EBME Forums Forums Maintenance Technical Assistance earth to neutral voltage

Dear Sir,
Could any one help us with the acceptable earth to neutral power line voltage requirement for medical equipments?
Regards
Sajeev

The easy (but flippant) answer would have to be zero, Sajeev! Ideally, and for reasons of safety, the protective earth should be physically connected to the neutral bus bar as close to ("behind" the) power outlets as possible (usually at the nearest electrical distribution panel). In a properly engineered power distribution system, the "earth" line is electrically connected to physical earth at various points all the way back to the generator (or power station), where neutral and earth should be the same point.

Obviously, at the socket outlet, and into our equipment, the neutral to earth potential should be "tending towards zero", and as low a potential difference as practically achievable. But, for all intents and purposes, earth and neutral should be "the same".

Back in my days spent "overseas", I have encountered all sorts of lash-ups. The most common being 220 volts taken across two phases of a three-phase supply (OK in itself), but with the so-called earth taken off the centre-tap of a star connection, placing it at around 110 volts or so above the (similarly so-called) "neutral" (it also provided 110 volt supply of course, which was the primary intention, I should imagine). I have also many times seen no earth at all (not good), and earth wires strapped to plumbing with Jubilee clips ("needs must", and not so bad, electrically at least). And lastly, the famous stake driven into the ground, ideally copper, but usually something a little less attractive to gentlemen of the light-fingered variety!

As I may have mentioned before, the wise tech (aka the survivor) always keeps his multimeter at the ready when venturing into such uncharted waters!

As a practical note, I have found that certain electrical safety testers (for medical equipment) refused to pass self-test unless the the earth to neutral potential was (if my memory serves me well ... and it was a long time ago) less than 15 volts.

Dear Sir,
Thanks for your reply.We have purchased a new ultrasound machine and we are planing to connect it to our hospital central UPS, we are currently having an earth to neutral voltage of 6-7 V but the vendor is not willing to install the machine unless we brought down the voltage below 5V ( he is asking for 3-4 v),if we force him for installtion with this electrical power there will not be any warrenty for the machine if there is any electronic part failure.He is explaning that if the voltage is above 5v it will cause noise in the image.did you come across any similar situation?
Tech manual requirement is not mentioning about the neutral to ground voltage but it mentioned about a good medicl grade earthing

Kindly advice

First of all, the very basic question one would ask is will it cause harm to your system based on your vendor's disaproval of the difference in PD.

If they are not then why is the big fuss over it. As with the norm, the PD should be as close to zero.

Hi Sajeev,

From browsing the net, found the tidbit below ...
http://ecmweb.com/power_quality/electric_clearing_neutraltoground_voltage/index.html



That one is the US standard, and if we follow the calculations, for 220V the voltage of 6V will be realistic. Is the ultrasound at 220V ?

I think ECRI recommendation for N-G voltage is less than 4V (kindly correct me if i'm wrong).

What is the recommended maximum N-G voltage as per your local electrical code?

As a last resort, you can get an isolation transformer. If i can remember correctly, with this setup the secondary neutral of the transformer is grounded.

As for the noise in the image, yes, ultrasound machines are susceptible to noise in the line. when i used to work for suppliers, we bundle isolation transformers with our high end machines offers, it made a difference in number of installations.

Yes, the vendor is right. It's not so much about damaging components, as obtaining a decent image. Do you mean to tell us, Sajeev, that your users won't complain about poor image quality, but will just shrug and accept the fact that you've got a lousy earth?

But, is the problem only with the UPS? What's the situation without the UPS? If you get a good earth there, why not look into a dedicated UPS for the ultrasound unit (and running an earth connection "around it" if necessary)?

Meanwhile, the jury's out (as they say) as far as I'm concerned whether an isolation transformer will help. An isolation transformer is essentially intended as a safety device when working with ungrounded equipment. Neither the L1 or L2 outputs from an isolation transformer are tied to earth (or "ground"), so an "experimenter" should be able to safely touch either L1 or L2 when the circuit is live (but, it's best not to try this at home, folks)! It is the core of the transformer that is connected to earth.

It sounds like your electrical engineers need to get a decent earth system sorted out. I suspect that the room in question will not be the only one that needs to be looked at. Does anyone there ever get a decent ECG?

Dear Sir,
Thanks for the reply.
We were getting a neutral to earth voltage of 3v during off peak hours and 6-7v during peak load time.Is it due to harmonics ? or poor conductors or some other issues ? is there any device to protect the main UPS from harmonics?

Geoff at al,
I think the emphasis needs to be on Either and Or.
I knew a "technical" person who thought that, as it was isolated from the mains, you could not get a shock.
I tried explaining potential difference and absolute potential but he was convinced a shock had to go through you to earth for it to be a true shock.
Rather negates the effectiveness of ECT and defibs then......

Robert

Oh yes, either and or were the words used. Naturally, it would not be wise to stick one finger on L1 and the other on L2. You can "safely" assume that 230 volts happily await you there!

Shock versus "true shock"? The mind boggles.

Are we talking about an "ex-technical" person, by the way? Whomever he was, he obviously hadn't been trained at Arborfield!

Hi
I observed an "Electrician" using a copper pipe (AS THE CONDUCTOR) to return a neutral to the Fuse box (days before RCD's where standard)???
When I challenged him he told me "There is only a few volts on the neutral, what's the problem?". To which I handed him a hacksaw and suggested he cut the pipe to find out! lol.

Eddie