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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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 04 December 2017 05:01 PM
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dixonn

Posts: 4
Joined: 07 April 2003

Hello, Im hoping to clarify the requirements for the use of a changeover switch and portable generator at a domestic TN-C wired property prior to the electrician undertaking the works.

A double pole, (single phase) changeover switch is proposed to be installed between the meter and consumer unit. With the portable generator feeding the alternate input to the switch. As the property is rural, fed by overhead lines, the earth appears to be provided by TN-C where the 100A incoming breaker is fitted.

Is there a clear guide to be followed to ensure the earthing is correctly connected? In this case, should an earth rod be fitted and wired to the earth bar? Does the N and E on the generator need linking too?

If anyone can say for sure what is needed that would be helpful, as there is conflicting information, especially around isolation of the DNO supplied earth, whilst running on generator.

Thanks!
 04 December 2017 05:58 PM
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davezawadi

Posts: 3954
Joined: 26 June 2002

Hi Dixonn

Is your supply fed from a close by transformer (which may feed several properties) or from overhead lines for a village for example. What I am trying to establish is whether the supply is actually TT or TNC-S. Supplies can no longer be safely assumed to be TN-S even if an earth connection is supplied by the DNO, and if this comes from the incoming supply DNO box is almost certainly TNC-S. It may come from a separate incoming G/Y earth wire, or from a split concentric type cable but you cannot tell by inspection very easily and a N-E resistance reading with a low ohms meter is needed. You should have a main RCD if the supply is TT, and a connection to an earth electrode. In some circumstances you may find no earth at all or dependence on a water pipe if the property has not had any attention for a long time.

The generator produces a supply which is effectively TN-S, and one side of the output (Neutral) is bonded to the earth system as well. This will enable RCDs to operate correctly when on generator supply, although with a small generator the circuit breakers may not function as inadequate fault current may be available.

Can you check the necessary information and I will give exact instruction as to what is required.

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 04 December 2017 07:14 PM
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dixonn

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Thanks for the reply. The LV substation is ~500m away, and overhead 3P+N lines come to the nearby pole, where a single phase is then used to feed the property via some (recently installed) underground cable. Previously 2 lines came overhead directly to the property. There is no earth rod at the house, currently.
 04 December 2017 10:25 PM
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AJJewsbury

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should an earth rod be fitted and wired to the earth bar? Does the N and E on the generator need linking too?

Yes and yes.

Also consider that overcurrent protective devices (MCBs, fuses) almost certainly won't operate promptly on faults when supplied from a small domestic type generator - as the generator won't be able to provide the hundreds amps needed. So you'll need RCD(s) to provide shock protection under earth fault conditions. All wiring & equipment before the first RCD (looking from the generator) then has to provide shock protection via double or reinforced insulation.

- Andy.
 05 December 2017 07:39 AM
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davezawadi

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OK Dixxon thanks for the data.
Your supply is TNC-S then, and this has one snag, which is that there is a possible fault condition outside your property which could be dangerous. This can be overcome by the use of a suitable Earth electrode, and bonding to incoming metallic services, such as water, gas, and oil from outside tanks. As Andy says above, you also need to organise automatic disconnection of the generator supply in the case of a fault, usually something like the kettle becoming leaky to the element connections. This is done by means of an RCD (Resudial Current Device). The Earth connection must not be switched between supplies (Reg. 543.3.3), although both live conductors should be switched before the primary RCD (which may be a standard type of 30mA if no others are present or may be a time delayed type if there are other standard types present). Note that this RCD provides no protection against faults in the generator itself, or the cable to the switch, but with suitable choice this is not a problem.

Your installation sounds as though it may be somewhat old, and you should ensure that the electrician you choose knows all of the above information himself (without reference to my post) before he (or she ladies, generic term!) is engaged for the job. He may suggest an EICR, additional bonding cables and replacement of old equipment not to current regulations etc.; but no changes to your current installation are mandated, it is your choice.

Please come back if you have any difficulties or questions.

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 05 December 2017 08:05 AM
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gkenyon

Posts: 4981
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In terms of earth electrode, 20 ohms maximum earth electrode resistance is recommended in BS 7430 for the generator operating as TN-S (Andy's post).

However, if you're looking at protecting against potential rise for open-circuit PEN conductor in the DNO supply (Dave's last post), a much, much lower value is required- see Table 14.1 of IET Guidance Note 5 Protection against electric shock.

Note that the table in the current edition does not appear to take into account Cmax, and is limited to loads up to 7 kW - or about 30 A load curent.

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

Web-Site: www.gkenyontech.com
 05 December 2017 09:49 AM
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davezawadi

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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

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 05 December 2017 10:45 AM
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gkenyon

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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.


