Earthing of standby generators

[Rockingit]

That all makes sense to me, until the genny is wheeled away and used elsewhere with a single piece of equipment (such as temp power for tool or floodlight etc perhaps for an hour or two). Then you have one pole tied to the chassis of the generator and no earth rod...

To me it makes sense to do it on the customers installation, rather than the generator for the reason above. Also what happens when the generator packs up, customer buys new and just plugs it in?

Unless there's a downside to making the link at the origin of the customers installation that I'm missing? Is there any specific regulation that addresses this? What's accepted general practice?

Appreciate the replies. I'm not massively expereicned when it comes to generators, and most I've seen have been dangerous imo! Cheers

The trouble is that you can't win..... omit the earth and you end up with an IT system which can only be used under skilled or supervised.

 

[mattg4321]

Agree, but that won't stop people doing it!

I think my biggest worry is customer buying new generator and just plugging it in. That's why my thinking is it would be safer to make the N/E link at the 32amp appliance inlet on the wall of customers house. That way any generator plugged in will have N/E linked.

Am I missing something that would mean this arrangement is unsafe?

 

[loz2754]

A N/E link on the inlet socket to the house would not be allowed, as this would make it a TNC system on the consumer's side, which is against the regs.

 

[mattg4321]

A N/E link on the inlet socket to the house would not be allowed, as this would make it a TNC system on the consumer's side, which is against the regs.

That seems to be saying it’s the wrong thing to do on a technicality though rather than for any electrical reason. In reality it makes no difference if the link is in the inlet socket or the generator itself all being well imo.

What is the definition of ‘consumers side’ where standby generators are concerned? Serious question. Could be argued that anywhere the generator side of the change over switch is not ‘consumers side’. Could also be argued that the whole thing including generator is consumer side? I’m not saying I’m right and you’re wrong btw. I always just find things are done with regards to generators because that is the ‘accepted’ way of doing things and they sometimes don’t make much sense to me.

 

[mattg4321]

Afraid im going to have to dredge this one up again. Went to install the wiring for the generator today and all went fine until we fired it up.

No load connected to it I measured 115v between line and earth and also 115v between neutral (ok probably this is also line) and earth.

So it appears the genset has a centre tapped earth. Yet looking at the wiring schematic suggests it doesn’t. At that point I decided to pack up and seek further advice. I’m sure it’s a bad idea to supply a house with a supply with a centre tapped earth. Manufacturer claims it’s suitable for use as a standby generator though. Could do with some advice.

 

[pc1966]

Afraid im going to have to dredge this one up again. Went to install the wiring for the generator today and all went fine until we fired it up.

No load connected to it I measured 115v between line and earth and also 115v between neutral (ok probably this is also line) and earth.

So it appears the genset has a centre tapped earth. Yet looking at the wiring schematic suggests it doesn’t. At that point I decided to pack up and seek further advice. I’m sure it’s a bad idea to supply a house with a supply with a centre tapped earth. Manufacturer claims it’s suitable for use as a standby generator though. Could do with some advice.

Can you IR at 250V the output L+N to E?

It might not be centre-tapped, it might just be equal filter capacitors from L to E and N to E for the electronic inverter part leading to a balanced output voltage when nothing asymmetric to E is present. After all the schematic only shows protection breakers in the L paths to each socket.

 

[pc1966]

Thinking a bit more, that generator lacks and obvious RCD protection for socket outlets!

So either it is assuming as an IT supply it is safe enough (not sure how that sits with UK regs once several loads are present) or it has some electronic RCD-style shut-down in the inverter (so in effect an automatic insulation monitoring system).

 

[mattg4321]

Can you IR at 250V the output L+N to E?

It might not be centre-tapped, it might just be equal filter capacitors from L to E and N to E for the electronic inverter part leading to a balanced output voltage when nothing asymmetric to E is present. After all the schematic only shows protection breakers in the L paths to each socket.

That’s what threw me. I assumed the earth was not centre tapped after looking at the schematic before visiting site.

