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Topic Title: Standby Generators and RCBOs
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Created On: 25 April 2013 07:00 PM
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 25 April 2013 07:00 PM
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Avatar for Legh.
Legh

Posts: 3483
Joined: 17 December 2004

We were in the process of making adjustments to the SMP in a giant fridge/freezer when the lights in the kitchen flickered on and off then off. 30 seconds later the lights come back on. Oh dear....
The gardener rushes in and says some bloke called 'digger Pete' has just gone through the DNO cabling (Wavecon 25-35mm2) with his bucket. Ok, now we know why the 'beast' (Stratton Briggs 60kVA generator kicked in).
At this point we thought it would be a good idea to wait until the DNO were called and patched up the cabling and started work on a previous fault on the ground lights outside.
After cleaning up and a few measurements we decided that all should be Ok. However, my man was unable to reset the RCBO circuit to the ground lights.
So off to lunch we went and came back when the guys from the dark side had finished. The power was back on and beast had gone back into hibernation. The RCBO could now be reset.

There could be various possibilities here. AFAIK, the earthing system from the public supply is Ok, or should I say, all ELITs carried out on various circuits in the past had been acceptable but not tested while the standby generator was up and running. There is a problem but not, I think, the standard earth/neutral type problems usually encountered with RCDs, more like incompatibilities when running from different supply systems.

Any thoughts?

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 25 April 2013 07:37 PM
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Avatar for OMS.
OMS

Posts: 19628
Joined: 23 March 2004

How is the generator wound - where is the N-E link and have you tested for a voltage N-E when the generator is operating ?

regards

OMS

-------------------------
Failure is always an option
 25 April 2013 08:11 PM
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jcm256

Posts: 1861
Joined: 01 April 2006

If it was a TT system and you had an earth electrode at the generator connected to generator earthed neutral, the RCBO should work with the clay path between the two electrodes. Not properly earthed generator to the required safety standards,(as above) running on generator supply the RCBO does not recognise the mains earth as a return path.
 26 April 2013 12:00 AM
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Avatar for Legh.
Legh

Posts: 3483
Joined: 17 December 2004

Thanks OMS and jcm256,
Some good questions which i shall pigeon hole for further reference .
I'm not usually called out when supply faults occur or power cuts happen since there is a company who maintains the generator supply side.
When it is running it runs for a short time (8 hours max.) so it is rare to be present when it does kick in.

Next time it does and if I'm on site I'll endevour to measure the voltages across the neutral to earth.

I'm curious to know how the generator windings might affect the residual currents in the RCBO? A little research required I suspect........

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 26 April 2013 10:01 AM
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Avatar for OMS.
OMS

Posts: 19628
Joined: 23 March 2004

You could investigae the location of the generator N-E bond and reserch the winding arrangement at any time.

As an example - I had a 25kVA set at one site that had run happliy for years - no one had noticed however we had a centre tapped winding arrangement that put 110V on each leg -

You could also determine the earthing arrangements - do you have PME present for example and how many poles the changeover provides.

regards

OMS

-------------------------
Failure is always an option
 29 April 2013 12:05 PM
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AJJewsbury

Posts: 11387
Joined: 13 August 2003

I'm curious to know how the generator windings might affect the residual currents in the RCBO?

Is it the kind of RCBO that has a white (cream) lead to earth - many of those are intended to trip if they senses a lost N - likely to be detected by an unusual voltage L-PE or N-PE.
- Andy.
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