EBME Forums Forums Maintenance Technical Assistance Isolated power systems in Group 2 locations

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!