Following a reported fault of electrical failure alarms in theatre I was summoned to inspect a Sony video monitor model PVM1443MD.

The mains lead plug, wiring, and controls all were intact and nothing untoward.
Being a class one device, tested the unit with a Metron Electrical safety analyser to IEC 60601.1. test regulations.

The unit was additionally tested using a Seward PAT1000s electrical portable appliance tester.

Never did it under any circumstance display a fault with either of the two mentioned safety devices.

However upon return to theatres, plugging this monitor back into the Isolated Power Supply in Theatres, the ITPS monitor (IPSM002-MB) displays a killo ohm fault with constant alarm.
This then lacthes a fault conditon that has to be reset at the main power intake board (Consumer unit).

The device instaled at the intake (Consumer unit) is an IPSM002-MB.
What this device does is monitor the insulation resistance of the unearthed AC network, mainly found in group 2 medical locations.
The current temperature of the isolation transformer is permanently measured. It is in the form of a DIN rail mounted device.
Its function is to measure DC voltage that is superimposed on the network. Any Ohmic insulation fault path between the network and earth is detected.
If the reading is below the selected response value the relay K1 and the Kohm LED will operate. The bar graph indicatior permantly shows the system insulation level to earth in Kilo Ohms.
The current passing through the isoaltion tranformer is measured via an external CT. If the current exceeds the adjustable reference value, the relay K1 and the alarm LED operate.
The bar graph indicator permantly shows the percentage in use.

After consulting the manufacturer (Starkstorm) they are of the opinion that the SONY Video monitor I had plugged into the Theatres mains wall outlet is still at fault.

How can this be I argued if the monitor passed both IEC60601 and PAT testing??

Anyone here have similar IT power systems in their theatres giving spurious or odd results?

I'd be inclined to alter the reference value on the (IPSM002-MB) however Starkstorm do not recomend this as they state their intallation currently meets with IEC regs governing installation in Group 2 medical locations, and altering this value would void this regulation / installation ??

Bloomin suss that one eh ?

Chris-H

At an absolute stab in the dark there could be a remote possibility of....

If you are using the BNC connected Shielded connectors on the monitor then maybe a few loose strands from one of the video signal leads are finding their way onto the outer earth shielding of the cable and back through the chasis earth.

Might be an idea to see if it alarms with the cables removed so you can isolate the device from the interconnecting leads.

Just a stab in the dark I know, but its worth a try if you can think of nothing else, these nurses have a tendancy to run over cables and generally abuse them.

Chris

Another stab in the dark so to speak, does a trip occur in the mains monitoring device immediately after the Sony monitor is plugged in?

Based on a reply of yes, rather than later, have you looked at the mains rf/emc filter in the Sony monitor? Our theatres are not mains monitored and as such no tripping problems using Sony monitors.

Could be the mains filter at switch on, surge of the X/Y, particularly the Y capicitor in the filter?

Jim

Chris -

Can I ask a few questions ....

If I understand correctly, of the the three IT monitors - isolation, overload and transformer temp - it is the first of these that is triggering the alarm.
What is the kohm refernce value at which the isolation alarm is set ?
Can the alarm be cleared without unplugging the offending monitor ?
What measurements were included in yuor Metron/PAT-testing of the monitor ?

Thanks,
Ged Dean

Soo many questions and so little time/knowledge
1) Yes as soon as the monitor is powered up, whether it be from the switchable mains socket or either the power button on the front of the Sony monitor, results in it tripping. It is indescriminate.
2)Have been unable to try the unit as of yet with all BNC's dissconnected. However the BNC at rear of monitor was used with refence to exposed parts during electrical safety testing and passed with flying colours.
3) The Kilo-Ohm reference on the IPSM002-MB (Starkstorm bit of main supply monitoring) has a "resistance to earth setting" of 240 kilo-Ohm. with a "current setting" of 20 amps. This is what has been stated in the commisioning report.
4) Yes I agree, it could well point to an internal mains filter, however, surely this would have shown up in the Metron Electricval safety test, this was carried out with earth leakage current limit of 1000 micro amp(open supply and reverse mains) 500 micro amp(normally closed and norm closed reverse mains), enclosure limits of 500 micro amps and (100 normally closed and reverse mains) as per Default setting by Metron for class one devices IEC60601.1.
So I think not !
4)No the alarm cannot be cleared without unplugging the offending monitor. It is a latchable alarm that has to reset at the main intake (Estates sparkie).
5) Unfortunately I do not have the PAT results but I vaguely recall a limit of 0.5 ohm being entered ( As do not regularly use this bit of kit)

Chris-H

Chris, your isolation monitor sounds very comprehensive in what it measures. In this situation though, only the impedance measurement part is up for consideration of course. From the information you have given us, this is very definitely an equipment-derived (mains) line-to-earth impedance problem, unless you already have an isolation monitor/system problem (see testing method below).

If you have not already done it, even though such a low impedance (1K-ohms) is being registered when the video monitor is connected, suggesting the problem is almost certain to be fairly and squarely in the video monitor itself, I would suggest you do a quick check of the system integrity with respect to the overall ‘leakage’ of the system. Isolation systems can deteriorate unnoticed and should be checked on a regular basis anyway. This will give you to some extent ‘peace of mind’ that your isolation monitor is calibrated and doing its job properly, or not, as the case may be.

