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Topic Title: Wiring up a generator in a domestic home!
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Created On: 06 December 2012 08:54 PM
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 06 December 2012 08:54 PM
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Rum1

Posts: 127
Joined: 13 February 2009

Evening all

I'm sure this has been spoke about before but just wanted some feedback and advice please?

Been invited to carry out a fuse board upgrade. Home owner wants a portable generator as a back up in event of power cuts.

I've only been involved in one of these set ups (a refurb) before so experience is lacking.

I am thinking the best way is to install two new consumer units of which one is a mini RCD main switch consumer unit. This mini consumer unit will say feed one of the lighting circuits and one of the socket circuits (that powers the boiler).
This little CU will be supplied by the mains via a 3 position change over switch (yet to be sourced - recommendations please) (Mains / OFF / Generator).
So in event of a power cut, the generator (Recommendations please)can be wheeled out the shed and plugged into a comando 240V Plug (socket on end of generator lead). Then change over switch can be switched to generator. Fire up generator and off you go!!

All sounds ok to start. But what about earthing etc. I'm sure the old refurb i was involved with years back used the suppliers earth - may even have been connected via change over switch (3 pole) - but the memory is vague on this as it was many years ago. I just fitted new parts to existing.

Can you use the suppliers earth with a small generator? If not how would you supply an earth from a portable generator? - An earth rod to the 3rd pole of a change over switch? Usual procedure please?
Also what about protection - do the generators have built in protective devices (I.e. suppliers have a main cut out fuse) ?

Doing my homework and gaining knowledge before even considering this job! Understand the basics but not familiar with generators & requirements (will only ever deal with domestic back ups though)

Thanks in advice for recommendations, advice and your experience & knowledge
 06 December 2012 09:30 PM
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mikejumper

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I've installed a back up system once before, a long time ago, much along the lines you're proposing.
Domestic premises, TT system with suitably rated changeover switch, commando socket on external wall.
I think the generator had it's own protection system. I don't remember doing any separate earthing for the generator, which was a portable thing on wheels.
It was a noisy generator (not chosen by me) so it might be worth having a look at dB levels in the spec. if it's likely to disturb your customers neighbours.
 06 December 2012 10:41 PM
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Parsley

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I think you might be getting more requests for this sort of thing in the next couple of years.

http://www.cibsejournal.com/ar...an-keep-the-lights-on

Regards
 07 December 2012 01:58 PM
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Delbot321

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Hi - I've carried out test and inspections on loads of installations with generators - some like yours are small nursing homes right up to very large phase synced generator systems with automatic ACB (air circuit breaker) change over system. But I wouldn't portray myself as an expert in the high end area as they are all custom design jobs.

A lot of Fire Stations use the same principle you are using with a small DB with a change over switch supplied from one of their portable site generators they carry on a fire engine. They just feed a few critical circuits like the comms cabinet and a few lights - much like yours.

A lot of the regulations for generators won't apply as you have a manual change over switch so there is no chance of dual supply or back feed on to the distribution network even when the supply returns.

Your suggestion for switching the earth won't work as you will also need to switch the main bonding over which means that if the mains supply should return the installation would then not have the bonding connected. The solution that I have always seen/used is that the earthing is unswitched and installed exactly the same as you would normally therefore you have always maintained you Faraday cage principle with the property regardless of whether you are on mains or generator supplies.

You need to consider what will happen should there be a fault on the cable into the property and there is a loss of the earth as well as the live conductors. The generator should already have a neutral earth link within it so this achieves the TN part or the earthing although you will need to install an earth electrode - and - as there is the same possibility of a supply authority network fault causing fault currents in the earth conductor to the electrode it will need to be at least the same size as your main bonding conductors.

You will need to be able to achieve shock protection on the system which for larger generators can usually be achieved using the overcurrent device but this will depend on the impedance on the generator. As smaller generators often have slightly higher internal impedances I would suggest that you fit an RCD to protect the whole of the generator board to ensure this is achieved.

Whilst you should also consider voltage drop, harmonics, power factor and thermal affects the load you are likely to be supply is probably not going to be sufficient for these to present an issue.

There are only about 4.5 pages of regulations that are specific to this, but you need to consider the usual variety of fault conditions you do for any installation except that you need to consider them for both the normal supply and generator supply.

I hope this helps and doesn't confuse things. I dare say someone on here will tell me why the above is wrong but I look forward to constructive comments.
 07 December 2012 05:58 PM
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Rum1

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Thanks for feed back so far. The earthing system for the house is not a TT system - thinks its TN-S - need to look again.

