EBME Forums Forums Maintenance Technical Assistance EN 60601 neutral/earth connection?

Hi All, I really hope you can help.

I work for a transformer manufacturer, and we are currently designing a Medical Isolation Transformer to BS EN 60601. I have been given the dubious task of ensuring compliance with the standard, and I have a question concerning the output connections.

I have been told (and have read on the web), that the output voltage MUST be floating from earth (i.e the output neutral cannot be tied to earth). Is this correct? And where can I find this (rather crucial) information in the 60601 standard?

Any help would be greatly appreciated - thank you.

Of course the output “neutral” (sic) cannot be tied to earth, or it wouldn’t be isolated, now would it? But don’t just take my word for it, Mate - check out the competition, they’re even answering all your queries on-line and f.o.c. See http://reomed.com/mits.htm

There’s some nice background information available also at http://www.bender-de.com/news/downloads/Hospital-Brochure-03-2002-e.pdf

Thanks for the reply Geoff. The Bender info looks useful.

My problem is proving to our design engineers that the output must be floating - ideally by refering them to a particular clause in EN 60601.

I'm ploughing my way through the standard, but for a Mechanical Engineer, deciphering a document written by a committee of Electrical boffins is no easy task!

Hi

As the man said it wouldn't be isolated would it; to put that another way the L pin would be at 230V wrt to E and thus indistinguishable from any other live. If your "design engineers" don't understand that then ask them what they think the transformer is for.

Once upon a time our Estates dept made up some transformers for us and they found it necessary to join N to E - cos "if you didn't you couldn't tell if there was a L-E fault i.e. the cb they had fitted wouldn't trip"! I had to spend a while telling them (starting with there is no "live" or "neutral" just L1 and L2) and I am still not sure they understood even then.

Yes Grahame,

The term galvanic isolation may not mean much to them but a practical "hands-on" demonstration might, eh?

Hi morgan240469,

You haven't mentioned whether the application you intend to use your transformers in is for line insulation monitoring systems, such as those referred to by Geoff in his postings above, or for mains seperation in medical electrical systems, such as those referred to in 60601-1 and 60601-1-1. There are some subtle and important differences between these two applications that I'm not going into here. The point is that there are different standards applicable in each case.

Seeing as you refer to it and I assume it's standalone seperating transformers for use in medical sytems that you refer to; in my opinion, as far as I am aware, the BS EN60601-1 general safety standard for medical electrical equipment is applicable to medical systems, including seperating transformers incorporated into systems, that supply "isolated" mains to medical electrical equipment.

The collateral BS EN60601-1-1 standard for medical electrical equipment gives information on the requirements for distribution of the mains, etc. These two standards should also give normative references to the other applicable standards that should be adhered to in electrical systems and your designers should be fully conversant with, or at least aware of, such as those concerning industrial & consumer goods, mains distribution, transformer manufacture, electrical and mechanical installation, etc.

The mistake that a number of experienced Medical Engineers make regarding seperating (or isolation/isolating) transformers, in my experience, is that they think that mains isolation means that the isolated system no longer has a protective earth connection. Those new to medical electrical systems seem to find it difficult to come to terms with the necessity for medical equipment to operate safely under single fault conditions or the subtle differences between industrial appliance testing and those required for testing medical electrical systems.

Yes, Richard is right (as always). What is the application? Could it be, for example, as a power supply box sitting on the bottom shelf of a “video stack” cart for use in operating theatres? If so, there are already plenty of nice examples about, Mr. Morgan. Has your company really done its marketing, I wonder. What new feature are you hoping to bring to this (already crowded, in medical equipment terms) market. Not low price, I presume? Better tell your boffins that what we really want is a parallel port to USB interface. See this thread .

Morgan, permit me to be a bit basic here to start with. Apart from auto-transformers (where Primary and Secondary coils are physically connected together) power-handling transformers are “isolating” i.e. the primary is completely separate from the secondary. Besides the advantage of separating, to some extent, electrically what happens on either side of the transformer, isolation does ensure, for application in particular parts of the medical environment, that the electrical connection on the primary side between the neutral and earth wires found in non-isolated systems is not present on the secondary side. This isolation, theoretically, separates from earth the two secondary wires. This means that neither of the wires on the output of your transformer will have a “neutral” potential with respect to earth. They will now both be “live” (with respect to earth), call them line 1 and 2 if you like. Remember, a neutral in a non-isolated system is only neutral - ie not “hot” - because it is tied physically to earth. This you can confirm by measuring the voltage (potential-difference) between earth and neutral in a non-isolated system. There generally should be no more than a few hundred milli-volts between them.