Wish I had thought to IR test. I’ll have to do it when I go back. Should I assume I will get ‘open circuit’ when testing between L&N to earth if the earth is not centre tapped? In which case I can just link N and earth in the output socket on the generator and happy days?

Alternatively if I get 0.00 Mohm then it is centre tapped. What are my options then? Assume it’s get them to return the generator or modify it somehow by disconnecting the centre tapping. Neither option is appealing!!

Appreciate the reply. Thanks

 

[Bellendian]

I'm about to have to tackle a job similar to this. Have to say I 100% agree with Matt regarding the location of the N-E link. In the case of a portable generator that can be readily carted off elsewhere for use as a standalone genny it seems hazardous to make the link within the machine itself. My preference would definitely be to make the N-E link at the point of connection to the fixed installation, and struggling to understand why that might be a problem? Surely if they are linked at some point, it doesn't matter where from an electrical perspective?
Not convinced you'd be creating a tnc system either since there would be separate parallel conductors, albeit linked, but not using 1 conductor for both functions.

 

[mattg4321]

I'm about to have to tackle a job similar to this. Have to say I 100% agree with Matt regarding the location of the N-E link. In the case of a portable generator that can be readily carted off elsewhere for use as a standalone genny it seems hazardous to make the link within the machine itself. My preference would definitely be to make the N-E link at the point of connection to the fixed installation, and struggling to understand why that might be a problem? Surely if they are linked at some point, it doesn't matter where from an electrical perspective?
Not convinced you'd be creating a tnc system either since there would be separate parallel conductors, albeit linked, but not using 1 conductor for both functions.

I was thinking about this again today and one downside I thought of was the chances of someone plugging a centre tapped earth generator into a system with a linked neutral and earth. This would short it out and potentially cause damage. You can’t win either way and tbh my feeling is most generator installs I’ve seen don’t seem all that safe and the installer has probably just ‘winged it’.

Any thoughts anyone on a way forward for my problem? Do I need to tell them to replace the generator with something else? I find it amazing how little guidance Hyundai give in their instructions. They don’t even tell you what earthing arrangements it has. There is no rcd on the unit as far as I can see. I have fitted one adjacent to the inlet plug on the wall of the property, along with a 32amp mcb.

 

[Simon47]

I was thinking about this again today and one downside I thought of was the chances of someone plugging a centre tapped earth generator into a system with a linked neutral and earth. This would short it out and potentially cause damage.

Correct. In all the "what if ..." scenarios, you can't control what someone comes along and plugs in. If their new genny is either centre-tapped to earth, or more likely, is 55-0-165V, then linking N-E anywhere is going to cause a problem - and I suspect few of them have 2 pole overloads meaning that you'd have a shorted winding with no overcurrent protection.
If you made a "special" lead with the N-E link, then either it doesn't get used (a different lead without it leads to an IT supply), or that lead gets used elsewhere and causes problems.

You can’t win either way and tbh my feeling is most generator installs I’ve seen don’t seem all that safe and the installer has probably just ‘winged it’.

That's probably a fair assessment. I think the only thing that can be said for certain is that it's impossible to do in a generic way - i.e. where you aren't in a controlled environment where things can't get swapped about without someone with sufficient skill overseeing it for safety.
You either create a risk of shorting out a genny (put N-E link in the fixed installation, replacement genny isn't floating), or you risk having an IT supply (put N-E link in genny, replacement genny doesn't have it). About the only way I can think of doing it is to fit a N-E resistor in the fixed installation downstream of the RCD - if the genny isn't floating then the RCD will trip, if the genny is floating then the resistor will create enough fault current to cause downstream RCDs to trip (like using an RCD where there isn't a good enough earth electrode on a TT supply). But that won't be standards compliant either, and there's a risk of nuisance tripping if the genny does have a N-E link.

Note to self ... must check how my genny is wired.

 

[RH2022]

Did anyone get a solution to this in respect to connecting an inverter generators into a fixed installation? Particularly considering whether it is ok to create a neutral earth link somewhere near the generator when using an inverter type generator?

