I was testing a local paper disposal plant that uses Telergon isolators on the three phase motor control centres. Now obviously there is a mechanical interlock ensuring that the panel is de-energised before the door is open. However, after the door was opened I carried out an insulation resistance test on the supply circuit. I noted a low resistance between red and yellow phases. It appears that around 9.6ohms is present between these phases even with the door opened. This was apparent on every Telergon isolator in the plant. I had to disconnect the supply cables to obtain a reading. Because this was a relatively
pressured job, I really didnt have the time to assuage my curiosity! Any ideas?

Industrial stuff like this is a little out of my league. It sounds very odd though, as that sort of resistance value would equate to 40-ish amps at 415V.

What's the rating of the fuse/CB feeding each cabinet? Could it be some sort of extra belt-and-braces safety circuit, intended to trip out the supply if it should somehow become energized even though the door is opened?

(Microwave ovens often have an extra door-interlock switch which puts a short on the supply downstream of the main switch, just in case it fails.)

Paul,
The fuse protection was 200 to 630 amps. I concur that this may be associated with an additional electrical interlock, I never got the opportunity to explore as the plant manager was dancing every time a machine was down! Dont forget the 9.6 ohms was a resistance value which may have simply been the resistive part of a coil.

Actually you saying "coil" has made be think of something else. Is there any chance there could be a solenoid interlock on the doors, so that they cannot be opened when power is applied?

Even though there will be inductance present, under 10 ohms DC resistance still seems pretty low for a coil used in such an application though, so maybe that idea's way off base.

Lyle,
Just a little question.
Are there no control transformers across the phases to step down to a lower voltage?.
But as Paul also pointed out, at 9.6 ohms, it's going to be pulling some serious current.
400/24V transformers are pretty common here for control purposes.