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Topic Title: Inspection and Testing, Alternative Mains and Generator Supplies
Topic Summary: Extent of re-testing when LV mains supply is connected?
Created On: 22 January 2019 11:42 AM
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 22 January 2019 11:42 AM
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robertshepherd001

Posts: 12
Joined: 25 July 2008

HI,

Please can you advise regarding the following?

An office building, with circa 500+ occupants has a generator installed, which will supply the building until the LV mains supply is available.

The testing and inspection has been completed with the generator as the only source of supply.

When the LV mains supply is connected, what is the recommended extent of re-testing necessary, considering that the source of supply characteristics will be different when the supply is changed from generator to mains please?

The generator will remain as a back up supply to the building, though will not operate in parallel with the mains supply.

Thanks,

Bob.
 22 January 2019 12:44 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I'd say you'd normally only have to re-check those things that would be adversely affected by the change in characteristics.

Most dead tests (protective conductor continuity, polarity, insulation resistance etc) won't be affected by the supply characteristics anyway - so no need to redo those. Ditto for RCD functional tests.

Presuming that the generator has a lower fault current than the mains, and you've already verified all Zs values are adequate for the generator, then they shouldn't be any worse when connected to the mains, so I'd say no need to redo those either.

Off the top of my head, provided you check the earth loop impedance provided by the mains (Ze) and confirm it's no worse than that of the generator, supply polarity (and phase rotation if applicable) and confirm they match that from the generator, the only thing remaining that I can think of is to (visually) confirm that the breaking capacities of protective devices are adequate for the mains supply (which might necessitate a few loop impedance measurements around the place if the protective devices' breaking capacity is lower than the PFC at the origin).

- Andy.
 22 January 2019 01:31 PM
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gkenyon

Posts: 5354
Joined: 06 May 2002

Originally posted by: AJJewsbury
Ditto for RCD functional tests.
I disagree with the RCD tests - the Means of Earthing has changed. The RCD test has to be carried out downstream of the RCD to the associated protective conductor, so the original test is not valid.



Presuming that the generator has a lower fault current than the mains, and you've already verified all Zs values are adequate for the generator, then they shouldn't be any worse when connected to the mains, so I'd say no need to redo those either.
Again, the means of earthing has changed, so Ze is required - if this was smaller than before, then agreed.

... and ...

All provided, of course, that the switchgear and other electrical equipment were selected and the installation designed for the actual mains supply (I'm thinking in terms of PFC in particular) - which we'd assume from the OP, but it is an assumption.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 22 January 2019 02:25 PM
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robertshepherd001

Posts: 12
Joined: 25 July 2008

Thanks Andy and Graham, I appreciate your replies.

Is there likely to be any detrimental affect to the grading please?

Thanks,

Bob.
 22 January 2019 02:53 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I disagree with the RCD tests - the Means of Earthing has changed. The RCD test has to be carried out downstream of the RCD to the associated protective conductor, so the original test is not valid.

Interesting thought. I hadn't considered an RCD test to be a test of the earthing arrangements - after all c.p.c. continuity should have been proved by a R1+R2 test (or similar) before energisation (in the same way as for non-RCD protected circuits). Indeed there are times when the RCD test is done to the supply N rather than PE (to avoid tripping downstream RCDs) - so I'd be surprised at a formal requirement to test to the c.p.c.. If we've already provided continuity back to the MET (by dead tests) and earthing of the MET via a Ze & PFC tests on the grid supply and can tell from the reading's that the earth impedance is no lower than when using the generator, I'm not seeing how the validity of the RCD tests have been undermined.

Is there likely to be any detrimental affect to the grading please?

If by "grading" you mean the selectivity (discrimination) between different tiers of protective devices, then I suppose it is possible that the increased PFC could upset discrimination (especially if circuit breakers are used - fuses are usually more tolerant). But that's really a design issue rather than an I&T one.

- Andy.
 22 January 2019 02:55 PM
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robertshepherd001

Posts: 12
Joined: 25 July 2008

Thanks again Guys, Really helpful :-)
 22 January 2019 03:15 PM
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robertshepherd001

Posts: 12
Joined: 25 July 2008

Thanks Andy, yup, I meant discrimination. Been dealing with the 11kV side too much ;-)

Bob.
 23 January 2019 11:30 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

(to avoid tripping downstream RCDs)

That, of course, should have read upstream RCDs. Doh.
- Andy.
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