Re-connecting gennie


Postby jackhawk » Sat Sep 27, 2008 8:51 pm

Hi,

My mate's dad used to be supplied by his own generator as he was well in the stix. He is now connected to a supplier but want's his gennie reconnected just in case. The 17th edition states that an aux supply must not rely on the supplier's PEN connection. As the gennie has an earth spike, I would say that if the two earthing systems are connected at the earth terminal then the aux system does not rely on the PME. It uses it as a back up only. Notifiable yes, and poss a 100ma RCD main switch in CU if EFL too high. Any thoughts??

Many thanks
Jack
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Postby ericmark » Sun Sep 28, 2008 1:24 pm

This is a personal view and I may be wrong. I would consider where an earth rod is fitted with a PME supply the earth rod is not an earth rod but an extraneous-conductive-part. This means any earth cable will need to be or a size as required by an extraneous-conductive-part which is normally 10mm rather than 6mm in fact I would be more looking at 16mm.
The change over must be so it is impossible to connect both systems together even if contacts stick i.e. if for example a star/delta contactor was used it would need the physical as well as electrical cross link.
It seems we must comply with 17th Edition with anything our alterations supply so I would expect 30ma non delayed RCD would be required.
When measuring the EFL this must be to just the rod and not with everything else connected.
There is a complete section in the 17th Edition about using two supplies with solar panels and like this is becoming more common. I think it’s mainly in Part 5 there are allsorts of rules including what notices need posting. I would need some careful reading to ensure all is correct.
Eric
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Postby jackhawk » Sun Sep 28, 2008 2:15 pm

[quote="ericmark"]This is a personal view and I may be wrong. I would consider where an earth rod is fitted with a PME supply the earth rod is not an earth rod but an extraneous-conductive-part. This means any earth cable will need to be or a size as required by an extraneous-conductive-part which is normally 10mm rather than 6mm in fact I would be more looking at 16mm.
The change over must be so it is impossible to connect both systems together even if contacts stick i.e. if for example a star/delta contactor was used it would need the physical as well as electrical cross link.
It seems we must comply with 17th Edition with anything our alterations supply so I would expect 30ma non delayed RCD would be required.
When measuring the EFL this must be to just the rod and not with everything else connected.
There is a complete section in the 17th Edition about using two supplies with solar panels and like this is becoming more common. I think it’s mainly in Part 5 there are allsorts of rules including what notices need posting. I would need some careful reading to ensure all is correct.
Eric[/quote]

Hi ericmark,

Thanks for your reply.

The earth return for the gennie is 25mm2 and both supplies will be conected via a 100A change over so they can't be conected at the same time. Three settings, mains, all off and aux. All earth conductors will be 16mm2. As his instalation is 10 years old there is no RCDs which I have explained to him he needs, to comply. The only thing I'm not 100% sure on is the use of the PEN earth conductor as well as the earth spike together when the gennie is running. "Must not rely" tells me that I'm not relying on the PEN but only using it as a back up if an earth fault happens. This would make the disconection time within the regs. The gennie has not been tested YET but all thoughts will be taken from the results from the EFL of this supply.

Regards

Jack
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Postby ericmark » Sun Sep 28, 2008 8:31 pm

All seems OK. My son and I were talking about generator earths only the other day forget about mains supply for now if the earth and neutral are linked in the generator then even if the earth rod is completely disconnected then the earth loop impedance will be A1 as connected in the generator. So to ensure the real earth is good enough we can’t use an earth loop impedance tester from the generator supply.
Next question is what reading is good enough he quoted the 200 ohms from 17th Edition but although the earth leakage trip will trip I considered it should be the 21 ohms required outside the consumers installation for a TT supply as one must ensure if for example one is mowing the lawn remote form generator that enough current can travel through the ground to ensure the trip will operate. If we look at worse case scenario with supply cable severed then you are total reliant on the earth rod.
In real terms we see generators running everywhere with no earth rods and unlikely in real terms to cause any problem. But I wonder what any other tester may look for!

I had the argument also in T5 Heathrow over a 750KW generator supplying a tower crane my boss said I could test the earth rod with an earth loop impedance tester and I had a hard job to convince him we had no reference point and although where a supply authority has put in an earth you can compare it to their earth that can’t be done with a generator.

I am sure we needed RCD under BS 7671:1992 413-02-08 Zs < Uo/Ia so I make that at least 16 years and I think it was required before that and we had the old voltage type it yellow button it is a lot more than 10 years.
Eric
ericmark

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