Voltage on an isolated installation 1

[AusLee]

Hello,

I refer to a hospital istallation isolated using a 1:1 transformer. I do know know if it should be called IT or II...

During the testing phase, i touched either terminals of a "single phase" socket outlet and i did not get a shock. I used a "test" screwdriver on both terminals of the socket outlet and they both lit. So far no problem.

The only thing i did not measure was the voltage between one terminal and the earthing terminal provided for the medical equipment. Now i measure it and i find 150 volts between either of the two terminals and the earth.

In another room with a similar installation, the voltage reaches 190 V.

I have someone telling me: "i can hook a device between the 'phase' and the 'earth'". I told him you can also do the same in a normal TN instllation, you have difference of potential but no significant current will flow because of the discontinutity between the 'earth' and the second terminal. Actually the earth may be taken at a central point from the 1:1 transformer, but i am not sure.

Could you please tell me if these measurements are ok in such installations?

 

[itsmoked]

I would think any floating non-earth referenced conductor in a single phase situation would be BAD. Especially in a hospital setting!

While you may not get any significant current it may still be enough to kill someone.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith.
I designed an installation for a small hospital once. My first task was to identify, locate and purchase copies of the applicable (Canadian) codes.
There are situations where circuits are fed from an isolation transformer and are not grounded. The size is generally less than 1 KVA to minimize capacitive leakage. Monitoring devices may be used to verify that the maximum current to ground from either leg is below a very low threshold.
I am sorry that I can't quote the standards but they are in my other office, a very long way away.
AusLee;
What may be more important than the voltage may be the maximum current to ground. I suspect that your voltage reading may change if you use a voltmeter with a different resistance.
What is the line to line voltage?
respectfully

 

[Skogsgurra]

This is the standard method to supply power to medical equipment. It is not classed as IT or TT because it is not a complete wiring system for a building.

There are strict rules for the leakage current allowed and the risk of killing someone with the feable current is nil, even if current goes directly through heart or brain of patient.

I had a case with a surgical table recently where the leakage current limit was 50 uA. That was tough. And when we had it all right and transported it to the customer (not the hospital, the people that produce these tables) we had hundreds of microamperes. Sigh.

We could not get it right. Understood nada. Phone calls, lunch (always helps) and then it was OK again. The equipment had been standing on the truck overnightr. It was a cold and damp night so it had absorbed enough moisture to let current through like a sieve. Conditioning in room temperature for a few hours remedied it. That is also in the pertinent standard "After x hours conditioning" - but, who reads standards?

http://www.antrimtransformers.com/catalogue.php?sec=148

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Thanks for the info bill. I was under the impression the equipment was now required to provide the isolation not the power wiring. Thanks for the heads up.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

Isolated power systems in hospitals are something that has come and mostly gone, at least where I worked when I was doing that type of work. The isolated system had its heyday when flammable anesthesia was being used. The isolated system had various inherent liabilities and required constant monitoring and regular testing of the monitoring equipment. As non-flammable anesthesia came into wide spread use the hospitals couldn't get rid of the isolation systems fast enough. Now a modern OR has way more electrical capacity than an isolated power system could supply.

 

[AusLee]

Hello,

The installation is as you said, i have a monitor module on the door, it is set to 1.5 mA. The line to line voltage is 230 V.

The "earthing" of the installation is made via a mesh wire grid installed on the floor and on top of which they put a thin layer of something that looks like tiles.

During the test, i stood on a metalic plate with the form of triangle, contact with the ground in 3 points, and measured the isoltation between me on this plate and the "earth" (i think the device sent current through the electrode i had in hand, through the plate, divided by 3 into the the ground and then back to the earthing terminal. The result was several hundred kilo-ohms.

I just want to know (make sure) that it is "normal" in this kind of installations to see "large" voltage between any line and the earth.

 

[waross]

You would normally expect to see voltage to ground at one half of line voltage. Your volt meter may be interacting with the monitor module.
There is probably an associated protocol for floor cleaning. As I remember the codes, the floors could not be waxed or treated with anything that may form an insulating layer.
The floor resistance has to be in a range that is high enough to prevent shocks but low enough to discharge static.
Out of time, more comments later.
Respectfully

 

[AusLee]

waross,

Indeed, again as you said. I measure in the other rooms something like 105V to 130V, but one room was 190V which was "alarming". In anyway, it is neither "wrong" nor "dangerous", right?

 

[waross]

Hi;
I must state that my information dates from the early '90s.
At that time there were two issues with operating theatres.
The first issue was explosive mixtures of highly flammable gas. One of my resources was my step father who was the head of the anesthesiology department at the local regional hospital. Over the years he was the first to get up to speed on each new gas as it was introduced and then trained the other doctors. Although the local hospital had not had any incidents, Dr. David mentioned reports of serious accidents at other facilities. There were instances where the anesthesia exploded with injury to the surgical staff and death to the patient. An explosive mixture detonating in the lungs is not usually a surviverable event.
Fortunately these anesthetics were being replaced with safer gases by the early '90s. I sucured a commitment from the hospital that such gases would not be used. It made the design much easier.
The other issue is the reaction of an open wound to the passage of electric current. Apparently the effect of an electric current on the body is quite different depending on whether the current is entering the body through unbroken skin or through an open wound. The standards gave an explanation that a current through unbroken skin so low as to not be felt was sufficient to cause death if it entered an open wound. We installed an isolation transformer with a leakage monitoring device to feed the light over the operating table.
We had some heart monitors that were donated when they did not meet changing North American isolation standards. Fortunately they had back-up batteries and we were able to use them safely on internal battery power.
This was a small isolated hospital in the Moskito coast of Central America. We did the best we could with no budget and no resources and no standards. I think we did quite well considering.
I am in agreement with both David and Skoggs posts.
AusLee;
Normally I would expect the line to ground voltages to be affected by the impedance of the voltage measuring device and sum to less than line to line voltage. If there is some interaction between the monitoring device, the voltage measuring device and/or the transformer and circuit capacitance causing a phase shift, then the line to ground voltages may sum to more than the line to line voltage. At these small currents it doesn't take much to shift a phase.
The actual voltages don't matter that much considering that the recorded voltages may prove a measuring error.
What is important is the leakage current.
By the way, as I remember, the floor resistance measurements were a regular recurring issue with the flammable anesthetics but were not needed with circuits that were isolated for shock protection.
For shock isolation the leakage was measured to the case of the isolating transformer which in turn was connected to the system ground. The isolating transformers included a grounded shield between the primary and secondary windings that was connected to the system ground.
Respectfully