outbuilding supply from floating neutral portable generator

[steve098]

Hello,

I would appreciate your comments on my proposed solution to supplying a stone outbuilding from a portable generator.
I have read the numerous posts on this and other forums, and http://electrical.theiet.org/wiring-matters/39/portable-generators.cfm?type=pdf and distilled it all into the solution below.


The aim is to power the outbuilding (club hut) for an hour or two once a week.
The design criteria are that:

A) power is supplied from one of several peoples' personal portable generators (~3kVA type), taking the generator away each night for security
B) sighting the generator away from the clubhouse for noise reduction,
C) use an easy-to-coil lead running from generator to clubhouse.
D) have one single, simple connection to hook up generator to clubhouse

The outbuilding (stone shed far away from grid electricity) is wired with small consumer unit(CU) with RCD protected ring main and lighting circuit. This Consumer unit is therefore to be fed from a floating neutral portable generator sited some distance away via suitable rubber extension lead.


You will see that there are several issues with the above that I need to address, namely:

a) bond the earth and neutral at the generator, so that CU RCDs can operate from the otherwise floating-neutral generator
b) provide suitable ground rod for fault current path/ground reference for RCD
c) provide protection for the extension cable running from the generator to the CU (it is proposed to be rubber not armoured for ease of coiling)

I would appreciate your comments on my proposed solution:

1) bond earth and neutral inside a 16A 'commando' plug for connection to portable generator. This way the generator remains unaltered for the person to take home and use as normal.
2) connect this commando plug to an inline 16A 30mA RCD (e.g. http://www.toolstation.com/shop/p22113) via a short (10cm) length of SWA (hence this short length needn't have RCD protection)
3) the protected side of this inline RCD is then led to the clubhouse via a 15m rubber-coated 3 conductor extension lead (suitably sized). This way the entire length of the connection from the generator to the clubhouse is protected yet easier to handle than a whole reel of SWA.
4) the ground rod is installed at the outbuilding end and permanently wired to the earth bar of the CU, this way the connection can be made permanent and good, and the generator connected by plugging a single 16A plug into the generator.

My main concern is with the ground rod arrangement.
I would prefer to keep the ground rod permanently installed at the CU, so as to avoid having to make up a good connection from the ground rod to the generator each time the generator is connected (only one plug to connect)

QUESTION 1: Is it sufficient to have the ground rod permanently connected at the C.U. instead of close to the generator?
For the connection inside the clubhouse to the CU, for neatness I would like the extension lead to have plugs/sockets at each end so it can be disconnected and stored away from rodents (i.e. a pair of 16A plug/sockets, one on the wall for the CU, one on the cable)
My concern is that if someone plugs one end of the extension lead into the generator(with its neutral-ground link) yet neglects to plug the other end into the CU (and hence the ground rod), then the generator frame will be at a 'high' neutral potential and a shock risk? A ground rod at the generator end would negate this risk so long as the connection were made sound each time, but I'm trying to avoid this if possible as it relies on the user connecting the ground as well as the main extension cable

QUESTION 2: I assume that all floating-neutral portable generators have their frames connected to the earth pin of the output, and therefore the frame would be suitably grounded via the extension lead to the ground rod at the CU?


many thanks in advance for any observations/advice.

 

[ericmark]

Floating neutral is the main problem. Many generators do have internal connection between winding and earth not always the neutral I have seen with a Honda where the earth tapping was 55 volts from neutral polarity so any connection exterior to the generator between neutral and earth could cause a direct short with some generators on one of the output windings.

Therefore any connection earth to neutral must be in the generator not at the consumer unit. Having an earth rod is fine but neutral and earth should never be bonded outside of the generator or transformer. Using an isolation transformer as use to boats would be the only way you could bond neutral and earth.

I have thought about a large resistor between neutral and earth this is what is done where commercial premises have generators to stop a fault outside there premises from causing excessive current running through their earth rod system. It is between the two earths. So a 10W 5kΩ resistor (sufficiently high impedance) between neutral and earth would ensure 30 mA can flow but would not be a direct short if the generator is not truly floating. However this may cause other problems specially with inspection and testing it would be seen as a fault. However in real terms this is what is done with the earth monitors of an IT system.

So in real terms the only way to use a variety of IT (that's the name for floating neutral) is to wire as an IT system and use the special monitors made for an IT system.

What I am uncertain about is the laws and rules about using an IT supply in a building used by the general public which clearly a club house would be.

