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Topic Title: Earthing of HV Generator Cables for Upgrade
Topic Summary: Help with how to earth generator cables without disconnecting them first from the switchboard
Created On: 12 March 2014 07:04 AM
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 12 March 2014 07:04 AM
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pcrichton

Posts: 3
Joined: 06 August 2011

There is a 3.3 kV generator being taken out of service so that the alternator and cooler can be upgraded. For this to happen the HV cables are being disconnected from the alternator main junction box on the skid and pulled back at the field end. My question is related to the best practice for earthing the generator cables before disconnecting them. My thinking was to earth them at the switch board end, we currently don't have schematics for the incomer cubicles for the switchboard. However, I was unsure of how it would be possible to earth the cables from this end without disconnecting them from the board first. As far as I am aware, standard practice would be to rack out the breaker from the generator to the board at the board end. After this the CME would be applied to earth the switchboard bus bars. This would then leave the cables effectively hanging as they are not earthed due to the breaker being open. I did think to not open the incoming breaker first and apply the CME cables once the generator is offline, however I'm almost positive the interlocks would prevent this, for good reason.
 12 March 2014 11:29 AM
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Russell Barnes

Posts: 8
Joined: 25 July 2008

What sort of circuit breaker connects the generator to the HV Switchboard? Is there provision for integral earthing, or have you a separate earth / test truck?

-------------------------

Regds,

Russell W. B.
I. Eng., MIET
 12 March 2014 06:40 PM
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Russell Barnes

Posts: 8
Joined: 25 July 2008

I might also add that you should have a thorough understanding of the switchboard, all that is connected to it, and the implications of 'getting it wrong' before attempting to do what you describe. And when the work is undertaken, it should follow a switching schedule isolating at all points (including fuel, batteries, and / or air on your prime-mover), proving dead, earthing, and the issuing of a Permit-to-Work by an appropriate AP or SAP.

-------------------------

Regds,

Russell W. B.
I. Eng., MIET

Edited: 12 March 2014 at 07:20 PM by Russell Barnes
 13 March 2014 04:04 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

I'm trying to visualise the setup; but essentially you need a "zone-of-work".

Try googling Electricity Networks Association Distribution Safety Rules.

There will be some general guidance in that - but often the rules refer to "approved procedures". If you do not have a procedure, then stop work and employ a suitably qualified SAP to do the work.

Essentially, what you need to do is earth the HV cables at ALL points from where it can be energised. Then, you identify the cables using (again) an approved procedure; then you can work on the cables (using an approved procedure!).

Note that the cables need not remain connected to the circuit main earths whilst you are working on them - but you CANNOT remove the circuit main earths until all the work is complete.


But first things first, get a schematic and verify it.

If you cannot get a schematic, make one.

Without this, you cannot do anything.
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