ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades switching generator neutral

Well Kiwi,
I have come up with nothing.
Last night here we were just starting the meeting when all the power went off.
So that was Brian out of here.
The 3 66kV feeders that feed Ashburton all went out, the whole town, Tinwald, Fairton were all knocked out, street-lights and all.
Apparently a faulty cable termination caused it.
As we all know, when you lose a feed, the others tend to go out in sympathy, once the current builds up.
It was a nuisance, but at least Gordon had his Solar system fully charged and we could at least boil the kettle and have some light from his 12V fluoro lamps.

This from an e-mail:
That from Brian.


[This message has been edited by Trumpy (edited 07-15-2006).]

Trumpy I think the issue here is whether the generator neutral can be considered a "Live Conductor". Also the likelihood of circulating currents in the mains neutral if the generator is running while the installation is switched to mains power.

I can't see how switching a generator neutral can compromise the safety of an installation as long as its switched simultaneously with the phase conductors, i.e. a 4 pole switch.

One thing is obvious now though. The ESS and the inspectors don't have a uniform approach to this situation.

btw: Never switch a 3phase N simultaneously, but switch it off after the phases and switch it on before! Otherwise your equipment won't last very long.

BTW Kiwi,
If the generator is under 25kVa,
All you need do is demonstrate that it complies with AS/NZS 3010.

Kiwi,
How did that work out in the end?.
I was researching a thing for a mate of mine to do with generators when I thought of this thread.
He is looking to permanently wire a 4000kVa generator into wiring with remote start and stop wiring.
It was the transfer switching that caused me to bring this topic back up again.
I suggested an Appliance Inlet with cord-set.
I couldn't remember if the Neutral had to be broken or not at the transfer switch.

I will give you a Canadian electrical code interpretation and a thought on what the problem might be from a technical perspective.
The CEC would require a 4 pole transfer switch only if the neutral in the generator is grounded. If the neutral is isolated in the generator then a 3 pole xfer switch is ok.
This has to do with only grounding the neutral (grounded conductor) once. with the frame of the generator isolated from the neutral the current cannot flow to two ground points via the white wires in the transfer switch.

Technically I think the 4 pole transfer switch and a second set of ground electrodes installed for the generator are better for fault impedance reasons and possibly ground fault protection too. If the generator is located close to the distribution then a 3 pole transfer switch makes sense but install the generator several hundred feet away then a fault will see a lot of wire before it returns to the Ground/neutral connection point possibly increasing the damage from a fault. To mitigate any damage to the neutral contacts in the transfer switch a closed transition type is best and allows both grounds to be connected for a brief moment as the transfer switch moves from 1 source to the other.
We see very few 4 pole transfer switches here but almost all generators can have their neutrals isolated. Now if the generator is used for stand alone service and is only ocasionally used as a standby source say in a power failure then the neutral should not get isolated in the generator and 4 pole transfer switches should be installed. We do have some telcom sites that don't have a permanent installed generator. Instead they install a pin and sleave plug and transfer switch so they can trailer a generator when there is a power failure. These generators are also used for portable power in construction. they have a switchable ground connection for both duties IE connected for portable power and disconnected for use as standby power in a permanent location. If these were not utilities I don't think they could do this. They have a special code when acting as a utility but that is another discussion.

Hi Mikesh. A permanent generator cannot be isolated from ground whereas a trailer mount generator can.

I don't think there is any disadvantage in switching the generator neutral in either situation.

As long as you switch the generator neutral simultaneously with the live and as long as you don't switch the mains neutral.

Kiwi
Perhaps where you live a generator neutral cannot be isolated from the frame but every generator I have installed or inspected in Victoria has had an isolated neutral and connected to 3 pole transfer switches. In my 26 years here I have not seen a 4 pole transfer switch installed. As I said My answer is CEC perspective.
I started out as a motorwinder and the two generators we fixed while i worked there could be isolated but I cannot say if they were permanent or portable use.
I have seen that most Portable generators are bonded to the neutral and if used as a permanently installed generator then would require a 4 pole transfer switch. Now you have me thinking about a 5kw transfer switch and plug for connection to a portable that I hooked up 13 years ago. The generator is seldom located at the house but at the owners workplace. That generator might be bonded to the neutral??

Thanks for that Mikesh. I realise now that portable generators have the star point bonded to the metal frame of the gen. Whereas larger permanently installed gens have a seperately grounded star point.

So only portable generators need a 4 pole isolator ?

Portables are typically used in the cowshed situation I originally asked about.

Thanks again Mikesh. I think I get it now