The use of a twin pole overload may also be an answer in fact it may be the only way with an IT system as really you don't have a line and neutral as such but two lines.

However in real terms with an IT system touching either live wires will not cause the RCD to trip but neither would it give one a shock. Only when there is a second fault would anyone get a shock and with the second fault the RCD would work.

So I would tend to forget about some generators having a floating neutral or IT supply and wire and use as if there was a bond in the generator between earth and neutral and provide an earth clamp to earth generator to the building earth.

Once your actions make it obvious you knew you were dealing with an IT supply then it would make it hard explaining why you didn't have the isolation transformer and that would cost as much as your own generator.

Do wait for other answers as this is really something I have not had to deal with directly and it would take a lot of reading up to be sure as to the requirements.

 

[steve098]

Ericmark, thankyou for your detailed reply. I think the only way to satisfy their requirement for flexibility in choice of generators is The isolation transformer as you point out, if cost were no issue.

I can see your point that being unable to guarantee where a given generator is earth tapped will prevent N-E bonding outside the generator at the plug/CU.

I think their solution will be to forego their requirement of using one of many possible generators and dedicate one generator that is/can be neutral-earth bonded inside the generator for use at the outbuilding.

I need to think about an increased risk of broken neutral in the generator extension lead, given that it is a flexible type not static SWA. Given the rod will be bonded at the CU, the outbuilding would be protected, but the generator frame would need a protective earth rod too?

The requirement for a flexible (non SWA) lead from generator to CU can still be met with the inline RCD at the generator end, but I need to consider discrimination of tripping of the RCD protecting the generator extension lead and the RCD in the outbuilding.

It is interesting how imposing the requirement for a portable generator fed from a flexible lead make this a different installation from the usual generator-fed portakabin.

 

[JohnW2]

So I would tend to forget about some generators having a floating neutral or IT supply and wire and use as if there was a bond in the generator between earth and neutral and provide an earth clamp to earth generator to the building earth.

Can you help me understand that? If there were not an N-E link in the generator (but you 'wired it as if there were one'), what would be gained by earthing the generator (which would then be 'floating' wrt the supply)?

Kind Regards, John

 

[JohnW2]

I think their solution will be to forego their requirement of using one of many possible generators and dedicate one generator that is/can be neutral-earth bonded inside the generator for use at the outbuilding. ....
The requirement for a flexible (non SWA) lead from generator to CU can still be met with the inline RCD at the generator end, but I need to consider discrimination of tripping of the RCD protecting the generator extension lead and the RCD in the outbuilding.

Do I take it that your second paragraph above relates to the first - i.e. a situation in which there definitely was always going to be an N-E link at the generator? In the absence of such an N-E connection, RCDs would obviously give no protection. As for your concern about discrimination of RCDs, for a start, if there was one at the generator, you wouldn't actually need a second one in the outbuilding.

I need to think about an increased risk of broken neutral in the generator extension lead, given that it is a flexible type not static SWA. Given the rod will be bonded at the CU, the outbuilding would be protected, but the generator frame would need a protective earth rod too?

I'm not sure I understand what your concern is here.

As eric has said, my greatest concern would be about the regulations and responsibilities associated with an electricity supply to a building to be used by 'the public' - which is very different from a 'domestic wiring' situation.

Kind Regards, John

 

[ericmark]

If one connects an earth to the generator with a clip like used when delivering gas then what every the earthing system they will all be as same polarity.

The stating that a RCD gives no protection I see as incorrect. It will not protect against a single fault but will protect against a second fault and the question has to be asked is there a need to protect from the first fault?

A residual current monitoring device works very like the old DC cranes of years ago where two bulbs in series + to earth and - to earth were fitted with no fault both bulbs glowed with a fault one bulb became bright. I have used these devices on concrete batching plants should one remove the power one would never get them started again so it was men with shovels to dig it all out so rather than a RCD the RCM and IT supply was used and a big red bulb would light on the operators panel. In theroy he would just finish that batch then get fault corrected before starting next one.

If all the generators are really IT then this would work, but the point is some may not really be IT and this is where the problem arises. If the generator did have some bonding of the neutral then the warning lamp would light.

I tried to buy an isolation transformer for my sons narrow boat. Found it was cheaper to drop to 110 and then lift back to 230 than get an isolation transformer so for you in real terms cheaper to buy a generator used only at the club house than get an isolation transformer.

So the big question is:-

Will tripping with a second fault be good enough for insurers and others involved with the using of the building.

