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

... I must admit, Richard, that I was thinking along the same lines – I just didn’t want to be the one to say it!

If you're not convinced: Try performing the safety tests laid down in 60601-1 by testing an isolation transformer with the output winding earth-referenced (N connected to earth) and then perform the tests again for one that has no-earth reference, while both are supplying their rated power into a class I or II medical device. Under normal and SFC conditions with mains polarity forward and reversed you'll see the effect that floating the output has on leakage currents.

The design of seperating transformers is not covered in 60601-1; this standard refers to the requirements for medical electrical equipment and how seperating transformers can be connected to medical systems (60601-1-1). Incidentally, as far as I'm aware, any seperating device becomes medical equipment when it's connected to a medical device manufactured to 60601-1 as explained in 60601-1-1. Hence the combined system must pass medical electrical safety tests.

If the seperating device is intended for the medical market it's probably wise to manufacture it to comply with 60601-1 and 60601-1-1 as well as the other applicable standards for isolating transformer manufacture, e.g. 61558 and other normative standards referenced in 60601-1, 60601-1-1, etc. The requirements for things such as markings, methods of mains distribution, fusing and elecrical safety requirements for medical devices will soon become apparent if you read these standards carefully.

Up to now I have agreed with all that has been said about keeping earth and secondary windings of an isolation transformer seperated. However on a closer reading of EN 60601-1-1 I find a confusing statement in Annex EEE.
'The transformer construction with protectiveley earthed centre tapped secondary winding is allowed, but not required.'
Does not this say that connection between earth and secondary output is allowed?

I don't think anyone has stated that an earth connection to the secondary is not allowed. My concern is with the effectiveness of seperation in limiting leakage current from and between medical electrical devices that are interconnected by functional or protective earth connections and may be referenced by N being connected to earth, otherwise.

The issue of erth-referencing referred to earlier concerns the connection of N to earth. Earth referencing of a centre-tapped secondary may be allowed but it is only probably of use in mains isolation monitoring applicaations (although I'm not certain of this and certainly not an expert).

I think that's a good point and well spotted RICK. Worth having a think about.

Morgan –

Unfortunately, as is often the case, different branches of engineering use the same word but with different meanings. I think the computer cabling applications you refer to relate to signal isolation for purposes such as interference control*. Your original query was, we have all assumed, with regard to power isolation for supply to medical electrical equipment, in which case the purpose and implication of ‘isolation’ is different.

In the latter case, as others have pointed out above, ‘isolated’ implies non-earth-referenced; so don’t tie either side of the secondary to earth through a low impedance (though you would normally still provide the protective earth conductor connection at the output for bonding of accessible parts of the equipment it supplies).

Isolation transformers are principally used for 2 purposes in medical device applications. In an IT-type installation (ie: IPS) supplying a medical area (as in Wiring Regs BS7671 and Guidance Note 7), purpose is to make the supply to critical equipment more reliable (since a first fault-to-earth will not trip circuit protection devices - it merely earths the supply). The other application, (presumably the one you’re wrestling with), is the introduction of a transportable transformer between the wall supply and a particular medical device (or combination of devices) - in which case purpose is to improve electrical safety (specificaly to meet 60601-1 requirements), particularly with respect to earth leakage through the patient. For this application a separation transformer may suffice (see 60601-1-1) – I’m not familiar with the transformer standards but my understanding is that the requirements on primary-secondary insulation are less for ‘separating’ than for ‘isolating’ transformers. (Note that, in this application, both types are commonlly referred to as ‘isolating’ transformers by hospital staff).

These are safety critical applications so your engineers need to be sure they understand what they’re about before proceeding further.

(* signal isolation, or 'separation’ may also be required for safety reasons in medical systems – in which case considerations will differ to those in your computing example)

It seems (and please don't be offended), that there are many "in my opinion.." or "I think.." interpretations, and precious few references to actual applicable clauses in the 60601 standard.

As Rick has found, the standard is not 100% clear on the subject of output earthing, which leaves it open to (mis)interpretation.

I get the impression that seperation of the transformer output from earth is not an actual requirement of 60601 (hence no specific clause), but is an infered requirement necessary to comply with the low earth leakage requirement of the standard.

I will pass all of your much appreciated comments and opinions onto our transformer design engineers, who I'm sure have the competentence and ability to ensure that our unit meets the requirements of 60601.

BTW Geoff - for your USB to parallel adaptor try: http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1219598&CatId=471
http://www.picotech.com/usb_parallel_port.html
http://www.maplin.co.uk/Free_UK_Delivery...aptor_34448.htm

Hi Ged Dean

Thanks for your response, I hadn't read your post before my last comment.

The primary/secondary isolation requirements are indeed greater for a 60601 compliant unit than they are for standard isolation transformers.

It was just the output grounding issue that was worrying me.

Thanks again to all that have commented.

Thanks Mate, but they’re all USB to parallel. What we are looking for is the other way around (ie, the elusive parallel to USB).

Meanwhile, all the hedging you refer to (“in my humble opinion” etc.) arises as (most, at least) of your correspondents do not represent any learned authority, as it were (but are just trying to help, as always). But I doubt that any medical isolation transformer with either output line tied to earth would pass muster (as they say)!

I will leave you with a parting comment, however. The manufacturer’s plate on a medical isolating transformer that I noticed yesterday stated 1000 VA, with a maximum leakage current of 58 uA.

Hi Geoff, I thought I found them a bit too easily!!

I just wanted to give something back for all the help from you guys. How about this tranceiver for bi-directional parallel ports to USB?

http://www.shopping.com/xPF-Bencole_08305

Morgan,

May I ask how you came to this conclusion - where is the degree of "isolation" specified (in the medical standards) and what other standard (RE:isolation transformers) did you make the comparison with?