Hi Delbot

So are you saying an RCD main switch on the mini-consumer unit?

Where would you wire an earth electrode into so its seperate from the mini CU's normal mains supply & earth?
 08 December 2012 10:16 AM
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Fm

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Rum1.

You get earth bars with double disconnects, 1 side is the gen earth the other side is the rec earth the middlemconnections are the earths to the installation.

A 17 mm socket, lets you open and close either one, and chioose to disconnect therec earth, when running on genny mode.
 08 December 2012 06:56 PM
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ericmark

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In the past I have used a simple plug and socket arrangement. From main consumer unit fitted a socket into which the emergency supply consumer unit is plugged into. On power failure just unplug from mains supply and plug into generator instead.

To use change over switches or relays one has to consider time. Any fridge or freezer needs to equalise pressure before restart having to unplug and swap leads increases the time to change over so less likely to cause problems. It also means easy to swap generators as well.

An earth rod becomes an extraneous-conductive-part when the TN supply is connected.

As to generator the price is main point. A wispergen with sterling engine looks great but the price is something else. There are also inverter generators where the engine speed can vary with load. But again expensive. Falklands used mainly Lister powered at 1000 rpm or 1500 rpm which lasted for years the little generator running at 3000 rpm will not last as long. But one hopes it will only be used on odd time so is it really worth loads of money?

As long as the emergency consumer unit is supplied with one plug it can't be plugged into two things so no real need for any special change over switches or relays. Keep it simple.
 11 December 2012 05:31 PM
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AJJewsbury

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3 position change over switch (yet to be sourced - recommendations please) (Mains / OFF / Generator).

There's a selection here: http://www.tlc-direct.co.uk/Ma...r_Switches/index.html

Where would you wire an earth electrode into so its seperate from the mini CU's normal mains supply & earth?

No need to keep it separate. Conceptually you'll have one MET for the whole installation, connect the rod to that. In mains mode the rod doesn't do much - it's just an extra extraneous-conductive-part. In generator mode it references the generator (and hence the installation) to true earth.

Check that the generator really does have a N-PE link - a lot of smaller generators (intended to power hand tools) will be deliberately separated from earth - so you may need to add a link (e.g. in the first generator plug).

Have a search of the forum - I vaguely recall someone posed some links to HSE documents about typical generator setups.

- Andy.
 11 December 2012 05:41 PM
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Inrush

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Don't forget it is recommended in BS 7430 that the electrodes earth resistance should not exceed 20 Ohms for a generator.
 11 December 2012 05:47 PM
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OMS

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What's the earthing arrangement of the existing set up - you may not need to do anything other than provide the changeover switch and the inlet coupler.

Check that N-E link as Andy suggested though

Regards

OMS

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 11 December 2012 10:38 PM
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jcm256

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Below are two bits from the relevant document:

An RCD cannot operate as intended unless one pole of the single-phase electrical system is connected to a reference point so that fault current can return to the generator winding, so causing the RCD to trip.

The possible gains from deliberately earthing a small single-phase system, for correct operation of what is at best supplementary protection, are unlikely to outweigh the safety and economic advantages of using the system as a simple floating system in the first place.



">http://www.hse.gov.uk/...nte.....oc482_2.htm


Edited: 12 December 2012 at 08:44 AM by jcm256
 13 December 2012 07:54 AM
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dickllewellyn

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It's interesting to read these posts, it's something I've often wondered about, but never pursued.

We've seen some great generator set ups on some of the farms we look after, usually involving old lister or old single cylinder diesel generators, and often electricians shouldn't look at the change over procedure for fear of being blinded by the archaic!

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Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 13 December 2012 09:11 AM
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AJJewsbury

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Don't forget it is recommended in BS 7430 that the electrodes earth resistance should not exceed 20 Ohms for a generator.

Does anybody know how that's figure is arrived at? I can see the sense in it where the generator might be part of a TT system and so the rod would form part of the fault loop, but in the case of a local TN-S system where it's only providing an earth reference, why such a low limit?
- Andy.
 04 January 2013 09:34 PM
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KevinH2

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Most small gensets do not have an E-N link so an RCD will not work until two faults are present. If you provide an E-N link at the genset make sure your changeover switch in cludes the neutral changeover to isolate the extra E-N connection or you may trip other RCD protected circuits.

Completely isolated standby systems using independant distribution cords provides physical prevention from overload.