If you do not have an isolation system checker, then build yourself this quick piece of test kit I have outlined below. With this you can test for that 240K-ohms alarm-trip point you may still have your eye on adjusting! You can also measure the isolation leakage (current flowing) from both lines with respect to earth. (I will refer to ‘Lines’ now, not ‘live and neutral’ in the following explanation)

First off though, disconnect ALL the equipment in the theatre suite in question, from All the sockets in the wall/ceiling mounts etc. (Don’t forget wall-mounted viewing boxes, surgical angle lamps etc. etc. if plugged into the ordinary sockets i.e not hard-wired in). With this done, the isolation monitor should be reading a high impedance, say at least 4-500K-ohms? if the ‘trip-point’ of your isolation monitor is 240K. If the reading is still much lower, say around the 240 value, then you should attempt to find out why, as it might not take much to trip the alarm when extra equipment is plugged in later. Generally, isolation monitors are designed to alert the staff with a current limit of around 2 to 5mA. At your mains voltage and for this trip impedance (240k), this would equate to an alarm tripping point of just 1mA. (If it had not been showing as low as 1k-ohm when you plugged in your video monitor, I would have suggested you look for an accumulation of impedances that might have been just ‘tipping the balance’ over the 240K trip limit. But this reading is LOW!).

With the isolation monitor reading hopefully well above the 240K value and with all the sockets empty, I am assuming that if you plug in your video monitor now it will show approximately the same 1k reading and alarm - Yes?? If the reading stays low, and then returns very high when it is unplugged again before the alarm is reset, consider the video monitor as a very likely 'culprit’. Unplug the monitor again.

Now to test the system. Get a 240 volt light bulb connected to two wires and momentarily touch them across the two line (live & neutral) conductors in any mains wall socket to ensure it lights up. Then connect the two wires, one to a line conductor and the other to earth, in the same socket. It should not light up. Connect to the other line conductor and earth. In neither case should it light up. (If one line to earth does light, you do not have an isolated system any more i.e one side is already down to, or very near, earth potential. If this is so, stop here immediately and investigate the reason).

Note: The isolation monitor may alarm each time the bulb is connected from each line to earth, just reset and continue. Do not forget this light bulb check step.

If the light-bulb test gives the correct results, then next get an ammeter (it will be measuring only a low ac value) and measure the isolation leakage current (impedance-equivalent) at the same wall outlet, of each line to earth in turn. As you did with the light bulb, connect it between one line and earth (not between line and line in this test, of course!!) note the reading and then do the same between the other line and earth. Note that reading too. Now calculate the individual impedances from the individual current values of each line to earth. The highest current value (lowest impedance) reading should equate to the isolation monitor’s ‘estimated’ and displayed value of isolation impedance in the system. Note: for trouble-shooting purposes, the leakage current value you read in either line when doing this test, relates to the isolation value of the other line-to-earth and not the line-to-earth that you are measuring.

Now with the trip-point potentiometer tester you have built (see below), plug it into the same wall socket (or any other). Slowly reduce the pot resistance until the isolation monitor alarms on the line selected. When it alarms, note the impedance reading on the isolation monitor and then remove the mains plug and measure and note the resistance between the relative power pin on the plug and the earth pin on the plug. Now return the pot to maximum resistance and switch over to the other line. Plug it in again and do the same measurement and note the same two readings. Using these pot resistance readings you can firstly, with the help of dear Ohm’s law (and 240volts), simply work out the current that has tripped the alarm. It should of course be near the 1mA level in both cases. And then secondly, check that the impedance readings found on the isolation monitor compare favourably with the resistance readings on the pot. Assuming that the leakage current readings, you took above, were near enough the same, it should be approximately the same reading i.e trip the alarm around the same point on the pot. This will give you an indication of how accurate your isolation monitor is (in the impedance measuring mode only) and hence if you have a problem actually in the isolation monitor/system itself. The pot readings should be around the 240K-ohm mark if the isolation monitor is working correctly. (If not, it might also help to decide whether the pot you wanted to adjust really needs adjusting, though I would still recommend consulting the company first).

Now to return to the video monitor itself. As you said, you did the insulation impedance of both live and neutral together, to earth, on the video monitor and it passed OK. However, the main point to note here is that the Insulation Test is usually done with a DC voltage in testers, which might well leave undetected (as has already been suggested) say a ‘leaky’ interference-rejection capacitor (normally having mains-frequency voltage applied across it) in what in your case here (if the video monitor is not, like me, of the ‘old generation’) could easily be a switch-mode PSU.

I was going to suggest that you had possibly over-looked, due to perhaps limitations of your testers, doing the earth leakage test in the mains reverse connection. The reason being (for interest only) that in the normal mains configuration testing on a non-isolated supply, any caps etc. going to earth on the neutral side that might be leaky, would definitely not be detected due to lack of ‘electrical stress’ (normally produced by the live wire to earth) needed to produce a current and the earth-neutral link. But since you have done that particularly important test in this case, the two things that might be happening are firstly, that when you switch the video monitor on, the initial charge-up surge current (though very small) into the mains input filter caps (tied to earth) is causing the isolation monitor to momentarily trip but permanently latch the alarm - a condition the above system tests would be unable to reveal of course. (If the reading stays latched at 1k after straight away removing the video monitor from the mains socket, this might help to confirm the theory). If, however, the impedance reading does change back to high impedance when removing the video monitor whilst in the alarm condition, it might strongly suggest that you have a ‘hard’ fault to earth not a transient impedance dip. In this case it should be easier to find. The second possibility, and perhaps the more likely, (if it is an SM-PSU) is that one (or both?) of the MOV’s (metal oxide varistors) may be too sensitive (or too low a value) and momentarily triggering low impedance to earth at switch-on.

