EBME Forums Forums Maintenance Technical Assistance Unearthed insulation system

Indeed it is! I think that the measurement method you describe is used by the likes of RCCB's (Residual Current Circuit Breakers), but I believe LIM's actually inject a small test current and measure voltage modulus/phase angle to work out the impedance of each 'line' to ground....see here for example: How_it_Works
In other words, it doesn't actually measure actual leakage current as directly as the EST (which is what you said)!

Perhaps the laser equipment actually has a particularly high leakage current, and is not indicative of a fault as such. Whether it is wise to use in a patient environment..well, that is another question entirely!

Yes, but I was trying to keep it simple. It's the difference in impedances that is the crucial point. Traditionally, the LIM serves as a warning that excessive "risk" currents would flow should the protective earth be lost.

ELIS has headed up this thread Unearthed insulation system ... does he mean to say that there is no protective "earth" in the theatre in question, I wonder? Isolated power systems should still have an earth. It's just that the L1 and L2 power lines are not referenced to it (in other words, they are "floating", but with 220 volts - or whatever - across them).


... where else is it likely to be used?

Tony, Geoff,

Having looked at "how it works", it can work out if the load is resistive or capacitive and adjust the reading. "The LIM can
measure impedances that are either pure resistive, pure capacitive, or a combination of both resistive and capacitive faults".

What happens when it is an inductive load? Is the laser an inductive load?

There are many variations on the LIM theme, Paddy. Some of the early ones were not very sophisticated. Don't forget that inductive loads are not unknown in operating theatres. Great big Matburn suction pumps, for example.

But it's an interesting question that you raise. My guess is that most loads (including the laser) would balance out as "neutral". That is slightly capacitive, or effectively resistive.

As an aside, don't forget also that RF gets generated in theatre, too. All in all, it's a pretty "hostile environment", electrically speaking.

Geoff,

Agree with you. There could be a motor in there somewhere which is causing a problem. We'll just have to wait and see!

... or some hefty transformers. Let's see what news ELIS comes back with.

... and (so far, almost seven years later, albeit on "Back to the Future Day") we never heard from the guy again.

So we're still none the wiser.

I would have liked to have learned more about the problem. Maybe there was a big x-ray system (or CT, MRI or what-have-you) somewhere in the vincinity. Or even (as I have come across in one of my previous lives) a lift (elevator) motor kicking in now and again. Or even a big washing machine in the laundry. But I suspect that ELIS was simply looking at another "poor" installation - possibly lacking an earth! As so often seems to be the case, we never got the "complete story"!