Sorry David ... you said in your previous post:

Your supply is TNC-S then, and this has one snag, which is that there is a possible fault condition outside your property which could be dangerous. This can be overcome by the use of a suitable Earth electrode ...


I therefore pointed out Table 14.1 of GN 5 ... What's the problem with that?

No RCD within the installation will help with the lost neutral situation whilst "on-grid" - and whilst "off-grid", the supplier's neutral will be disconnected - provided there are no other properties fed from the same transformer, then there won't be a problem "off-grid"?

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.

Whilst I agree that the 20 ohms stated by BS 7430 is OTT for a small stand-alone generator:

(i) it's still greater than your "suitable earth electrode" to protect against PEN faults (for which Table 14.1 of GN 5 provides some guidance)? Or did you means something different?

(ii) we don't know the rating of the generator - nor the rating of other generation at the premises, such as solar-PV. If the total generation small (< 6 kVA)then perhaps the guidance in Section 9 of the IET Code of Practice for Electrical Energy Storage Systems relating to off-grid earthing, might be useful for increasing the earth electrode rating, but it still gives a higher value than Table 14.1 of GN 5


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.
You brought the DNO problem up ... but in the grand scheme of things, broken PME neutral safety concerns won't go away; I expect to see more in this space as time goes on.

I am more concerned that the install probably does not have bonding if required, and that the installer is fairly competent.
No problem with that at all.

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

Web-Site: www.gkenyontech.com
 05 December 2017 11:12 AM
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davezawadi

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Graham
I do not take issue with your comments, but one must be realistic, we are not designing a hospital back up system here. I regularly use mobile generators in the 50-100 kVA range and you might say that your comments are relevant. In fact they cannot be met in any way and I think that this is a major problem with the IET guidance notes. They are in many cases not written by practitioners, and simply follow what has been said before, often from poor sources. I take the 20 Ohms requirement which we discussed fairly recently, provenance unknown! The best advice always comes from a skilled and experienced practitioner, one of the reasons why I spend time on the forum giving a service to the industry. We all may occasionally disagree about details but I have never had anyone (even by PM) say the advice which is offered is bad or wrong, which is as I would hope having 40 years of serious engineering involvement with electrical engineering. A simple domestic generator of say 10kVA is more than safe with the advice I have given, provided the installation is competent. It is the regulations which make there any kind of doubt, and the BS7671 group is far from authoritative! I have recently given another forum member some advice for a new publication, partly to see if they needed some help in writing it, but also to understand the structure of the authors. It is as opaque as ever! The scope creep, and poor DPC recently are typical examples of the wrong people trying to be clever in response to something which they don't understand. Vested interests also have a huge say, which I consider to be verging on corrupt.

This thread is probably not the best place for this post, as it may confuse the uninitiated, but it needs to be said that good advice is invaluable, whilst bad is very expensive. In many ways KISS should apply to all posts.

I just noticed one thing and that is "the RCD will not help with on grid lost neutral", but it will! Bonding should take care of the danger, and the first fault will still trip the RCD, as there is still a difference in the L&N currents. In fact it may be made to trip by putting a VDR (voltage dependent resistor) or similar between neutral and the earth electrode, if the N drifts too far from the real earth the RCD will trip and isolate the supply. In fact I wonder why no one has mentioned that idea before? Oh well it is now public domain so I (or anyone else) cannot patent it!

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk

Edited: 05 December 2017 at 11:53 AM by davezawadi
 05 December 2017 11:23 AM
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gkenyon

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Originally posted by: davezawadi

Graham

I do not take issue with your comments, but one must be realistic, we are not designing a hospital back up system here. I regularly use mobile generators in the 50-100 kVA range and you might say that your comments are relevant. In fact they cannot be met in any way and I think that this is a major problem with the IET guidance notes.
So, that needs to be discussed ...
They are in many cases not written by practitioners, and simply follow what has been said before, often from poor sources. I take the 20 Ohms requirement which we discussed fairly recently, provenance unknown!
Can't be drawn in on that ... and of course, BS 7430 isn't BS 7671 ... nor IET guidance notes, but a long-standing British Standard in its own right.

The best advice always comes from a skilled and experienced practitioner, one of the reasons why I spend time on the forum giving a service to the industry.
With you there.
We all may occasionally disagree about details but I have never had anyone (even by PM) say the advice which is offered is bad or wrong, which is as I would hope having 40 years of serious engineering involvement with electrical engineering.
Agreed ... but as you have pointed out, there are things to discuss here ... particularly, what's "to standard" or "to guidance" perhaps being unfeasible, and what you do when it's not feasible.

A simple domestic generator of say 10kVA is more than safe with the advice I have given, provided the installation is competent. It is the regulations which make there any kind of doubt, and the BS7671 group is far from authoritative! I have recently given another forum member some advice for a new publication, partly to see if they needed some help in writing it, but also to understand the structure of the authors. It is as opaque as ever! The scope creep, and poor DPC recently are typical examples of the wrong people trying to be clever in response to something which they don't understand. Vested interests also have a huge say, which I consider to be verging on corrupt.