Now to the construction of the isolation monitor leakage alarm-trip tester:
Get yourself a linear (not log) potentiometer of say 470 kilo-ohms and a center-off, single-pole, double-throw mains switch. Connect a short piece of any size (a one or two-amp rating will do) three-core mains cable to the live, neutral and earth of a 13-amp plug (any rating fuse in it will do, as it will not be there for protection, only continuity) - or whatever type you use in theatres. Connect the slider of the pot to one of the other contacts on the pot as well as connecting a wire from this jumpered pair first to one end of an in-line fuse holder (with say a 100 or so mA fuse) and the other end of the fuse holder to the earth wire of the cable. Set the pot to read maximum ohms with respect to the other free terminal on the pot. Now connect this free terminal on the pot to the center pole on the switch. Connect now one power line wire in the cable to one of the two spare switch poles and the other power line in the cable to the other spare switch pole. Tape over any bare wires.
Note : Even though it is generally safer working on isolation systems, in certain circumstances (particularly across the two line conductors) it can still be dangerous, so use the usual amount of safety awareness when doing these tests.

Lastly for my own interest, did I interpret you correctly when you say that the isolation monitor is looking to measure dc current superimposed on the (ac) power circuit connected to the isolation transformer? Or did you in fact mean that it uses a superimposed dc to do the impedance measurements from the lines to earth ? The latter (if pulsed) to me would probably be more likely as it would need some form of a ‘changing’ current to measure not only purely resistive components to earth, but capacitive as well. If I am wrong, what dc is it measuring on the system? Measuring the temperature of the transformer in an isolation set-up was new to me as well. Interesting. Seems a very thorough monitoring device.

Finally, permit a note from an ‘old-hand’ on pot-adjusting. If the pot in question that you thought about adjusting is not easy to get at i.e. need to remove covers, it was good initial advice from the company. In fact as a basic technician tenet, it is never a good idea to adjust internal ‘pots’ in any piece of equipment as a way around problems, however tempting it may be, unless you have explored all the other avenues to try to correct the problem first. Although ‘pots’ can go noisy and upset readings (or be mis-adjusted by others), best wait, as one can easily end up with more problems than bargained for particularly in this case if you have to have the isolation monitor re-calibrated with specialized equipment later (at the hospital’s own expense?) having found the fault elsewhere.

If now you decide it is in the video monitor, (don’t rule out the small possibility that the isolation monitor may be reacting incorrectly to spurious switch-on dips), concentrate only on the PSU, (old style or smps) . If it is a switch mode, I would suggest (only to test of course) just removing the interference caps that go to earth and then MOV’s or vice versa.

Keep going.

We have recently started using new theatres with isolated power supplies and are starting to have problems with the mains triping out.
Can this problem be caused by plugging some equipment into none isolated power outlets.
Where can I get more information about the use of these isolated outputs.

Chris/Ken

Chris, historically we have had to fit isolation transformers to endoscopy trolleys due to high leakage currents associated with Sony monitors. Would appreciate you highlighting the eventual solution to your problem.

Attached are a couple of references that may help illuminate this topic. Looks as if we will all require to gain more understanding in this area.

www.brandon-medical.com/nletter/issue4/safe_electrics.htm

IEC INTERNATIONAL STANDARD 60364-7-710 First edition
Electrical installations of buildings – Part 7-710:
Requirements for special installations or locations – Medical locations
Reference number
CEI/IEC 60364-7-710:2002

Bill

I think that what is normally being monitored in IT-systems is in fact insulation, rather than isolation. (I’m told that in some parts of the world the practice is to measure isolation impedance rather than insulation resistance, but then the measurement is normally given as a prospective mA current value rather than a kohm value).

If your insulation resistance is being measured by a dc monitoring signal then any errant dc present (eg: from a faulty s-m power supply) will add or subtract to the measured value, causing over- or under-sensitivity respectively. In the former case this could result in nuisance alarming, the latter in loss of safety margin.

Potentially relevant to your case is that in the Bender sales literature, (Bender are also manufacturers of IT-power systems), they report having found a particular problem with a number of makes of VDU appliances. Apparently a certain internal EMC fix - of which unfortunately I have no knowledge - used in some of these devices results in small dc current flow of the type described above, which then affects results of the IT insulation monitoring as described. If your Starkstrom IT-system uses a plain dc monitoring signal then this might be the source of your problem. (I believe Bender now use a dc signal which is pulsed at very low frequency, which then enables the monitor to detect the presence of any other dc signals and correct results accordingly).

p.s.: With regard to comments on IT-system monitoring above , insulation, overcurrent and temperature are all requirements of IEC-60364-7-710 so will now be the norm. (The relevant chapter in the IEE’s Guidance Note 7 (2003) supporting the ‘Wiring Regs’ is based upon this IEC Standard).

