EBME Forums Forums Maintenance Technical Assistance Theatre Medical IT System - Alarm causes

Currently IEC 60364-7-710 (electrical installations in medical locations) is being reviewed and I wanted to know what are the underlying causes of the medical IT system insulation monitor alarm operating.

So when electrical equipment connected to the medical IT supply causes the alarm to go off, what is the usual failure in the medical electrical equipment?

Is the main cause when liquid gets spilled onto equipment?

When the alarm goes off is the equipment still working?

How often does an item cause the medical IT system fuse to trip out or overload? (E.g. using a portable x-ray unit or plugging/un-plugging equipment stacks that uses a built in isolating transformer.)

As many examples as possible would be appreciated.

Are we talking about isolated power systems with a LIM?

In my experience the LIM alarms (as it is designed to do) when a piece of kit with lots of leakage current is plugged in ... and (or) when many items of kit are plugged in at the same time (bearing in mind that all kit has some leakage current, and the LIM clocks up the accumulated amount).

Yes ... unless it is actually faulty, the kit itself carries on regardless; which, of course, is the main reason (expensive) isolated power systems are back in fashion in operating theatres.

But the usual cause of "failure"? How about overloading (and high leakage) caused by the dreaded "trailing multiple socket outlets", sometimes lubricated by buckets of whatever is available? Not really equipment failure, IMHO; but more like "poor practice" in the OR.

Meanwhile, in a well designed set-up, mobile x-ray units (and C-arms etc.) and other "heavy duty" stuff (lasers, maybe) should be served by their own dedicated (earth-referred) supply circuits (suitably marked for that purpose). Non-medical equipment (floor scrubbers, and WHY) should also use a "normal" power outlet, and not simply plugged in at the pendant, for example. Such a "domestic" outlet is best placed near the door, and appropriately marked.

I've never heard of an isolated power system actually tripping out per se, and if my memory serves me well, the best systems include an element of "load shedding" before anything catastrophic is allowed to occur.

The few responses seem to indicate very few actual 'real world' events where the insulation alarm (LIM is really an american term) goes off.

Generally medical equipment conforming to IEC 60601-1 will be very reliable and I would not expect to see any increase in equipment leakage current over it's working life. The only failures would be the where a mains cable was run over or damage, in which case the power would probably have been lost on the device anyway. Alternately, some fluid spill inside the equipment, which would render it useless and dangerous to continue in use.

I will wait and see if anyone else can add some 'real world' events.

As previously mentioned the LIM will eventually kick off as more and more kit is plugged into the isolated system.

The "drill" when the alarm off goes is to un-plug the last bit of kit that was plugged in - to see if that makes any difference. It usually does (the alarm can be cancelled). "Fault finding" (that is, discovering which piece of kit is drawing heavy leakage) is generally straight-forward enough - as long as you have the theatre to yourself! By observing the meter on the LIM you can usually see what's going on.

I am not interested in the alarm going off just because too many devices have been plugged in. I am interested in the cases where an item is faulty or fails, for example by a bag of saline being spilt over it.

The idea of using a medical IT is to maintain continuity of supply, so when a first fault to earth occurs the supply is maintained, whereas in a TN system the fuse (or RCD) trips and the supply is lost.

If the circuit had 20 medical devices connected and an MCB tripped then all would lose their supply. In a medical IT system the same fault to earth keeps the supply on to all 20 but my guess is the one with the fault stops working due to the fault. I can't think of a fault where the unit with a fault to earth can continue to operate.

Remember a medical IT system limits the total A/C leakage current to that of the transformer itself (0.5mA max), irrespective of the total leakage of all the devices plugged into the IT system. Only the DC resistance is effectively measured, so is most affected by devices with heating elements. (Measure the insulation of most modern equipment and you measure infinity ( >100K ohms).

I think you mean 5 mA.

Yes, kit will generally fail if you tip a bag full of saline over it - regardless of whether it's plugged into an isolated system, or a normally earthed one (or even one without an earth at all)! But something like that (a live to neutral short, in other words) would hopefully pop the fuses at the equipment mains input.

In fact kit doesn't really care where it's plugged in; and the only things we could say about kit failing in the operating theatres are:-

1) The type of kit: eg, electrosurgical units tend to be used exclusively in the OR-type environment. So if they fail at all, that's where they will fail.

2) The age-old biomed dilemma (or should that be paradox?) of - although being a critical area, with plenty of scope for dramatic incidents - theatre kit is not always well looked after (or maintained), with "lack of access", and "unsocial hours" being the usual moans (excuses).

3) The tangle of cables, tubing, and general confusion that can occasionally build up in operating theatres presents another opportunity for hazards that can sometimes bring the kit tumbling down (as well as the tray full of instruments, or what-have-you); slip, trip, yank, drop, kick ... I'm sure you can imagine the scene.

Lastly, are you sure you're not getting insulation and isolation mixed up?

No, the allowable leakage current for a medical IT transformer, which must meet the requirements of BS EN 61558-2-15, is 0.5mA. From BS 7671:2008 (2011) section 710.512.1.1:

The leakage current of the output winding to earth and the leakage current of the enclosure, when measured in no-load condition and with the transformer supplied at rated voltage and rated frequency, shall not exceed 0.5 mA.

The 5mA (as seen in the USA LIM systems) relates to the insulation resistance alarm, which is set to go off when the insulation falls below 50K ohm. (This is equal to 5mA of fault current assuming a worse case 250 V supply.)

What I need to know is how many devices, which cause an alarm due to a technical fault with the device allow the continued use of the device.

The lack of response seems to indicate ether there are never any equipment faults that cause the alarm to go off or when the alarm goes off the equipment has failed so is unusable.

So you're asking for (over a given period, the year-to-date, say):-

1) List of equipment that "failed" in the Operating Theatre(s)
2) The cause of the failure (the actual fault)
3) For those listed at 1) - did the LIM alarm sound?
4) For those listed at 3) - did the equipment carry on working?

I doubt that any database has that kind of detail readily retrievable. Although I grant you that any really significant failure is likely to have been retained in Human Memory (as long as it happened recently enough).

Some databases have details such as Fault Codes etc., whilst others may not. But I have yet to come across any with a "LIM alarm" flag!

What you might get away with is asking folk to interrogate their databases with a query such as:-

1) List of non-PM jobs in the Operating Theatre(s)

As well as the essential question:- "do your Operating Theatres have isolated power supplies"?

And then delve a bit deeper into any records that "stand out" from that.

I would have expected a fault, which caused the insulation alarm to operate, would have been recorded since this is a serious equipment failure.

The fact that so far no one has offered up any examples would suggest these faults are very rare indeed! (Which is what I would have expected for modern, well maintained equipment, being used by professional people.)

As an aside ... I wonder if it is common practice these days for Theatre Staff to report (to Estates*, or anyone else) every time the LIM alarm sounds. Maybe modern systems keep their own log - to be recorded on the Building Management System (BMS), for example.

My guess is that they (the Staff) would tend to press the Cancel button, remove the offending equipment (usually the last one plugged in); and carry on "blazing away" (with the ESU).

* "Facilities", Engineering ... or whichever Department the electrical engineers inhabit.