Hi Medical Engineers
I have problem. We used in operating rooms the device with pulsed consumption. There was tested the leakage current and it was OK. But somebody connceted the device to the unearthed system and the insulation monitoring device showed the insulation fault so the resistence was not stabill the resistance was moved it seems with relationship of pulsed consumption of the device.
Has the device any defect? Can we claim the device on the manufacture so the device is damaged? The defect will not appeared if the device will not be connect into unearthed system.
Thanks for your reply and advices .
ELIS

What machine/device is it? How does it monitor the earth system?
Your questions are difficult to answer unless you provide more details.

OK ELIS ... so you're using a certain piece of equipment in an operating theatre which has an isolated power system, right? And, when that equipment is running in a certain mode (pulsing), it triggers an alarm (over-leakage-current) at the LIM (line isolation monitor)?

OK, it's leaking a bit of current at that stage (probably due to RF leakage if it's an electro-surgical unit, or a motor being in circuit, perhaps, if it's a pump of some kind). But it may not be faulty. What other equipment is plugged into the IPS at the same time? Don't forget that the LIM shows the sum of the leakage currents of all equipment plugged in. Try disconnecting all other items and see what happens. Also, is the LIM correctly set?

Naturally, the item will be assumed to be working normally (and I suspect that it is) when plugged into a "normal" (earthed) outlet ... because then there is no LIM to report the leakage. It will still be there, though. Try testing it with your Electrical Safety Tester. If it passes the EST, then basically it's OK.

But Bong is right. We would be interested to hear what this equipment is. My guess is an electro-surgical unit. Am I right?

..or perhaps it is something with a heating element using some type of proportional control?

ELIS, please tell us what the equipment is, we would all like to know!

The equipment is the laser for lithotripsie. The insulation monitoring device shows the lower insulation resistance when only the laser was connect to the unearthed system.
ELIS

OK, ELIS. More questions:-

Is this a new piece of equipment, recently commissioned ... or something old, and rarely used?

Is it excessively humid in the theatre?

Have you checked out the kit with your Electrical Safety Tester?

If there is a fault, take a close look at leads and cables (especially those that may have been "run over" by wheeled equipment) ... then take a look at the power supply unit (big fat capacitors?).

Thank you Goeff you wants to help me
The laser is new approx. two month. I think in the operating room is not any excessively humid.
The laser was check by medical electrical safety tester and the leakage currents are in standard value.
The insulation monitoring device monitor the resistance between the active conductors and earth. Regarding to our low, the critical level is 47 kohms. The problem is so when to the insulation system is connect only the laser, than the gauge and his needle is not on the value "infinite", but the needle is moved between value "infinite" and critical value ( the laser is connect only and switched on without activation).
ELIS

H'mmm. On the face of it, it does indeed sound like excessive leakage current there somewhere. It could indicate a defect of some kind. Or perhaps the thing just draws a heavy current anyway, upsetting (or confusing) the LIM. Odd, though, that you get comfortable readings with your electrical safety tester. You may find that you need to provide a special (that is, standard, "earthed") power outlet in the theatre just for this unit (as sometimes needs to be done for other items, like mobile C-arm x-ray units, for example). But as the unit is still "new", perhaps you should first of all seek the opinion of the supplier!

I agree with Geoff that you should talk to the supplier of the laser.

It is strange that the LIM and the safety tester seem to disagree...or do they?! What is your supply voltage? What leakage current readings did you see?

Yes, that's an interesting topic in itself:- LIM versus electrical safety tester (EST)! Put simply, doesn't the LIM essentially look at the imbalance of currents drawn at L1 and L2 (as we can't really say line and neutral when talking about an isolated power system, can we)? It then regards the difference as leakage. But, maybe it's not current leakage within the equipment itself ... perhaps there's a parallel circuit somewhere!

And, of course, the EST measures the true leakage current(s) value(s) by interrupting the protective earth conductor (and all the rest).

It would be "educational" to get to the bottom of this one. My next question to ELIS would have to be:- how much supply current does this equipment draw (consume)? Is it something hefty?

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"!