If a hospital main switchboard has an emergency generator transfer switch, does this switch have to be a 4 pole on the generator side to isolate the generator neutral ?

The switchboard would have an earth-neutral link ( MEN ).

I'm thinking that the generator neutral should be switched.

Can anyone shed some light ?

kiwi,
Darned good point mate.
Let's see what we have here.
A Hospital would have a Double Redundancy system to ensure that there is no break of supply under any circumstances, not even for a nano-second.
Medical systems are the cream of Electrical system.
As in the Tech's that work on them are trained for that work.
Kieran,
if you want the email of the local Hospital Tech, who is also an Electrician, e-mail me at the above address.
He would be the guy to talk to.
Hope this helps.

BTW Kiwi,
The Local Hospital was Timaru.
Would there be a bond to Earth at the genny itself?.
If there was, I'd say don't switch the Neutral.

If you had a bond at both the generator and the panel, you'd have a parallel path for neutral current on the earthing conductor.

I don't know if large generators like that are bonded to earth.

For reasons similiar to Pauluk's I thought that the generator neutral should be switched.

In a medical situation, shouldn't all practical steps be taken to ensure that the neutral and earth voltage can't float up to a dangerous level ?

Neutrals at a genny switch-over should be switched, simply because the neutral is a current carrying conductor; a fault can put significant voltage on the neutral.

Another thing that hasn't been mentioned here is the fact that most Generators are bonded to Earth through thier fixing bolts.

Hi guys, long time no post ("darn, he's back" I hear some say!).

Let me tell you of a fault I investigated recently; The gen AND panel had N-E bonds. The circulating currents throughout the installation were enormous! (the main current carrying conductors could carry up to 1000A each).

The solution was a single N-E bond in the panel and all Neutral and Earths met at this point.

However, the gen can also run in "augment" mode (supply the national grid with extra juice to stop the voltage & frequency falling too low) but this causes high Neutral currents (harmonics etc.). So in this mode the Neutral to the gen must be switched so as to stop high Neutral currents in the gen.

However, this raises another nasty. If you have a Neutral switch fault (open circuit) while in back-up supply mode, then the voltages that can appear L-N throughout the phases could be anything from subdued to almost 400V (taking 230V norm), and there situations where all three phases can suffer high voltages in this mode (I'm not going to go into that explanation now).

Does this make sense?

M.

Here in Auckland we synchronise generators in on the grid to do maintenance on the 11 kV ring mains units RMU's to have a no break situation for the customers.

This can be anything from 200 to 1000 kVA.

The neutral and earthcables from the generator are connected to the transformer Neutral / Earth bars. The 3 phases are switched through an isolator plus motor operated contactor arrangement via synchronoscope.

In brief:'

Only the phases are switched. The Neutral not.

Thanks Rodalco. After I posted this question I did a bit of reading on this subject and you are right. A seperately derived generator does not need a switched Neutral at the transfer switch if it is connected to a MEN system appropriately. I.E starpoint and metalwork of generator connected to Earth at Main Switcboard

Our European cousins however do things differently and have 4 pole transfer switches.

Thanks Y'all for your help on this. I couldn't get advice like that if I paid for it. Long Live ECN ! !

Kiwi,
We are probably quite lucky down here as far as inter-connecting generators is concerned.
We don't have the problem where if the power goes out in the winter, people just blindly back-feed thier switch-boards, like has occured a time or two in the US.
One question though, is it acceptable to have a seperately derived generator feed into a small part of a house, where that part of the house is fed through a transfer switch?.
I'm talking the generator feeds that section, through an appliance inlet, PDL IP56 job, and the genny is mounted on a concrete pad, with a cage over it, to prevent un-authorised starting of the said generator.
A local guy here wants to be able to keep his 3 freezers going in the event of loss of supply, he keeps deer velvet in them.
Any drop in temperature would ruin the velvet.
He want's to know if this is feasable?.
What I want to know is if I need to drive an earth stake and/or bond the Neutral at the genny frame.
Losing a freezer load of velvet is like losing NZ$100,000, if not more in one shot, it will only tolerate a temperature rise of 5C from -15C, once frozen.
Any ideas??.

Trumpy, a transfer switch should never let the supply be connected to the auxilliary supply.

In N.Z. I don't think its possible to connect a generator to the grid without a transfer switch unless a suicide lead is used.

Not to say that it doesn't happen though !

Kiwi,
Granted mate.
Instead of a transfer switch, I was thinking a pair of contactors, mains failure opens the main circuit to the freezers, closes the secondary supply (12V coil), also having an alarm contact that closes, to alert the owner to start the generator, fed from a 12V SLA cell, kept charged from the usual 230V supply?.

Trumpy, I think a pair of contactors does constitute a transfer switch.

Motorised circuit breakers are a more reliable method though as the contactors require a constantly energised coil which can fail unexpectedly.

The generators we use have motorised circuitbreakers.