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Topic Title: Portable generator earthing for domestic property
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Created On: 04 December 2017 05:01 PM
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 06 December 2017 08:55 AM
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gkenyon

Posts: 4981
Joined: 06 May 2002

Originally posted by: sparkingchip

Quite, supplying one double insulated hand tool from a portable generator is not a comparison to be made.



I am picturing this as a lone rural property.



I will suggest a complete disconnection from the DNO cable is appropriate to avoid being connected to a faulty distribution network, reducing the risks to both users of the installation and DNO workers when the generator is in use.
The IET CoP for Electrical Energy Storage Systems certainly recommends disconnecting all live conductors. Will prevent "feed-back" of voltage into the DNO network for the safety of people working on it.

However, this CoP recommends remaining connected to the supplier's means of earthing. This is because, most of the time, benefits of leaving it connected (improved system earthing) outweigh the risks. There are certainly benefits where the means of generation operates in parallel with the supply in addition to a switched alternative (as with Electrical Energy Storage Systems).

But of course, you are free to do your own risk assessment on this for particular properties. And, as you say, the opinion on a single property in the middle of nowhere may not be the same as a property on a housing estate in a town.



Then spiking the installation so that when the generator is in use the earthing system is TNS referenced to earth.
Yes, separate means of earthing is required by BS 7671 where ADS is used, regardless of whether you choose to let the supplier's means of earthing remain connected.



Aiming for a Ra of 20 ohms or less would appear to be good engineering practice particularly if the spike is to be permanently connected.
That's what BS 7430 recommends. Alternatively, depending on your risk assessment, where the total site generation is small, use the guidance in the CoP for Electrical Energy Storage Systems based on the differential current rating of the RCD at the generator.

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

Web-Site: www.gkenyontech.com
 06 December 2017 09:45 AM
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davezawadi

Posts: 3954
Joined: 26 June 2002

Actually sparkingchip you need to think through the switching the means of earthing. You have a potential for the switch to carry a large current if the DNO neutral fails and there is bonding in place, or perhaps you want to put the bonding after the switch. Unfortunately this is potentially worse as you could use a smallish switch to connect a large fault current which would be very dangerous. There really is no need to switch the earth and so you should not do it, a suitable fail safe switching arrangement will cost more than the generator. Switching just the live conductors makes nothing less safe than without the switch, and has the advantage of being fairly simple so that the next electrician can understand it!

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 06 December 2017 10:32 AM
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mapj1

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Joined: 22 July 2004

'd agree that nearly always, the sensible thing to do is to leave all earths connected, just switch lives and neutral, and in any case of doubt just do that, and certainly in the farmhouse gen-set situation that is what I would do..

However, at the risk of muddying the water, there is a problem in that the regs don't really distinguish between bonding to things that may carry a full load current, where the effective loop impedance is very low and the PSSC is potentiality dangerous to the wiring - a water pipe shared with another property with PME from the same megawatt substation for example, and earthed objects where the current is limited - such as a bond to a short length of oil pipe going to an external tank in the yard.
It may be heretical to say so, and certainly goes against current regs, but there is a safety argument that can be made for having a deliberate weak link in the earthing (I'm skirting around using the word 'fuse') to minimise damage in those few cases where the diverted currents may be many kA. If something is going to let go with a bang anyway, it is sometimes best to at least be in control of where.

-------------------------
regards Mike
 06 December 2017 11:21 AM
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gkenyon

Posts: 4981
Joined: 06 May 2002

"Fusing" bits of the protective earthing system I don't think would help - would just increase the currents in other parts of the earthing system, until you were left with just a perhaps lethal touch-potential somewhere (or in a number of places).

Monitoring the currents in the main earthing conductor, and disconnecting the public supply live conductors (perhaps even before the PE is branched off the PEN would help at least one installation, but this reduces the "fortuitous earthing" of the PEN) - but there's the problem of identifying what is "problem current" and what is "normal current" as some current travels through E in "normal service" in PME systems - nuisance-tripping galore unless we get it right.

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

Web-Site: www.gkenyontech.com
 06 December 2017 01:45 PM
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sparkingchip

Posts: 10178
Joined: 18 January 2003

Originally posted by: davezawadi

No Andy and Graham, he is not going to get either of those, and doesn't need to try. The RCD is perfectly adequate even with a lost neutral and accidental contact, just like TT (but not) with 1-200 ohms is fine. You do try to over complicate things sometimes, BS7430 is not really the right document for a domestic small generator. As long as the earth rod provides some current diversion the RCD will operate normally, even if the ground potential is higher than one may like. We are not trying to prevent a tingle, which is safe enough, just not a long and big jolt. Remember all this discussion is looking at a very unlikely issue with a DNO problem. I am more concerned that the install probably does not have bonding if required, and that the installer is fairly competent.



Regards


It's a bit of a leap from not needing a low resistance earth electrode, to the switch might blow off the wall due to diverted currents if we try to switch the main earth out with the DNO distribution system in fault condition and the generator is brought into use.

Andy B.
 06 December 2017 02:33 PM
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AJJewsbury

Posts: 16095
Joined: 13 August 2003

Switching the supplier's earth connection is probably a futile gesture in many cases anyway - as any extraneous-conductive-parts shared with a neighbour will ensure a solid connection with a supply CNE via main bonding even with the switch open.

We probably shouldn't just assume that the whole system will be TN-S when running off the generator either - certainly it would normally be for the building that the generator is supplying; but it's increasingly common to TT outbuildings (and EV charge points etc) these days - which will normally remain TT even when running off the generator unless something very special is done (like running an additional c.p.c. out to the outbuilding and switching it to the MET when the generator is running). There might even be several outbuildings each forming their own TT island. In such cases the generator's electrode would carry both earth fault currents and standing leakage currents from those TT islands - and have a corresponding p.d. between the MET and true earth. While I agree that in most cases an Rb of <200 Ohms shouldn't be a problem - there may be the occasional case (e.g. a farm with many TT'd outbuilding, high leakage current and high rated RCDs) where a bit more careful calculation might be needed.

- Andy.
 06 December 2017 04:25 PM
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sparkingchip

Posts: 10178
Joined: 18 January 2003

Can you TT a domestic generator, if you can is the maximum Ra still 20 ohms?

Maybe those remote buildings require a hardwired main earth conductor back to the generator MET?

Andy B.
 06 December 2017 04:56 PM
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OMS

Posts: 22428
Joined: 23 March 2004

As a generator, you need the neutral reliably at or around earth potential - which usually means a solid connection to an electrode less than 21 Ohms

The alternative if big earth fault currents are an issue (in reality they won't be) is to make the system an IT based arrangements and use a resistor (or reactor) to connect to earth - it would also give advantages under first fault conditions

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 06 December 2017 11:07 PM
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sparkingchip

Posts: 10178
Joined: 18 January 2003

Originally posted by: mapj1

I have read that article a few times, and have not spotted that the image on the front page is not a generator. Now you point it out, it is actually pretty clear it is one of these

As you say, not the right object. Mind you I am reminded me of an imported paperback 'clone' of Agatha Christie's "Body in the Library" with a picture of a public library, and horn rimmed librarian,rather than the reading room of a stately home with comfy chairs that it should have been, on the cover.

So the IET are in good company alongside the cheap copy book printers. Makes you wonder what else is not fully checked.


There is of course the water pump further on in the article, just to add a bit more variety.
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