The statement that "you can run anything in the house but not everything" is not understood when you wire a 2kva genset straight into a full house board. When the fridge cuts in pushing the load over and the lights go out.

To plug in the fridge or the jug is then a matter of the number of power outlets avaIable and with small distribution boards having their own overload protection the lighting can be given priority.

I know it sounds obvious but ensure the genset is outside and well veltilated, we even had a death last year cause by a genset running inside a house so the fridge could be plugged in directly.

Kevin H.
 04 January 2013 10:52 PM
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ady1

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Well i'm with Ericmark - keep it simple with your plug and socket idea.
What i dont understand, is why must we worry too much about a genny rod, when the lead will carry the genny earth to the MET which will be miles lower impedance than a rod ?

Regards
Ady

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Resistance is futile.
 06 January 2013 07:22 PM
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jcm256

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The Generator should have the following parts electrically bonded together to form the Generator frame bonding system: the engine frame, the Generator frame, all exposed metal enclosing electrical equipment or wiring, the "earth"terminal of all socket outlets and any residual current device, and the main frame terminal.

If the generator is a three-phase

The Generator will have via the windings neutral or star point which is normally connected to the frame bonding system through a removable connection or link. As a minimum, a Generator should be provided with a circuit breaker with over-current and short circuit protection. If you also wanted to fit a residual current device , the connection between the frame bonding system and the winding neutral or star point is necessary to permit correct operation of the device. However, this device is unsuitable for connection to a fixed MEN (Multiple earthed neutral) connection (PME) (where the neutral and earth are connected together on the load side of the device) because the device would trip on load due to the diversion of current between the neutral and earth connections to the installation. Could, be the same for single phase.

Refer to 551.6.2 for information on TN-S system.

Not 100%sure; but 20 ohms maximum for generator earth electrode, would be the just a continuation of a MEN 20 ohms or mains transformer neutral. Generator stake lower (cough) would carry most of the fault current of a MEN system should a fault occur.

Edited: 07 January 2013 at 06:17 PM by jcm256
 07 January 2013 06:20 PM
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ady1

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Thats interesting jcm
Again though, in a simple set up as Ericmark suggests, wouldn't the loss of Neutral trip the RCD on the generator anyway as it wouldn't then be a balanced load ?

Regards
Ady

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Resistance is futile.
 07 January 2013 09:39 PM
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jcm256

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For independent small scale generators HSE 482/2 states Par 11 states It is unusual for small, single phase generators to have built in RCDs

Did you come across many small scale generators fitted with RCD, of course you could fit one yourself. But for example, would you want your freezer in the garage, security system or fire alarms disabled because of a fault on your lighting circuit or kettle. No I would use the circuit breaker with over-current and short circuit protection that comes with the generator. Besides don't RCDs only go up to 125amps after that you are into big money for 100ma,300mA protection.
Not going into Generators operating on different supply systems well covered in BS767. But one other additional comment in connection with the earth electrode, the same HSE document Par 22 states: In order to obtain the full benefit from an RCD, either one pole of a single phase generator, or the neutral terminal of a 3-phase generator together with the relevant metallic enclosures and protective conductors, should be connected to true earth.

.
 08 January 2013 07:25 PM
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ady1

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Thanks JCM that does seem quite clear, however it still seems applicable also to my 2 scenerios that require a generator.
Basically, A remote house at the end of a supply. Was TT then converted to pme (although Ze a little high), but suffers from regular power failures - probably due to the infrastructure of the little poles falling down !
Customer just wants a genny kept in a vented bunker just outside the house that she can 'swap over' to run some LED lights and a single socket in the kitchen and her boiler controls, as Ericmark suggested.
She has a 3kw genny with rcd and earth and neutal are linked.
(nb. I have a similar one, for use in my house in France - where the power is always going off too)
I feel that this setup does not contravine the HSE regulations youv'e stated. (although i've not seen the document).
A simple lead into the house from the genny to swap over the little separate board still seem very cost efficient.. what do you think ?

Kind regards
Ady

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 08 January 2013 08:32 PM
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jcm256

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Yes, better you have a look at information on Wiring matters rather than more writing from me download Operation of portable Generators.


The paragraph you should look at is for class 1 installations


http://electrical.theiet.org/wiring-matters/2011.cfm

Hope above helps,
Good luck, with your project, I see now that your circuits from your additional consumer unit will be very separate from your main consumer unit when you switch over the changeover switch, you will be ok when you test your consumer RCD with the generator running.

jcm
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