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.

 

[Rockingit]

Anymore thoughts on the best place to make the N/E link?

As above I’m not convinced the generator itself is the best place. If it gets wheeled off and used for temporary power at a neighbours house or something its probably best to have a ‘floating’earth imo as there probably won’t be a proper rod used, if any!!

You can only really make a PEN at the source. Normally just lug the two terminals together.

 

[Km1777]

Cheers guys For the advice. Looks like I’ll have to find a suitable place to drive in earth rods and link n-e at source. It’s a pretty straight forward job apart from the rods side of things

 

[mattg4321]

You can only really make a PEN at the source. Normally just lug the two terminals together.

Which terminals are you meaning if the generator has a 32a commando socket as it’s outlet?

The terminals in the socket that’s part of the generator itself? Or the terminals in the lead to the generator from the house?

 

[Rockingit]

Which terminals are you meaning if the generator has a 32a commando socket as it’s outlet?

The terminals in the socket that’s part of the generator itself? Or the terminals in the lead to the generator from the house?

In an ideal world as upstream as possible. Remember that ADS and RCD’s are both affected by this so if there’s access to a primary bus then that’s where to do it.

 

[mattg4321]

In an ideal world as upstream as possible. Remember that ADS and RCD’s are both affected by this so if there’s access to a primary bus then that’s where to do it.

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

 

[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.