Sorry can't answer that question.

If it was me I would pretend I didn't know some of the generators had a floating earth but sand gets everywhere when you bury ones head in it!

 

[JohnW2]

If one connects an earth to the generator with a clip like used when delivering gas then what every the earthing system they will all be as same polarity.

That's true, albeit the issue in terms of gas deliveries is static electricity.

The stating that a RCD gives no protection I see as incorrect. It will not protect against a single fault but will protect against a second fault and the question has to be asked is there a need to protect from the first fault?

Yes, that's often said, and it is, in some situations, true. However, I think it would probably be wrong to give the impression that any pair of faults would result in operation of the RCD. I think the truth is that only a few, very specific, pairs of 'first fault' and 'second fault' would actually result in operation of an RCD - so one has to consider whether one is perhaps contemplating 'the vanishingly improbable'.

So the big question is:- Will tripping with a second fault be good enough for insurers and others involved with the using of the building. Sorry can't answer that question.

Needless to say, I can't answer it, either. However, as above, I don't think it's even as simple as that, since I don't think one could say that it always would 'trip on the second fault' - since that would depend upon the nature of the first and second faults.

As we've discussed before, I think the most important thing is that any exposed-conductive parts of equipment supplied by a floating (or possibly floating) supply are joined together by CPCs - whether those CPCs are connected to true earth (which may well increase hazards in some situations) or not.

Kind Regards, John

 

[steve098]

Hi John,

Do I take it that your second paragraph above relates to the first - i.e. a situation in which there definitely was always going to be an N-E link at the generator?

indeed - to eliminate doubt over the type of the generator used, we would specify the use of only generators with N-E link so that RCDs would work.

As for your concern about discrimination of RCDs, for a start, if there was one at the generator, you wouldn't actually need a second one in the outbuilding.

Good point, removing the RCD at the building and siting solely at the generator might be our solution...


re: concern over broken neutral in supply lead:
I think I have satisfied myself now this wont be an issue. I was likening the situation to an exported TN-C-S to outbuilding where a loss of neutral would impose voltage on the earth, which would not trip the RCD in the outhouse. Given that we are putting RCD inline at the generator, and specifying ground rod at the CU, loss of neutral in the supply lead would trip this inline RCD.

However, with the only rod at the CU I wonder how a break in the earth from the generator to the CU might make the generator frame at a higher than ground potential that isn't protected by the RCD...?


Adapting the solution to incorporate this feedback, but lose some of the initial design criteria:
N+E linked generator, ground rod at generator, inline RCD at generator, rubber supply to CU. (ground at CU for good measure), no need for RCD in outbuilding.

As eric has said, my greatest concern would be about the regulations and responsibilities associated with an electricity supply to a building to be used by 'the public' - which is very different from a 'domestic wiring' situation.

True, but as with many posts to these forums, forewarned is forearmed when it comes to contracting third parties.

I value your feedback, thankyou.

Steve

 

[JohnW2]

As for your concern about discrimination of RCDs, for a start, if there was one at the generator, you wouldn't actually need a second one in the outbuilding.

Good point, removing the RCD at the building and siting solely at the generator might be our solution...

That would certainly make sense.

re: concern over broken neutral in supply lead: I think I have satisfied myself now this wont be an issue. I was likening the situation to an exported TN-C-S to outbuilding where a loss of neutral would impose voltage on the earth, which would not trip the RCD in the outhouse.

The 'lost (broken) TN-C-S neutral' issue is very different. The problem (in terms of possibly raising the potential of the installation's 'earth' to well above true earth potential) only exists if the break in the neutral is upstream of where the installation's earth is derived from it. In your case, there would certainly (hopefully!) be no connections between neutral and earth beyond the generator - so that situation can't arise. If the neutral in the cable between generator and house were to break there would be no immediate hazard (i.e. 'in normal use') - the supply to the outbuilding would simply stop working. Only if people started 'playing with the wiring' and touched the neutral (and earth, assuming the supply was earth-referenced) without first switching off the L supply (and/or stopping the genny) would there be any hazard - and if it were an earth-referenced system, taht ought to cause the RCD to operate.

Given that we are putting RCD inline at the generator, and specifying ground rod at the CU, loss of neutral in the supply lead would trip this inline RCD.

Loss (break) of neutral (I assume you still mean between genny and outhouse) would not, per se, cause any RCD to operate - no current would be flowing in either L or N.

