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Topic Title: Generator back up supply to new dwelling
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Created On: 23 February 2014 12:04 PM
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 23 February 2014 12:04 PM
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smoke1811

Posts: 18
Joined: 18 December 2010

Hi,

I have read all of the treads on backup supplies that I can find but still have some questions that I hope somebody could help me with. I have detailed the installation and questions below:

Background information
1. The incoming supply is a single phase 100a TNCS supply
2. The incoming supply is situated externally outside the utility room
3. The consumer unit and generator change over switch are proposed to be installed within the utility room
4. The generator is proposed to be used within the detached garage
5. The cable between the generator wall plug (garage) and the change over switch (utility room) is proposed to be 10.0mm 3 core SWA. The cable will be routed up an internal wall (behind plasterwork) and through ceiling void (beam & block ceiling with suspended plasterboard finish)
6. The proposed generator is 9kVA

I have the following queries:

I propose installing an earth electrode at the garage so that if the PME (TNCS) earth is broken the installation will have its own earth, is this correct ?

I have read that the rod is require to have a resistance of lower than 20 ohms & also lower than 200 ohms (I know that BS7671 states for TT systems that lower than 200 ohms is required for stability) do you know the correct figure ?

I propose installing a 100mA time delayed RCD within the garage to meet ADS should the DNO earth be broken, as the resistance to the rod alone would typically be too high to disconnect an MCB protecting the SWA cable to the change over switch should a fault occur on this cable, is this correct ?

I propose installing a 40a MCB with the garage to protect the 10.0mm SWA against over load ( a larger generator could potentially be used in the future), is this correct ?

As the earth rod would be an extraneous conductive part when the TNCS system is functioning correctly, I assume that the rod require bonding, Am I right in assuming that the 10.0mm SWA from the utility room to the garage is adequately sized to bond the earth rod ?

Do I need to install a neutral/earth link ? if so where would be best to install this link ? Would it be in the 63a wall plug underneath the consumer unit within the garage ?


Thank you for any help in advance. I have tried to attach a drawing of the proposed installation

/Users/adrianbrudenell/Desktop/Genny change over.pdf
 23 February 2014 01:10 PM
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alancapon

Posts: 5840
Joined: 27 December 2005

Originally posted by: smoke1811
. . . I propose installing an earth electrode at the garage so that if the PME (TNCS) earth is broken the installation will have its own earth, is this correct ? . . .

Yes.

. . . I have read that the rod is require to have a resistance of lower than 20 ohms & also lower than 200 ohms (I know that BS7671 states for TT systems that lower than 200 ohms is required for stability) do you know the correct figure ? . . .

Why TT? I would expect the generator to be TNS.

. . . I propose installing a 40a MCB with the garage to protect the 10.0mm SWA against over load ( a larger generator could potentially be used in the future), is this correct ? . . .

Can your generator provide enough current to operate a 40A mcb? Can your generator provide enough current to operate all the mcbs you are fitting in the consumer unit?

. . . Do I need to install a neutral/earth link ? . . .

Yes. This will be a requirement of the DNO, and it will need to be accessible to them.

Regards,

Alan.
 23 February 2014 04:25 PM
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anastasis

Posts: 588
Joined: 01 September 2009

Originally posted by: smoke1811
4. The generator is proposed to be used within the detached garage


This isn't electrical, but an alarm bell sounded when I read that. Unless it is designed for indoor use (and would need a professionally installed exhaust), there's a huge risk of carbon monoxide poisoning. I read a report recently (think it was on the HSE web site) about a death from using a generator inside.
 24 February 2014 12:58 PM
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AJJewsbury

Posts: 11784
Joined: 13 August 2003

I have read that the rod is require to have a resistance of lower than 20 ohms & also lower than 200 ohms (I know that BS7671 states for TT systems that lower than 200 ohms is required for stability) do you know the correct figure ?

