Earthing of standby generators

[Km1777]

Afternoon guys. Recently been asked to do a generator changeover switch for a friend it’s a 6.0kva standby generator . The earthing system is tnc-s . Just seeking a bit of advice with regards to earthing requirements as it’s a portable gen . Also it will be sitting on top of block paving . Can’t see an easy way to drive an eart rod in. The output wiring is neutral and earth are separate . I’m a fully qualified electrician have all up to date qualifications just haven’t had any dealings with standby generator equipment . Any advice would be appreciated.

 

[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]

Champion 3600 Watt Frame Type Inverter

Champion 3600 Watt Digital Hybrid Petrol Generator. Portable and easy to move. Wireless key start. Perfect for backup power, home use and even camping in the UK.

www.championpowerequipment.co.uk

 

[James]

Instructions say it is a floating N but nothing else.

 

[pc1966]

Looking at the diagrams it has a schematic at the end:

https://cdn.shopify.com/s/files/1/0624/4745/2318/files/CPG4000DHY-eu-technical-drawings.pdf

Looks like a 3-phase alternator and the "control unit" might be the inverter that converts rectified DC off that to good sine-like single phase AC. However, there are no RCD on the outputs and I'm guessing the floating supply approach is the means of reducing the risk of shock on contact with a live conductor instead of ADS from a RCD.

Without modifying the generator (not a trivial task) you can't really win:

• Earth the neutral some place and you have no additional protection on the generator's socket outlets, against the regs. Also if you ever use a properly engineered generaotr with RCD it will trip as a result of the link.
• Don't earth the neutral and the house has an IT supply without skilled supervision, also against regs

Also the instructions are clearly for the USA market, in spite of it being sold here with UK outlets:

Use a ground fault circuit interrupter (GFCI) in damp areas and areas containing conductive material such as metal decking.

Connection to your home’s electrical system requires a listed 30A transfer switch installed by a licensed electrician and approved by the local authority having jurisdiction. The connection must isolate the generator from the utility power and must comply with all applicable laws and electrical codes.

Do not use generator for medical and life support uses.

In case of emergency, call 911 immediately.

That also fits, as the USA typically has N-E links in the incoming panel/DB, so the issue of a floating supply is not happening. I don't know if they have any similar blanket RCD rules on socket outlets as we do.

 

[goasis]

Looking at the diagrams it has a schematic at the end:

https://cdn.shopify.com/s/files/1/0624/4745/2318/files/CPG4000DHY-eu-technical-drawings.pdf

Looks like a 3-phase alternator and the "control unit" might be the inverter that converts rectified DC off that to good sine-like single phase AC. However, there are no RCD on the outputs and I'm guessing the floating supply approach is the means of reducing the risk of shock on contact with a live conductor instead of ADS from a RCD.

Without modifying the generator (not a trivial task) you can't really win:

• Earth the neutral some place and you have no additional protection on the generator's socket outlets, against the regs. Also if you ever use a properly engineered generaotr with RCD it will trip as a result of the link.
• Don't earth the neutral and the house has an IT supply without skilled supervision, also against regs

Also the instructions are clearly for the USA market, in spite of it being sold here with UK outlets:

Use a ground fault circuit interrupter (GFCI) in damp areas and areas containing conductive material such as metal decking.

Connection to your home’s electrical system requires a listed 30A transfer switch installed by a licensed electrician and approved by the local authority having jurisdiction. The connection must isolate the generator from the utility power and must comply with all applicable laws and electrical codes.

Do not use generator for medical and life support uses.

In case of emergency, call 911 immediately.

That also fits, as the USA typically has N-E links in the incoming panel/DB, so the issue of a floating supply is not happening. I don't know if they have any similar blanket RCD rules on socket outlets as we do.

Thankyou for your detailed response.
I'm thinking I may have bitten of more than I can chew here.
Option A, create a neutral earth link in the plug inlet on the house. There's no additional protection on the outlets of the generator but that is the case anyway?
If the cable from the inlet to the changeover switch is installed in a way which doesn't require rcd protection would that then be OK?

 

[James]

I would go with option A
consider the inlet on the house as the origin of supply and link N to E at that point.

 

[UNG]

Champion 3600 Watt Frame Type Inverter

Champion 3600 Watt Digital Hybrid Petrol Generator. Portable and easy to move. Wireless key start. Perfect for backup power, home use and even camping in the UK.

www.championpowerequipment.co.uk

Shows the level of quality when the warranty is limited to 1000Hrs for domestic use and 500Hrs for industrial and commercial use