However, with the only rod at the CU I wonder how a break in the earth from the generator to the CU might make the generator frame at a higher than ground potential that isn't protected by the RCD...?

You're not thinking of deriving the the outhouse's earth from the neutral, are you? - that would definitely be a no-no!! A break in the earth (CPC) between generator and CU would do nothing (unless, perhaps, other faults wre also present) - certainly would not cause an RCD to operate.

Adapting the solution to incorporate this feedback, but lose some of the initial design criteria:
N+E linked generator, ground rod at generator, inline RCD at generator, rubber supply to CU. (ground at CU for good measure), no need for RCD in outbuilding.

Possibly. One thing we haven't asked you - are there any metal pipes or such like entering the outhouse and/or does the building have any structural metal? If so, those metal parts (which are likley to be at true earth potential) would need to be adequately 'bonded' to the earthing system of the building.

Kind Regards, John

 

[steve098]

I tried to buy an isolation transformer for my sons narrow boat. Found it was cheaper to drop to 110 and then lift back to 230 than get an isolation transformer so for you in real terms cheaper to buy a generator used only at the club house than get an isolation transformer.

interesting...using a step-up transformer as isolation transformer...
something like
230v IT generator -> 110v step-down transformer -> inline 110v RCD -> cable to outbuilding ->230v step-up transformer -> CU (earth rod at CU, no RCDs)

OR
110v IT generator->cable to outbuilding->230v step-up transformer->isolate earth from generator, CU earth rod (i.e. TT to outbuilding), RCDs in CU

so the system is IT up to the CU, then TT beyond.

all the 'portable' generators I have seen are floating wrt ground. Is there a requirement for N+E bonded generators to be labelled as such?

would be interesting from a DIY point of view, can't see it being approved for public use.

 

[steve098]

Possibly. One thing we haven't asked you - are there any metal pipes or such like entering the outhouse and/or does the building have any structural metal? If so, those metal parts (which are likley to be at true earth potential) would need to be adequately 'bonded' to the earthing system of the building.

True - It's a stone outhouse but with with a tin roof.

 

[JohnW2]

Possibly. One thing we haven't asked you - are there any metal pipes or such like entering the outhouse and/or does the building have any structural metal? If so, those metal parts (which are likely to be at true earth potential) would need to be adequately 'bonded' to the earthing system of the building.

True - It's a stone outhouse but with with a tin roof.

I wouldn't think that is an issue - it's only metal which is in contact with (real!) earth that is the issue. No water pipes or suchlike?

Kind Regards, John

 

[steve098]

Possibly. One thing we haven't asked you - are there any metal pipes or such like entering the outhouse and/or does the building have any structural metal? If so, those metal parts (which are likely to be at true earth potential) would need to be adequately 'bonded' to the earthing system of the building.

True - It's a stone outhouse but with with a tin roof.

I wouldn't think that is an issue - it's only metal which is in contact with (real!) earth that is the issue. No water pipes or suchlike?

Kind Regards, John

No - no plumbing at all. It's a very rural old stone Scottish croft building with new-ish tin roof.

 

[steve098]

an update to this topic.
The requirement for converting/using a dedicated N+E bonded generator was too much of a restriction. The isolation transformer approach gives us the most flexibility in this respect and is the chosen solution. Power demands are modest and a ~3kVA IP67 isolation transformer is just a few hundred pounds.
It is proposed:
IT Generator-->Isolation Transformer with N+E Bonding on output -->inline RCD -->rubber extension to outbuilding CU.
Ground rod at outbuilding permanently to CU Earth.
Would a single ground rod at the CU be sufficient, or would there be a requirement for another ground rod at the isolation transformer output side too for some fault condition caused by a break in a conductor in the extension from the RCD to the outbuilding?

 

[JohnW2]

... Isolation Transformer with N+E Bonding on output -->inline RCD -->rubber extension to outbuilding CU. Ground rod at outbuilding permanently to CU Earth. ... Would a single ground rod at the CU be sufficient, or would there be a requirement for another ground rod at the isolation transformer output side too ...

I may be off-track, but ..... If there wasn't an earth rod at the isolation transformer, what exactly would you mean by "N+E bonding on output [of transformer]? I would have thought the question would be whether you should ('also') have an earth rod at the outbuilding CU. Do I take it that there will be a CPC in the cable from tranny to outhouse, such that (with an earth rod at tranny end) it would effectively be a TN-S setup?

Kind Regards, John