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Topic Title: Neutral Earthing in Standby Generator Applications
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Created On: 23 May 2013 11:04 AM
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 23 May 2013 11:04 AM
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clarky280

Posts: 83
Joined: 10 June 2008

Hi,

I am after some confirmation regarding neutral-earthing arrangements in a standby generator application (an old topic I know, but can't find anything that answers this specific question).

We are designing a switchboard that is supplied by a 11kV/400V transformer in normal operation, and changes over to a 400V generator when the mains fails, via an open-transition (no paralleling of mains and generator) transfer. The transformer and 11kV infrastructure is all privately owned and there will be a single independeint earthing system (mat or electrodes or building structure).

The idea is to have only a single neutral-earth connection on the main busbars of the switchboard, and use four-pole circuit breakers where the sources connect to the switchboard. This will ensure that the neutral is not disconnected from earth when the transfer switch opens the mains breaker (hence avoiding problems of a floating neutral), and also ensure that one can fully isolate either the generator or transformer from the system neutral (as they are four-pole devices). The generator and transformer chassis will both be bonded to the buliding earth.

There are two questions with regard to this approach:

* As this is a private mains supply (i.e. the mains feed into the switchboard is not by the utility company), there is no need to provide separate earthing systems for the generator and the transformer. There is only one earthing system. Is this correct?

* During starting up, the generator star point will not be connected to earth. The generator itself is still earthed (via the bonding to the chassis). Will this create problems for the generator during starting... i.e. will it have a suitable reference point?

Thanks.
 31 May 2013 10:58 AM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Hi Clarky,

You are correct that only a single neutral earth link is permitted to be connected at any one time per regulations however I don't understand how you can achieve this with 4-pole circuit breakers and only one N-E link on the main switchboard.

Normally if you are going with 4-pole breakers you will need a neutral-earth connection at BOTH the transformer and the generator. Interlocking prevents both breakers (and therefore neutrals) being closed at the same time. However there's a diagram by Siemens here that states their preference for a common neutral (3-pole breakers) and just a single N-E link at the switchboard.

In answer to your questions:
1. A common earthing system for the LV part is generally the best approach. Common HV/LV earthing is acceptable when the total earth termination network is less that 1 Ohm.
2. Floating star points are best avoided for high powered machines (although very common in ship installations - but that's another matter). Like I said above if you go with 4-pole interlocked breakers you should have a neutral-earth connection at both transformer and genny infeeds.

Regards, Tim

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