So you now might be asking …if most transformers are isolating what makes the difference to us and why not just use any normal (isolating) transformer? Well the answer lies in the "degree of isolation' from the hospital earth. Extra isolation has to be designed into a medical-grade isolation transformer so that we can use it in particular clinical areas, which is the important advantage to us in the medical field where we need to keep earth-currents extra-low on patient-oriented equipment. Although I said that “theoretically” in an isolated system there is no electrical connection between the two "line' wires (on the secondary side of the isolation transformer) and the earth wire (also on the secondary side – yes we still use and need an earth connection here) that goes back from the secondary side power-socket outlet system to the main earth on the non-isolated system (on the primary side of your isolating transformer), there is still a certain amount of coupling between them. This is provided by “stray-capacitances” all over the place. So when you design your transformer for the medical environment, in order to keep earth-currents low you need to make sure that the stray capacitances between the wires and the core of the transformer are kept to the barest minimum. The reason for this is that the customer's environment will inevitably have a considerable amount of additional stray-capacitance that will be added to any inherent capacitance found in the design of your transformer. All this will reduce the effectiveness of the overall isolation and hence electrical safety for patients.

If you measure the potential difference between each line and earth (when the transformer is installed at the customer's site), you might see that each line to earth may well be around half the normal live-to-neutral reading (e.g. 230 volts) you have on the primary (non-isolated) side, i.e say a reading of around 110 volts. But equally, this reading may be totally different as it is only a “floating” voltage now since there is no hard connection to earth on the secondary side and no such thing as ideal isolation. The resulting voltages will completely depend upon the relative capacitances between each line conductor and the system earth.

Lastly, because of uncontrollable build-up of capacitance in critical areas of the hospital as more equipment is added to power sockets on the output of your transformer in the medical working environment, an isolating transformer can not provide electrical safety for us from “Micro-(amp) shock” hazard (only Macro-shock – live wire touches earth for example) but it does keep the overall leakage currents that we are concerned about lower than if we did not use the transformer.

Medical-grade isolation transformers were originally put into areas such as theatres in order to minimize the risk of an explosion of anaesthetic gases due to say an electrical spark created in the event of an equipment malfunction.

I hope this back-ground information clarifies things for you.

Read some of my earlier postings for extra information.

Hi All, thanks for the replies. A few things are now a lot clearer.

I was concerned with some the comments stating that connection of the secondary 'low' output to earth (to create a grounded neutral), meant that the transformer was no longer an isolating transformer.

My understanding (and interpretation of EN 61558) is that isolation (or separation) transformers are so called because of the galvanic isolation between the primary and secondary windings, and that this is irrespective of output earth bonding.

The company I work for supply standard Isolation Transformers (for the UK) with the output tied to earth (unless the customer specifically requests a 'floating' output). This is because many IT (computer IT - not earthing IT) applications require a neutral line which is at zero potential (wrt earth).

I know that for an IT Isolated Power System (IPS), the isolation transformer must be ungrounded as clearly stated in BS7671 and referenced in IEC 60364-7-710 Annex to MEIGaN.

And (thanks to Nick et al), I now appreciate that grounding the secondary 'low' output, means that an isolation transformer is no longer suitable for Medical applications. And I believe (my understanding is getting stretched now!) that this is because the Earth Leakage Current must be kept to a minimum for patient protection.

But I still can't find it written in the 60601 standard that the output from a isolating supply transformer must be floating (that would make my life so much easier!!).

BTW - has anybody experienced any problems attaching PCs etc to an ungrounded Medical Isolating Transformer?

Thanks again for all your comments.

Morgan,

Actually galvanic isolation (output to input) is essential to enable the seperation of the "floating" output winding on the secondary from earth-referenced input-windings on the primary of medical isolating transformers. Thus the output is floating wrt earth.

The idea is to provide a mains output that is no longer directly referenced to earth, i.e. neutral is no longer connected to earth and line is therefore no longer referenced to it. This gives no direct and much reduced capacitive return path to leakage currents via earth reducing the maginitude of earth leakage and enclosure leakage currents under normal (N) and Single fault condition (SFC) for example.

When I commented that the earth connection still exists I meant that the chassis, etc, still has a protective earth tied to the non-isolated (earth-referenced/connected) input side of the seperating transformer for electrical safety purposes.

The transformer primary side still needs to be protectively earthed and is earth-referenced if the device is class I but the mains output is "floating". As I said earlier even experienced medical engineers have problems with the concept that a device such as an isolating transformer can still have an enclosure that's earthed.

Perhaps someone at your company needs to buy-in the services of an expert in the design of medical systems.