This seems like the right answer for a conventional generator according to BS7430 but there was concern with an inverter unit in posts further up this thread?

 

[Simon47]

My guess is ... NO
I'm thinking that the only "generic" way to do it is to provide a 3 pin inlet, provide a local earth electrode, and stick a big notice next to the inlet indicating the requirements for connected genny. Something along the lines of :
Generator Input
Local earth provided via inlet, Ze=n.nn ohms
Generator must provide earthed neutral
It's something of a cop out, but it shifts the responsibility to whoever connects a genny - and I guess many will just ignore it. But even then, it's probably better than them using a widowmaker.

 

[RH2022]

My guess is ... NO
I'm thinking that the only "generic" way to do it is to provide a 3 pin inlet, provide a local earth electrode, and stick a big notice next to the inlet indicating the requirements for connected genny. Something along the lines of :
Generator Input
Local earth provided via inlet, Ze=n.nn ohms
Generator must provide earthed neutral
It's something of a cop out, but it shifts the responsibility to whoever connects a genny - and I guess many will just ignore it. But even then, it's probably better than them using a widowmaker.

Thanks for your reply Simon. I agree with providing an appropriate sign stating the connection requirements to meet the design.

I was really wondering if any of the people on this thread who have said they were asked to install an inverter generator for a customer have resolved the issue with it appearing under test to have 115V L-E or N-E; and if this is correct, the issues it would present when you provide a neutral earth link as per BS7430? If this is indeed how they are wired then I am not convinced they are suitable for supplying fixed installations in the UK as a back up home supply, while meeting the British Standards.

 

[Simon47]

Section 7.1.3 ? That's nice and short

Maaany years ago, a friend was responsible for some holiday cottages - two of which were remote and had no electrical supply. As someone who knew a bit about lecky, I got asked after the local DNO had quoted something like £70k (that's not a typo, and in 1990s money ) to connect them. These cottages were in a large piece of land they owned, and after hearing about the generator they'd tried, I suggested they consider a submain from another property (1300 m away) - stressing they'd need to get someone who knew what they were doing to size it.
I do recall that this friend had gone out and bought a small genny without getting any advice other than from the person selling it. They were a bit put out when I pointed out that if run continuously it would need one plug and two oil changes/week ! As it happened, I was told the electrician needed to re-wire it, and the first time they tried it - the connectors melted as they had put it in a small outhouse. It also consumed silly amounts of gas.
So they went down the submain route

I would suggest that if asked to connect such a genny, it would need re-wiring to a floating output so it can have the N-E link added. Otherwise (if it couldn't be rewired) you'd need to keep it as a split-phase supply and include a suitable 2 pole breaker in the genny connection to protect the genny from N-E faults.
But as per the discussion, unless it's a "known" genny, and hardwired, then it is always going to have to be down to whoever connects the genny to sort this out on a case by case basis. Good luck educating the average householder on that

 

[goasis]

Hi,

I've been asked to look at installing a generator changeover switch at a property, so that they have some level of backup in the event of power cuts.

Having started to read up on this i'm beginning to think that i dont have the knowledge or experience to do this work.

I don't have a great deal of experience with generators.
The generator they are looking at is 3500w, with a 16a outlet and a "floating neutral".
Can anyone give me some advice as to what this means, and therefore what is required in terms of earthing and protection.

My understanding so far,
-earth rod connected to the main installation MET
- Linking the CPC and neutral together somewhere, inside the 16A inlet socket seems the best option?
Thanks in advance

 

[pc1966]

The generator they are looking at is 3500w, with a 16a outlet and a "floating neutral".
Can anyone give me some advice as to what this means, and therefore what is required in terms of earthing and protection.

This is exactly the sort of issue covered in posts #7 and #8

Basically if the generator is a floating supply then you have an IT power source and that, when used with more than one load (so excluding shaver transformers, etc) is only permitted with skilled supervision, etc, as the usual ADS of MCB/RCD are largely ineffective (certainly not behaving as normally expected).