We are trialing a new camera system and associated components that is wired through an isolating transformer.It is since we began to use this equipment that our problems started. Unfortunately the system does not immediately trip when plugging the camera system into the mains supply, but this is the only common piece of equipment being used when the problem occurs.
All leakage tests on the camera system have given the expected results. It could be that the monitoring system is faulty and I am awaiting for this to be tested.

We have found that the isolating transformers on our equipment trolleys are causing the mains circuit breakers to trip out because of the high inrush currents.
Has anyone else experienced this problem.

Ours seem to stress the fuses untill one time you switch on and they blow - both of them - big time ! The older stacks with isolating transformers are kept in the older theatres - where I think they replaced the circuit breakers with six inch nails a few years ago.

We only have one theatre with all this fancy isolation monitoring equipment fitted and we did have some problems with alarms going off during the first few days. The company came back in and adjusted things and I don't think we've had any problems since. There are video scope stacks in use in that theatre area which have Sony monitors on the top, so I'd be surprised if they were not used in this particular theatre.

Ken,

The first thing here to mention (since you requested a bit of background) is that in an area where there is a dedicated isolated system, such as your theatre, strictly speaking there MUST only be isolated outlets within this area. The simple reason is historical - that with the isolated system the earth-neutral connection is broken and therefore in the case of a fault-current to ground occurring (say within a piece of equipment itself) in the patient environment, there should not be a chance of a spark igniting explosive anaesthetic gases /agents (rarely found now in theatres). Obviously, with the normal supply present (non-isolated), if a fault-current to ground occurs, a large current can flow and hence this could be potentially dangerous if a spark were to occur. Back to the isolated area. All power to sockets within the isolated area comes from an isolated transformer. All earths radiate out to sockets from a common point at the transformer to keep potential differences low at the distant earthing points (equipment enclosures - plugged into the outlets) and then this single point at the transformer is tied to the main hospital earth - it is not an earth-free environment as some would suggest!

So if your theatre system was built professionally, you will be isolated throughout the room and no chance of isolated/non-isolated mix-ups unless however, theatre staff are, unknowingly, violating the safety integrity of the theatre by bringing a long ‘extension cable’ in from outside (under the swing-doors?) to power this camera assembly. I am assuming you mean this as you talk about mixed supplies in theatres and “trailing” camera systems? If this is so, why are they having to do this i.e. not able to plug the camera system (plus your isolation transformer) in the isolated sockets in the theatre itself? If it is being done because, as per your latest comment i.e. you feel that the in-rush current to the transformer on the camera system is causing a circuit breaker (CB) to trip on the isolated system, then I suggest you investigate the CB being tripped and the typical associated loading if possible, it’s sensitivity (and perhaps the camera system’s transformer?) as per suggestions at the end of this article. In-rush currents to good isolation transformers on trolleys should not normally be a problem for the standard CB. The effects of in-rush and working current, as far as the local CB feeding that equipment is concerned, will be the same whether the equipment and CB are in an isolation system or not. As has been mentioned, it is often a practice to use isolation transformers as leakage current reducers. If using them were a problem for CBs we would not be able to use one as a solution to the problem.

Extension cables aside (as that is another issue of safety concern by itself), if you have decided that the “trailing” camera equipment has to be fed from non-isolated power from outside the theatre, going to your trolley with its seperate isolation transformer system inside the theatre, then strictly speaking, you must observe the following two points.

Firstly, make sure that your isolation transformer on the trolley is of high quality. The reason being that if it is not and you get a break-down to earth on the primary side of a cheap transformer, as stated above you run a much greater risk of an explosion hazard through sparks ‘flying’.
(The R.S. Components have good well-insulated and potted high quality isolation transformers that do not have a safety earth/screen for protection and will therefore do very well in your situation - as well as in leakage current reduction situations. Having said all this, it is more than likely that your hospital does not use explosive anaesthetic agents any more, but play on the safe side if you have to have this arrangement!

The second safety problem is that the earth wire feeding your camera system will have come via a different route (although still connected at a distance) to those earths in the theatre itself and therefore will almost certainly not be connected directly to the particular theatre system’s earth. This, in itself, is a potential hazard (particularly in Cardiac Theatres and Cath Labs) by being able to produce high potential differences between individual earths within the patient environment. You will therefore have to tie your camera system’s earth to the theatre earth. The easiest way is to use an equi-potential strap from your system to the nearest wall connection whilst the camera system is in the theatre arena.

Your other point re. your “ mains tripping out”. In the case that it is not the actual MAIN CB that is tripping but an individual lower current CB, then the only situation that will cause your low current CB’s to actually trip-out in an isolated system is obviously a ‘line to line’ over-current situation. (In the case of a line to earth fault in an isolated situation, it is not normally possible for a local CB to trip. As already mentioned, only an alert signal is given to the staff, in the form of an alarm, that ‘trouble is brewing’ i.e more than a milliamp or so may be flowing. The very rare exception to this is a double line-to-earth fault, i.e. a fault occurring on both sides of the lines, such that there is actually a low impedance path - or at worst, a short circuit - between the lines via the earth).

