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Topic Title: "Power supply system" can be a generator as well?
Topic Summary: Regulation 411.4.2
Created On: 10 March 2017 10:39 AM
Status: Read Only
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 "Power supply system" can be a generator as well?   - Nakhil - 10 March 2017 10:39 AM  
 "Power supply system" can be a generator as well?   - jcm256 - 10 March 2017 02:18 PM  
 Power supply system Can be a generator as well   - mapj1 - 11 March 2017 01:59 PM  
 Power supply system Can be a generator as well   - Nakhil - 12 March 2017 01:21 PM  
 Power supply system Can be a generator as well   - mapj1 - 12 March 2017 03:54 PM  
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 10 March 2017 10:39 AM
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Nakhil

Posts: 45
Joined: 11 April 2014

BS7671:2008+2011 says



411.4.2 The neutral point or the midpoint of the power supply system shall be earthed. If a neutral point or midpoint is not available or not accessible, a line conductor shall be connected to Earth.

Each exposed-conductive-part of the installation shall be connected by a protective conductor to the main earthing terminal of the installation, which shall be connected to the earthed point of the power supply system.


Could the "power supply system" also be a standby generator installed as changeover switched alternative source to public power supply?
 10 March 2017 02:18 PM
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jcm256

Posts: 2300
Joined: 01 April 2006

411.4.2 The neutral point or the midpoint of the power supply system shall be earthed. If a neutral point or midpoint is not available or not accessible, a line conductor shall be connected to Earth.

Yes badly put that paragraph, could cause one to begin to lose the grasp of the meaning. I think what they mean, if a midpoint of the generator supply is not available i.e. (it is not a three phase generator) , then one leg or line of the single phase generator should be connected to earth (i.e. the frame of the generator ) also connect to mother earth by using an electrode with Ra no more than 10/20 ohms.

To the rest of your question :Yes.
 11 March 2017 01:59 PM
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mapj1

Posts: 9363
Joined: 22 July 2004

If you are doing this for real, for example connecting a standby generator to take over from mains, the gen-set needs an N-E bond that is not accidentally switched out while it is still generating, and unless you do it very carefully, you don't want two NE bonds in the system, and certainly not when switching over to a PME supply - so usually this means switching the neutral as well as the phases.
As an aside you don't want to rely on the supply company earth while running on genset - the whole point is that the supply cable may be broken

-------------------------
regards Mike
 12 March 2017 01:21 PM
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Nakhil

Posts: 45
Joined: 11 April 2014

Isn't switching the neutral along with the phases already a requirement of the Regulations?


444.4.7 Transfer of supply
In an installation forming part of a TN system the transfer from one supply to an alternative supply shall be by means of a multipole switching device which switches the line conductors and the neutral conductor, if any.
NOTE: This method prevents electromagnetic fields due to stray currents in the main supply system of an installation.


And if the genset has to have its own earthing (earth electrode, protective conductors, etc). then that would mean the genset's PE conductors would be connected to the same earthing terminal (of the switchboard, for instance) where the DNO's PE conductors are already connected, and so after the changeover the DNO's earthing will remain connected to the earthing terminal while the genset is operating. Is this acceptable? Or is it required, on changeover, to switch the PE as well as the neutral and phases?
 12 March 2017 03:54 PM
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mapj1

Posts: 9363
Joined: 22 July 2004

Agree, and the same goes for PNB and TT also. There are normally no problems having your own metal in the ground that is permanently in parallel with the supply company earth - after all gas and water pipes do it all the time In some countries a PME supply is required to have a local electrode at the point of entry to the building, but the UK does not require this.

In general, breaking CPC connections, except perhaps by plug and socket, as you may on a caravan, where you know all live poles have to break first, is not a good idea, and certainly not if they may be interrupted with the power still on.
My point was really supposed to be that you always need a local electrode to the genset, as during a supply fault, depending what has happened, the supplier mains PE may well be disconnected.

-------------------------
regards Mike
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