EBME Forums Forums Maintenance Technical Assistance mixing isolated and grounded power supply in OR

Could I know if there is any safety concern when having the two different power supplies in a OR.
We have a OR that is not frequently use as wet location so the consultant adviced we could supplement the isolated power outlet with grounded power supply.
Would appreciate any feedback.
Thank you.

If for some reason the isolated power supply was floating at a high voltage compared to the grounded supply you could get a shock. I know it has happened to me.
We had video feed from a grounded fluoroscopy system and I was connecting it up to something that was on the floating supply and got a shock.
Robert

Not a Consulting Engineer, hopefully.

And why is "wet location" the issue here? The origins of isolated power systems in ORs was to do with the prevention of fires (or explosions) due to electrical sparks when flammable anaesthetic gases and (or) agents were used. Hopefully, you are no longer using those. Anti-static flooring, and all the rest.

These days, (expensive) isolated power supplies are generally specified primarily on the grounds (no pun intended) of being able to carry on regardless of the LIM blaring out a warning that a high (in our terms) leakage current is being delivered somewhere.

If I remember rightly, wet locations were generally "protected" by a GFCI (although I am aware that RCD and RCCD are terms that are used as well - generally speaking they are "the same" device). In the case of large fixed items like a hydrotherapy tank there may be only one GFCI (or whatever); whilst other rooms may have many (one on each power outlet, for example), and I presume that your pendant situation is like that.

The mixing up of normal (grounded) power and isolated power systems in the the same environment (room) is something that I would never recommend. In fact, personally, I (would) regard it as a dangerous idea ("an accident waiting to happen"), and frankly would have nothing to do with it.

The only proviso I would go along with would be a special (dedicated) grounded outlet for heavy current equipment such as mobile x-ray units, some lasers etc. that may be needed occasionally in the OR - but such outlets would be of a different configuration (ie, 30-A plugs) - and non-interchangeable with the others in the room.

Also, there needs to be a grounded outlet nearby (I would suggest just outside the room in question) for use "out of hours" by cleaning machines, maintenance power tools, and what-have-you.

Even if all outlets are clearly marked ("isolated", "grounded" etc.) this may (will) not mean much to the staff, who are just as likely to be lulled into a false sense of security thinking that all is well as long as the LIM is not sounding.

By the way, I am aware that many new builds have isolated power supplies in "intensive care"-type areas (and sometimes in "dialysis"). Again, I would need to have "deep mulls" about all that, too. For instance:- why? What is the point - non-interruption of supply (why not use UPS)?

As Geoff points out it is perfectly permissible to have isolated (medical IT system) supplies and none isolated supplies (mainly TN-S in the UK but TT also possible). The reason for plugging items into an isolated supply is for first fault resilience. That is when a first fault to earth occurs it will not blow a fuse and cut off the power to vital equipment. If a first fault to earth occurs on a TN-S supply (where the Neutral is referenced to earth) then the fuse will blow and disconnect all equipment connected to that supply.

Medical IT systems are generally automatically connected to a backup power supply (with a maximum switch over time of 0.5 seconds) but is also possible to have an earth referenced (TN-S) supply which have the same backup switching. So a single item of medical equipment connected to single TN-S supply (which is RCD protected) is no less safe than if connected to a medical IT system. If a fault to earth occurs it will not disconnect other items of equipment only the affected device. Since this will almost certainly be due to a catastrophic failure of the device (e.g. a bag of saline poured inside) it is unlikely to work even if on a isolated supply!

One myth about isolated supplies is that they reduce touch voltages, which arise from earth leakage when the PE goes open circuit. However since the medical IT transformer only limits this to 500uA, the same as the medical equipment leakage, it really has not made any difference.

An isolated supply should not "float" to a high voltage since it is referenced to earth by the capacitive coupling in the transformer and is limited to 500uA. That is why on a 220V isolated supply you measure 110V between each phase conductor and earth.

Although now eleven years old, this .pdf may be of interest ... if only in that it spells out some of the terminology that Mike likes to use!

For example:-


Meanwhile, I have always considered NFPA 99 to be the definitive document for all this stuff (and *more), myself.

* It's a very comprehensive (and concise) document; and one that I believe should be found in every biomed's library (regardless of the fact that it is American in origin). If you need the latest edition, you can view it on line (once you have registered) or buy a download here.

Geoff,
If it is not recommended how would you address the power supply for a Hybrid OR where the Angio system are typically grounded but the anesthesia medical pendant outlet are all isolated power supply?

There is no issue with using grounded and isolated supplies in the same location! It is only essential that the right supplies are used for the right load! In other words the Angio X-ray equipment will be supplied from a TN-S (grounded) supply and the pendent socket-outlets will be from the isolated supply, maybe with the addition of a marked socket-outlet (or special type of socket) for a high current supply such as a mobile X-ray system or Laser etc.

As this would be a group 2 medical location (theatre) then the TN-S (grounded) supplies would need to be RCD protected.

Metal case of equipment plugged into isolated outlet on pendant (or wherever) is placed on exposed metal of x-ray table (normally grounded, of course).

What is the status (electrical safety wise) of the equipment now?

Will the LIM sound?

Thinking that they are "doing the right thing" ("belt and braces", and all that) - the staff get carried away with connecting Equipotential Terminals between (for example) a monitor plugged into an IT (isolated) outlet and some other equipment plugged into a TN-S (grounded) outlet.

Any objections here?