In this case, look first to which sockets are generally used to connect the high current-demanding equipment. It could be that there is a current accumulation on the same low-current CB circuit such that with certain equipment connected together, the circuit is sometimes drawing just a little more than the rated amount causing a trip. If it is not always tripping, it may be because they move the equipment around the sockets. A reason for the trip not going when you plug it in could be that it only happens when there is an extra high current call such as say another blood-pump or cooling-mattress compressor, starting up, that causes the trip to actually go. It is unusual, but quite possible, that you actually are experiencing a sensitive circuit breaker instead. If you cannot find any anything that is looking suspicious, ask your maintenance to replace the particular CB for a new one.

In the event that it is the main CB tripping, if your isolation monitor is a modern one (as talked about in the original entry above by Chris) it could of course be due to an overall heavy current demand, though I would feel this (under no-fault conditions) unlikely as it should be generously rated. It could perhaps be control-tripping, as mentioned, due to an over-temperature being detected in the transformer, resulting possibly from an over-current situation or poor ventilation etc.

You may actually find you have an earth leakage (ELCB) circuit breaker (together with? or) instead of a line isolation monitor at the main switch panel in theatres, in which case this may act as the main CB and trip out if earth current increases more than a specific amount (in a normal - non-isolated - application this is around 30mA’s).

Finally, in order to have your camera system plugged inside the theatre and avoid CB trips, see if you can get a dedicated line or two put in either for the other equipment that have high running currents if those equipments can stay in one place, or for your camera system. If that is not possible, ask the staff to use the same sockets for the ‘heavy’ equipment and work out an equipment distribution plan for the other items. In any case try to avoid the situation you seem to currently have i.e only have equipment in the patient environment that are plugged into the dedicated isolated theatre sockets.

Is it just the one monitor that causes the problem? Or all of the type? Our Sony monitors have a few M resistor (maybe 5 but I can't remember) between the live circuit and the earth. You should be able to detect this on an insulation test. Don't see why this should be a problem though.

Nick
Thanks for the information. I have just moved into a brand new hospital and it is the first time I have come across these isolated supplies in a theatre environment. The old hospital was wired with fused supplies, nails and bits of wet string.
In each theatre we have 4 seperate circuits wired back to a MCB (type B)which are tagged onto the Isolated Power Supply. Thera are also non isolated outlets for plugging in X-ray viewers, cleaning equipment etc. If there is a standard that says that isolated and non-isolated outlets should not be mixed then please let me know.
On investigation it has been proved that isolating transformers on brand new and existing equipment racks can cause the MCB to trip.
I have spoken to another hospital (same PFI as this one) and they have had similar problems although I do not know what type of MCB's were fitted there. They have even had situations where equipment with toroidal transformers have tripped the MCB and they have had problems in CCU as well as in theatre.
I have spoken to one manufacturer who is aware that this is becoming a more common problem and they are going to fit soft start isolating transformers to their equipment trolleys.
The problem at the other hospital was solved by fitting a different type of MCB (I think type C which can handle higher inrush currents))and I assume we will follow the same route.
At the end of the day it may well prove an inappropiate choice of MCB and this could be a problem at many hospitals.

Ken, Unfortunately I do not have access to up-to-date rules and regs. concerning Isolation systems in England/Europe. It seems it is actually a controversial issue in some other parts and in Canada, for one, they have been discussing (on an informal basis?) whether having them, for the little benefit offered, really warrants the cost of them, both capital and running expenses. Probably because the original reason for them (avoiding explosion) has all but gone now. Since it cannot be relied upon to help in micro-shock (leakage current) reduction, this leaves nothing much of advantage, except macro-current reduction. If one does use flammable agents then the reduction in macro current is a plus (spark prevention). Also in the same fault scenario i.e. in the event that a machine does actually go live to ground internally say, a whole power circuit, with its associated equipment connected to it, does not get switched off as it would if the same thing happened in a non-isolated area. In this case, only the offending piece of kit needs to be pulled out of the wall socket when the alarm goes off. Very convenient in theatre situations.

As to your new theatres. I would expect the NHS to have fitted out their "electricals' commensurate with the latest guidelines set up for isolated areas. As the new theatres have come with (hopefully a restricted?) mixture of supplies, then it would appear to be acceptable. Not working either in England or in the NHS I do not know if they now insist on dedicated (alternative socket / pin arrangement) power sockets for floor cleaners and portable x-ray units. Floor cleaners being heavy-current "dudes' and not used, for obvious reasons, during procedures, then it would make sense to have these powered separately using a non-isolated supply. Additionally, if different sockets are used to the normal type, they could not have medical equipment plugged in by mistake. In the case of the x-ray viewers, I would imagine that the inductive circuits of these florescent lights cause unwanted extra leakage current-to-earth in isolated systems. If the x-ray viewer uses the same type of sockets as the other medical equipment in the theatre (even though the associated socket is likely to always have the viewer plugged into it) you perhaps should advise the staff, on the grounds of safety, not to plug medical equipment into it/them unless they are absolutely desperate for a socket. Presumably, all these non-isolated sockets are labeled as such? If however they are not marked, it would be a good idea to mark them clearly.

