about all those ground conductors. I will use a car stereo amplifier and an “inverse” current clamp to induce a modulated 1 kHz current between N and PE and then use a search coil to listen to that tone to find inadvertent connections between the two.

Why? Because the terminal is operating 24/24 and a black-out is a big NO-NO.

Will report back if and, when we know more.

I just had a switchboard, destined for an IT system and fitted with a Bender unit, under factory test, and it dutifully informed us of the presence of the MEN link. Of course, when we finally went to commission the board, the Bender worked just fine.
Your issue (and troubleshooting method) sounds much more involved, and fascinating.