I suspect you're getting the 200 Ohms figure from BS 7671 for TT a consumer's electrode in TT systems and 20 Ohms from BS 7430 (code of practice for earthing) for source electrodes. On paper the first figure isn't relevant (as it's neither TT not a consumer electrode), so as a source electrode on the face of it 20 Ohms is appropriate. However, that is (to my belief) based on large systems where the consumer might be TT (i.e. there'd be a second electrode in the system), in your setup - almost certainly TN-S where the generator electrode isn't going to form part of the earth loop, but is just to reference the system to earth, the resistance of of a lot less significance. I'd suggest that whatever you can reasonably achieve will be fine - so sub 200 Ohms for stability would be a good.

I propose installing a 100mA time delayed RCD within the garage to meet ADS should the DNO earth be broken, as the resistance to the rod alone would typically be too high to disconnect an MCB protecting the SWA cable to the change over switch should a fault occur on this cable, is this correct ?

Yes and no. Right answer, but for the wrong reason! When running on generator the earth loop will be via metallic c.p.c.s to the N-PE link - no rods involved. It's the generator's inability to produce large fault currents (conventionally we'd see that as a high impedance in the windings) that makes the RCD necessary.

Do double-check the generator's configuration, it's not unknown for larger units to have the N-PE link already in place.

- Andy.
 24 February 2014 09:44 PM
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smoke1811

Posts: 18
Joined: 18 December 2010

Hi,

Thanks for the replies & information. The more I read on this the more I seem to confuse myself.

I have read section 551 in BS7671 & section 2.4 in the onsite guide. Am I right in saying that because the generator will be supplying power to an area where there are extraneous conductive parts and exposed conductive parts that the generator will need reference earthing by means of an earth rod ?

I propose to install a small consumer unit within the garage with a 100mA time delayed RCD and an MCB. MCB to prevent over loading and RCD to provide ADS to the SWA cable between this consumer unit and the main consumer unit in the house. Underneath this unit would be a 63a 230v wall plug, for connecting the generator. Would it be acceptable to connect the CPC from the earth rod into the consumer unit along with the CPC from the 230v 63a wall plug and the CPC & armourings of the SWA cable to the main house consumer unit ?

Would somebody be kind enough to explain the need for the Neutral earth link and where it would be installed.

Thanks,
Adrian
 25 February 2014 09:54 AM
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AJJewsbury

Posts: 11784
Joined: 13 August 2003

Would somebody be kind enough to explain the need for the Neutral earth link and where it would be installed.

The system needs a N-PE link so that L-PE faults create a short circuit and allow automatic disconnection to happen.

I did a couple of not-very-pretty pictures for this thread: http://www.theiet.org/Forums/f...tid=205&threadid=55987 - I don't know if that helps at all.

- Andy.
 27 February 2014 07:44 PM
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TVRBryan

Posts: 6
Joined: 19 February 2014

Just a gentle word of warning here, I strongly suspect you will be required to run this by the network operator in your area (i.e. UK Power Networks). They may have there own specific requirements & in my experience they can get extremely twitchy over people installing back up generator supplies, the danger being that a back feed could cause someone to be electrocuted working on a network cable which is thought to be dead
 01 March 2014 07:45 AM
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SherlockOhms

Posts: 332
Joined: 05 April 2011

Fit one of these.

">http://www.justgenerat...o.u....._Switch.htm


This has the N-E link hard wired internally. It's simply an earth lead crimped and fitted in with the N of the incomer from the genny at the RCD terminals. Mark the output plug at the genny as N-E linked.

From the transfer box run an earth to the house MET. Obviously check all bonding is sound.

Earth rod at the genny (better still outside under a tree ) to connect the frame.

Done.

Edited: 01 March 2014 at 09:26 AM by SherlockOhms
 02 June 2014 05:38 PM
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paulhumph

Posts: 3
Joined: 28 February 2014

Hi,

Can anyone suggest what test certificate you would issue for this set up, and what you would put in the cert for supply characteristics and particulars of origin.

Cheers
 02 June 2014 06:06 PM
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Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Well, you have two sources of supply - the PME service from the DNO and the Gen set

So you need to provide additional pages to the EIC to define the characteristics of the gen set - ie voltage, frequency, rating, earthing characteristics, primary overcurrent protection, etc etc - exactly as you would for the "mains" in fact

Regards

OMS

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