I would be interested to know if anything else is allowed, officially, to be connected to your non-isolated sockets, besides these two items. Hopefully the staff are restricted in what they are allowed to plug into which power system??

Yes it does sound as if there are a few MCB's that need re-assessment in the area of delayed trip time as it looks, judging by the amount of adverse findings noted in this forum alone, that isolation systems are not reliable, which should definitely not be the case.

If, in your hunt to sort out your particular problems you come up with any interesting info/references on rules and regs of isolated systems. I, for one, would be interested to know too, thanks.

Ken

Re your statement "If there is a standard that says that isolated and non-isolated outlets should not be mixed then please let me know."

The IEE Special Locations Guidance Note 7, chapter 10, page 65, says "Socket0outlets of other systems (TN-S or TT) within a Group 2 location should be clearly and permanently marked to distinguish them from the IT system sockets outlets"

We have done exactly this within our newly refurbished NICU, where the sokets are labeled for non-medical use e.g. cleaning equipment etc..
Isolated sockets are then labeled medical equipment use only as cleaner equipment would cause unwanted tripping.

Mark

Joe,
The unit was eventually checked without any BNC’s connected. Still the fault was apparent.
Mark,
You are correct; these sockets should each individually be clearly identified.

WOM, Ken, Nick, Ged, Grahame and Roy
The eventual resolution was for the company have offered two solutions to the problem.
1) To replace the 14-inch monitor with a 21 inch one. Is seems the 21 inch monitor did not have this problem.
2) To fit an isolation transformer to the trolley, therefore being able to use the current 14-inch monitor.

During a visit by the Starkstrom engineer several weeks ago (Who I must add was extremely helpful) came up with a probable theory.
If you recall my initial posting
“The device installed at the intake (Consumer unit) is an IPSM002-MB.
What this device does is monitor the insulation resistance of the unearthed AC network, mainly found in group 2 medical locations.
The current temperature of the isolation transformer is permanently measured. It is in the form of a DIN rail mounted device.
Its function is to measure DC voltage that is superimposed on the network”

His theory is that this particular model of Sony monitor emits a DC voltage back onto the line. This in itself interferes with the Isolated mains monitor system (IPSM002-MB) installed by his company?
This theory seems to ring true, and we proved it by using an isolation transformer fed from the theatres supply with the 14inch Sony monitor without any hindrance.
However this kind of defeats the principle of having this ITPS monitor (IPSM002-MB) power system installed in the first place eh?
Using an isolation transformer to isolate it from the Isolated (IPSM002-MB)Power system ?

Chris-H

P.S I did post an earlier reply on this subject, however I believe it was at around the time the server was backed up by HUW following the crash.

A bit late for the answer but I think the offending problem lies in the de-gaussing circuitry of the Sony monitor. When switched on, it initially will degauss the screen, this normally involves a PTC via the ac supply and coils around the back of the CRT. This is normal for this monitor and have seen this when safety testing.

On the IPS system issue we have a problem with the system tripping with an Isolation TX on a stack system. (high inrush current probable cause)

My question/s is this, can a stack system with a leakage current over IEC601 guidelines without an ISO TX fitted be used on the IPS system?
What prevenatative measures have or can be put in place to prevent use of the stack on non isolated IPS supplies?

Appreciate any views on the subject!

All will be well when modern flat-screen monitors become the norm for surgical video stack systems!

Font

I am afraid the answer to your interesting 'mind-teaser" query about a stack or, for that matter, any piece of medical equipment that has a leakage current higher than that allowed, has to remain a definite "no it can not". Even if on the IPS, and it is almost certain to be so, it reads well below any danger level of leakage current without the isolation transformer present. Immediately though, there is one obvious very good reason why it should not be allowed to be a 'yes' anyway, and that is purely due to the possibility that the stack could be wheeled/moved into another area that is not isolated, and then it would be in violation of safety standards if the Biomeds were not informed in time to adapt it, or worse, not informed at all.

However, there is a more subtle scenario that needs to be examined. The answer lies in a scenario where there are two different pieces of equipment in an IPS area, each of which has a single-fault condition. (Safety limits for all equipment being based only upon a single fault condition occurring in that piece of equipment under test). Let us say that one piece of equipment in the IPS area has accidentally got one of it’s internal main input power lines (let us say L1), shorted to the chassis due say to insulation breakdown. The important thing to recognize here is that now all the equipment in this IPS area is no longer isolated from the secondary side of the isolation transformer i.e. the two power lines L1 & L2 (live and neutral in a non-isolated system) are now referenced to the area earth and the IPS system has reverted to a non-isolated system.

At this point, this single-fault condition would only be a minor problem as having one power line connected to the area earth is, of course, just like having a neutral connected to the earth. In this case, the only thing that would happen is that the isolation monitor should indicate, by an audible alarm, that it has detected a lower-than-normal impedance between one line and earth. This would not cause the power to be tripped but just be a warning to the operators that something was wrong in the system and it needs checking out ASAP.

(If we were to continue to follow this further, it would become obvious that there now is a potentially very dangerous situation if this insulation break-down is not discovered soon. If the other conductor (L2), either in the same, or any other, piece of equipment in the IPS area has a similar case of insulation break-down, consequently shorting to it's earthed chassis, and since all machine earths are still present (and therefore connected together) in and IPS system, there will be a major line-to-line short and a CB should trip, of course. This is one of the minus marks where IPS systems are concerned).