This thread is probably not the best place for this post, as it may confuse the uninitiated, but it needs to be said that good advice is invaluable, whilst bad is very expensive. In many ways KISS should apply to all posts.
I won't make this post into something it shouldn't be seen as.

As you say ... there are things to discuss.

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

Web-Site: www.gkenyontech.com
 05 December 2017 01:17 PM
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mapj1

Posts: 9689
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Is there a clear guide to be followed to ensure the earthing is correctly connected? In this case, should an earth rod be fitted and wired to the earth bar? Does the N and E on the generator need linking too


Without becoming embroiled too much in the above, whcih you may take as what guidance there is is not always clear....
Yes, the generator will need an N-E bond, on the generator side of the isolator, so it is not in circuit when running on mains . But before adding one, make sure there is not already one internal to the design.
If N-E linking is omitted, RCD operation will be compromised when on genset.

And yes, during a fault you cannot rely on the suppliers earth connection (after all it may be that reason the power is off, is that their cable is cut through), so as a minimum there should be an electrode at the genset, and there is nothing stopping you having more than one.

And it is wise to have at least the genset supply to the house routed via a high value RCD (perhaps 300mA) unless the short circuit capacity of the genset is known for sure to be able to trip the largest breaker reliably without it.
If you know the genset rating and model we may be able to advise this too.

-------------------------
regards Mike
 05 December 2017 06:05 PM
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Fm

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 05 December 2017 09:08 PM
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sparkingchip

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I presume this is a petrol, gas or diesel backup generator for use when there is a fault on the DNO distribution system.

Do you want your generator earthing connected to a faulty DNO cable whilst it in use.

Can you avoid such a situation?

Andy B.
 05 December 2017 09:37 PM
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davezawadi

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No you cannot stop this because of Reg. 543.3.3.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 05 December 2017 10:04 PM
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sparkingchip

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Originally posted by: davezawadi

No you cannot stop this because of Reg. 543.3.3.


543.3.3
A switching device shall not be inserted in a protective conductor except for the following:
(I) as permitted by Regulation 537.1.5
(I) multipole linked switching or plug in devices in which the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected.

537.1.5
Where an installation is supplied from more than one source of energy, one of which requires a means of earthing independent of the means of earthing of other sources and it is necessary to ensure that not more than one means of earthing is applied at any time, a switch may be inserted between the neutral point and the means of earthing, provided that the switch is a linked switch arranged to disconnect and connect the earthing conductor for the appropriate source at substantially the same time as the related live conductors.

So back to my previous post:

"I presume this is a petrol, gas or diesel backup generator for use when there is a fault on the DNO distribution system.

Do you want your generator earthing connected to a faulty DNO cable whilst it in use.

Can you avoid such a situation?

Andy B."
 05 December 2017 10:13 PM
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mapj1

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You can switch a CPC, so long as it is first in last out. Very much the same thinking as the longer pin on the mains plugs.
Personally in the second case if shifting NE bonds, I would interpret " at substantially the same time" as allowing a brief but deliberate moment of having 2 NE bonds, rather than risking a brief but possibly very expensive moment of not having any at all.

-------------------------
regards Mike
 05 December 2017 11:10 PM
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sparkingchip

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Why is it that articles get published with incorrect stock photos?

Rotopokers don't generate electricity, the only thing they have in common with a generator is an engine.

It might as well be a picture of a lawnmower or a motorcycle.

Andy B.
 05 December 2017 11:38 PM
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mapj1

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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.

-------------------------
regards Mike
 06 December 2017 07:53 AM
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gkenyon

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There's a huge difference between deciding whether to "earth spike" a portable generator for a few bits of temporary equipment, or simply bond that to local extraneous-conductive-parts (noting 411.3.1.1 and 542.1.3.3 which may put a bit of a damp squib on that if the extraneous-conductive-parts happen to be exposed-conductive-parts of another installation), and one supplying a fixed installation.

Earthing of the fixed installation is recommended for a number of very good reasons, although perhaps if there ALL of the following are true, it could be an option:

- No incoming metallic services (including copper telecoms)
- Few or no other extraneous-conductive-parts
- No LPS, and no lightning risk from adjacent structures
- DNO earth is disconnected when off-grid in accordance with 543.3.3 and 537.1.5
- RCD provided at generator (but there may need to be one anyway to achieve disconnection times).
- SPDs are not required

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

Web-Site: www.gkenyontech.com
 06 December 2017 08:36 AM
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sparkingchip

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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.

Then spiking the installation so that when the generator is in use the earthing system is TNS referenced to earth.

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.

Andy B.
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