Generators intended for powering significant installations usually have N & E linked internally, just like the supply substation. Above has a bit of a debate as to how best to achieve this with smaller and or nastier generators. If a captive cable with commando plug was tolerable (so cable is not re-used elsewhere, it is only ever for this one generator job) then linking N-E in the commando plug would be one solution as no PEN (separate PE and N in cable) and you are not linking N & E "within" the installation (OK, maybe pushing that). Only big concern is if some future generator has a centre tapped 230V supply but that ought to be rare, usually it is the 110V building site transformers & generators that have that.

Also you must have a DP changeover switch for the generator and grid supply so the N-E link at or in the generator is not present when on grid power.

You still need an independent means of earthing as under loss of supply the DNO's earth may be gone as well. That can be a rod to the MET, it need not be on the generator and it can be permanently present (of course it is "extraneous" so should be 10mm bond if PME supply normally). Note that this rod does not make it a TT system, it is TN if the N & E are linked in/at the generator. but the very definition of "neutral" means it has to be reference to earth!

 

[goasis]

This is exactly the sort of issue covered in posts #7 and #8

Basically if the generator is a floating supply then you have an IT power source and that, when used with more than one load (so excluding shaver transformers, etc) is only permitted with skilled supervision, etc, as the usual ADS of MCB/RCD are largely ineffective (certainly not behaving as normally expected).

Generators intended for powering significant installations usually have N & E linked internally, just like the supply substation. Above has a bit of a debate as to how best to achieve this with smaller and or nastier generators. If a captive cable with commando plug was tolerable (so cable is not re-used elsewhere, it is only ever for this one generator job) then linking N-E in the commando plug would be one solution as no PEN (separate PE and N in cable) and you are not linking N & E "within" the installation (OK, maybe pushing that). Only big concern is if some future generator has a centre tapped 230V supply but that ought to be rare, usually it is the 110V building site transformers & generators that have that.

Also you must have a DP changeover switch for the generator and grid supply so the N-E link at or in the generator is not present when on grid power.

You still need an independent means of earthing as under loss of supply the DNO's earth may be gone as well. That can be a rod to the MET, it need not be on the generator and it can be permanently present (of course it is "extraneous" so should be 10mm bond if PME supply normally). Not that this rod does not make it a TT system, it is TN if the N & E are linked in/at the generator. but the very definition of "neutral" means it has to be reference to earth!

Thank you, an excellent summary and speedy as well!
If you were to create the link in the male outlet (the means of connecting genny to house) that would only become part of the house installation when the changeover switch was operated, so would that be thought of as part of the installation? Or the source?

 

[pc1966]

Thank you, an excellent summary and speedy as well!
If you were to create the link in the male outlet (the means of connecting genny to house) that would only become part of the house installation when the changeover switch was operated, so would that be thought of as part of the installation? Or the source?

Good question! The basic idea is there should only be one link between N & E and it should be such that a cable failure, etc, is not at risk of an "open PEN" style of fault. The DNO are allowed TN-C as they are expected to work to higher standards than installations that are poorly regulated, DIY'd etc.

Having thought about it there is no simple and really good solution beyond having a proper generator arrangement. Having a N-E link at the house inlet (or maybe cable as I thought) will cause a properly designed generator to trip its RCD.

If you link N & E only after an RCD you have no additional shock protection on that outlet/cable (as leakage to true earth is not causing a current imbalance in the RCD, etc).

If you have a floating generator (with or without RCD as it won't be effective - as above) with a large and leaky installation that is not earth-referenced then the external cable again has little 'additional protection' beyond a shock being limited to the 10mA or so the house normally might leak, and you have ineffective ADS, etc, on any faults. Hence why IT systems are normally for special cases and under skilled supervision (ship electrics, operating theatre, etc).

Do you know the model of generator and if it claims to have any RCD protection on the outlets (floating neutral suggests not)? Have they bought it already?

 

[goasis]

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