However, let’s now assume that the staff have heard the alarm produced by L1 in the faulty piece of equipment but decided to mute it and carry on with their procedure, and combine this single-fault scenario on the one machine above with your stack (minus transformer) that we will say has a single-fault condition whereby the main power feeder cable has it’s earth-wire detached inside the mains plug going into the wall socket. (This could be due to the staff always pulling the plug out of a socket by the cable alone). Now attach the stack system to a patient on say an operating table. Due to the excessive (over-the-limit) earth-leakage current in your transformerless stack, we now have a potentially dangerous situation waiting to be exposed. If say the patient’s arm or hand touches a grounded point through say the frame of an earthed bed, or chassis of another piece of equipment nearby to the patient, all the leakage-current from your now non-earthed stack has the potential to pass through the patient to earth via any of these points. Since the system earth is now connected indirectly (via L1 in the other piece of faulty equipment) to the secondary side of the IPS transformer the likely results are now no different to those that would have occurred if this had been a scenario on a non-isolated system, i.e. if it is a cardiac patient on the table then your stack might be mainly responsible for an RIP situation?
Hope this clarifies things.

As to the second part of your query, could you please clarify your question as it is not clear as to why you should wish to prevent the use of stacks in either situation and which type of supply in particular you wish to avoid using the stack ie non-isolated or ISP.

I have worked for over ten years with camera systems and I am glad someone has highlighted this problem .I have found that all makes of medical grade CRT monitors and all sizes (21 & 14 ) causec this problem on CP systems and the problem gets worse as the monitors get older but they all pass EST I have got both Sony and Panisonic involved and they confirmed that there was nothing wrong with their monitors .But nobody could tell me what the leakage sensors should be set at and should they be set with all the equipment in the CP room or not .The awnser I always got was put an isolating transformer on the camera system but to me this is only hidding the problem and not finding the reason also it seems silly to have a VERY expensive CP system and then use isolating traffos on the camera systems .Some people suggested that there could be and resistor beteew the monitors mains traffo and earth and this increase in value as it ages!!!!!!!! anohe reply is CRT monitors are old hat and flat sreens are the new thing but tis dose not awnser the ? what is the value settings for the leakage sensors

Hi Nick,

Appreciate your long answer! My advice before and after introduction of these IPS supplies has been to fit an ISO TX. I was wondering whether anyone had fitted dedicated style plugs/sockets to stack systems/IPS supplies so they could only be plugged into the IPS supply so there would be no need to fit an extra ISO TX. We all know staff like to move these systems around so it would not be practical.

We are going to look at the ISO TX's supplied by Monmouth Surgical that have inrush limiting and integral IEC sockets to see if it improves the situation.

Font, your suggestion about the use of a dedicated plug is a good one.
However, if you sort out the issue of the in-rush current either with a transformer designed to reduce this or, alternatively, replace the CB in the panel (at the source of the IPS power–circuits) relating to the wall-socket where your stack will be connected, with a dedicated less sensitive CB (a much cheaper option if technically agreed to by your hospital electrical engineers), then you will not need to fit a dedicated plug and so allowing the equipment to still leave the IPS area if need be.

John, the contributions from this forum seem to show categorically that in the latest IPS system requirements, both in-rush currents and unexpected DC currents are candidates for mischief-making with the detector circuits. If your displays are giving a dc current into the mains/earth line for whatever technical reason (Meg resistors to ground etc!), then If your IPS system does not use say a (high-frequency?) pulsed-dc to monitor the insulation-to-earth of the mains wires, you will have to isolate this with a transformer to block the dc component. Blocking the external pathway for the display’s dc component will in no way affect the well-being of the display itself but if not blocked is obviously a no-go for safety monitoring in your area. As to in-rush currents, see what Font comes up with through his investigations and that should reduce/eliminate completely this nuisance-factor to the detector circuits and, of course, still allow the equipment to be used outside the IPS area. If you choose a low in-rush type of transformer this will equally prevent CBs potentially tripping in non-isolated areas too in which you might be using your display. Could be a case of “killing two birds (these two issues) with the one stone (ie the transformer)”

Now to what value the “leakage-setting” control (for your sensors) should be set to (as I am assuming it is this setting that you are concerned about and not a setting for the in-rush detection circuits – not that I would be expecting to hear that there was one for this!). Well if the manufacturer has provided you with no information for guidance then the conventional IPS leakage trip is generally set to trip around 2 to 5 milliamps. In lieu of any other value, you can quite happily use this. If your setting-control has an ohms scale then this current-equivalent value would equate (for the low current-trip point of 2mA) to a reading of 115 Kilo-ohms with your mains voltage of 230, and an ohm’s value of 46K for the higher 5mA trip-point For anyone with a mains voltage of 115 volts, the corresponding current readings are halved, of course

With regards to how to set it up, “no worries mate”. Just set the trip to a value within the above range (normally a starting point of 2mA /115K will be fine). As to equipment being connected to wall-sockets in the area when you set this up, you do not need to take this into account at all. However, just remember that the way an IPS works is that the more equipment that is added to the system (ie plugged into the wall sockets in the area at any one time, the lower the overall impedance of the system will be (reduction in impedance being provided by the aggregate of all the equipments resistances and reactances to earth). This means that if, in the rare event, you have so much equipment in the room that the lower trip-point (2mA/115K) is almost reached, when you add another piece of equipment the alarm may sound. Temporarily turn up the control setting to compensate. Obviously if turning it up does not cancel the alarm then you know you have a real over-load from a piece of equipment or just too much equipment connected in that area. Note too that this detection circuit is not protection against our “earth leakage currents”. An IPS is not capable of providing such protection.

This is a very interesting thread to have just read as we are experiencing very similar problems with a famous manufacturer of camera stacks.
On this particular stack it has a new flat screen monitor and the usual items of equipment that the surgeons always complain about. In our hospital this stack is being moved from one theatre to another and is the common link in causing the 32A current trip to trip! It seems to happen predominatley first thing in the morning, but has been known to fail at other times. The stack has a isolating transformer with an in-rush PCB and when we have tested the stack it only pulls 2.5Amps on swith on when the fault occurs. Our test kit is not good enough to see the raising edge of the current spike we think is the problem at switch on. What we are attempting to do is by-pass the manufacurers isolation on the base of the stack and introduce our own RS isolation trans while we await the manufacturer to fix this fault.
Is the problem caused when the system is cold first thing in the morning and the isolation trans acts like a short cicuit, hence the current spike or is it the in-rush PCB?

Double D.

Yes, this is indeed an interesting thread, and worthy of resurrection, surely? Especially as it ties in with recent discussions about theatre isolated power systems, isolation transformers on theatre stack carts, and other such intriguing matters (including, unfortunately, cases of people still using mains extension cables in operating theatres).

Hopefully, DD has things squared away by this time, and so perhaps we shall hear about the final resolution of the problem.

For timid souls who may shy away from Nick's "home-made" approach to LIM testers ... ready made versions are available!

New link.

Other LIM testers may be (and indeed are) available!

If you are serious about testing medical isolated supplies and their associated LIM then take a look at the Metrel MI 3110 http://www.metrel.si/products/electrical...est-im-new.html

There's a lot of nice test kit about these days.

Today's young biomeds don't know how lucky they are!

Any transformer will experience a high inrush current at switch on and the bigger the transformer the bigger the inrush. Almost all isolated supply will be protected by an MCB with a type B rating. Ideally, if large inductive loads are being used, a type C rating would be expected.

The problem is even if the transformer is fitted with in-rush protection this is usually via a NTC (high resistance when cold and low resistance when hot). So once the unit is powered on the NTC has a low resistance and is effectively bypassed. If now the equipment is quickly turned off and on then the in-rush protector resistance will be low so a high current will occur. This can happen when plugging equipment in or out, as the contacts in the plug may make and break rapidly. Remember it can take a few minutes for the NTC to cool fully and restore its effectiveness.

The same applies for most switch mode power supplies so these can also be a new source of problems!

Sounds like a good enough reason for a GFCI at every isolated outlet.

Although I'm now wondering if the real answer isn't RCBO:- "Residual-current Circuit Breaker with Overcurrent protection"*. I'm not sure if these are available for single circuits (individual outlets), however.


I wonder what became of Type A?

1) Source

2) Many others are available.

* An RCBO is a combination of an MCB and RCD and provides both overcurrent protection and earth fault current protection in a a single device.

Things are getting (even more) complicated now Mike!

OK, the in-rush limiter is basically a NTC thermistor. Where does it actually sit in the overall circuit (of the type we're discussing)?

Do we now need yet another component ... an "in-rush limiter cooler"? How about using a Peltier device? ... I'll get on it straight away! Ha, ha.

Maybe in the future we'll be looking at real-time monitoring of individual supply circuits (outlets) - via computer algorithms I guess - with all sorts of protection device options set at various tripping levels (and response times) depending upon specific applications (and, set up by program variables - or maybe just good old DIP switches)! Expensive though (and let's hope they also remember to include fuses as the line of last resort)!

By the way, "in-rush" may well have been the problem here (as well as in many other mysterious - and often "one-off" - faults on high-current (switching) equipment).

Clearly any NTC in this case sits at the input (primary) of the mains isolation transformer (or often any large transformer). It is also possible to perform this function by using a resistor that is bypassed by a relay contact after a time delay. It is also possible to have more sophisticated electronic devices that reduce transformer in-rush.

Please note that it is not allowed to install any such device in the primary of a medical IT system (IPS) transformer!

As for complex monitoring, there are moves by the manufacturers to try and make such individual supply monitoring compulsory for medical locations! The question no one can answer is what is a "normal" monitored level and what is abnormal? If a socket-outlet one day has a residual current of 1mA and the next 2mA is this dangerous or normal? Will such equipment enhance safety or simply drive up costs? Already we have lots of additional IPS units that may be doing nothing but often the infrastructure behind these units is not all it should be. For example I know of cases where all the hospital power went off for hours, even the IPS supplies! The doctors carried on via torchlight!

Good points; and good questions!

Maybe it would be a wiser course to have any complex monitoring incorporated (where need be) into the mains input side of the individual items of equipment themselves.

On your last point:- sounds like (yet) another case of lack of maintenance (which should include regular testing) of the standby generator(s).