20kVA single phase generator grounding question

[Chris Wilson]

How should grounding be made to a 20kVA single phase generator set feeding a domestic property via a manual change over switch? At the generator's alternator neutral is bonded to earth. The generator will be some sixty feet from the mains input box of the property fed by SWA cable. The property's mains is a "protected multiple earth system" according to the sticker in the outside box.

Thanks.

 

[pc1966]

You have to have a separate means of earthing, i.e. an earth rod, as under loss of supply conditions you can't assume the supplier's earth will still be present. Note this does not make it a TT system, it is still TN-S due to the generator's N-E link internally, it merely references that to the Earth itself.

Also you are not allowed to link N-E within the installation, so any change-over switch must switch both L & N between the two sources.

The risk some worry about is the connection of a large metal object to the CPC of a TN-C-S system due to the small risk of an open-PEN fault making it live. One way to help mitigate that is to make the means of earthing a mat or set of wires under the region of the generator so the voltage between it and the soil immediately next to it is kept down. A simple option would be to put it on some sort of insulated platform, but given a 20kVA generator is not far off the weight of a small car that needs a bit of thought.

 

[pc1966]

Also the bonding from your means of earthing to the installation has to meet the size for PME applications so 10mm copper or equivalent. That would be 80mm for steel which most likely the SWA will not meet. Going 3C 25mm or similar is one easy an option, or possibly cheaper just to run 10mm in parallel.

 

[Lucien Nunes]

It is still TN-S due to the generator's N-E link internally, it merely references that to the Earth itself.

Entirely true and an oft-misunderstood point (not perhaps by the OP but by many who talk of generators and earth rods.) I am going to go even further and state explicitly that it is TN due to the N-E link creating a neutral by earthing one of the live conductors, and then specifically TN-S if there is an earthing conductor present from that link to the MET of the installation, as there invariably is.

 

[Chris Wilson]

Is there a valid reason not to use a galvanised scaffold tube as an earth rod, inserted perhaps six feet below ground level with decent clamping and weatherproofing for a 25mm squared pure copper cable ground wire to the generator earth termnal?

My thanks for the replies, they are appreciated!

 

[James]

I can’t think of a reason why that would be a problem.

 

[pc1966]

Is there a valid reason not to use a galvanised scaffold tube as an earth rod, inserted perhaps six feet below ground level with decent clamping and weatherproofing for a 25mm squared pure copper cable ground wire to the generator earth termnal?

I can't see any problem with buried galvanised scaffold tube in ground that is not especially acidic. Should be cheap and effective.

Copper to zinc (the galvanised layer) contact will accelerate corrosion of it so you would need to exclude moisture from around that joint. Often the top of the rod has an inspection box so it is not permanently wet, and you can use Vaseline or dedicated contact greases (e.g. Noalox) to help with corrosion.

However, you still need the 10mm copper equivalent from the generator back to the installation, as any open-PEN fault currents from the supply would flow that way to your rod.

 

[davesparks]

Is there a valid reason not to use a galvanised scaffold tube as an earth rod, inserted perhaps six feet below ground level with decent clamping and weatherproofing for a 25mm squared pure copper cable ground wire to the generator earth termnal?

My thanks for the replies, they are appreciated!

Well its a lot more effort to sink a scaff tube far enough into the ground to be effective than it is a copper bond earth rod!

 

[pc1966]

That is a good point, you would almost certainly find it easier to use a conventional rod and the SDS-drill like accessories to make driving it in easier:

Earth Rod Pro Drill Bit (Pack of 1)

Allows rapid earth rod installation when used with Earth Rod Pro drivers - sold separately [1LERPD] and [1LERPM]

www.cef.co.uk

Earth Rod Pro SDS+ Driver

Allows rapid earth rod installation when used with Earth Rod Pro drill bit - sold separately [1LERPB]

www.cef.co.uk

Those let you turn the standard 16mm copper-clad steel rod in to a giant drill bit in sections that can be extended in 1.2m sections to get your depth.

Of course you should check what is below before drilling a rod so you avoid service pipes and cables!

 

[Chris Wilson]

Thanks for the further replies. I have a smallholding, and am also into amateur radio, so use the post knocker on the tractor to sink scaffold tube earth rods, there are dozens of them around the place

I have another naive tech question if I may?

OK, the generator is grounded from its dedicated earth terminal to a long earth rod via a permanently connected hefty copper cable.

The cable from the gennie to the change over switch in the meter box outside has live, neutral and earth conductors. The change over switch changes both live and neutral, isolating both from the incoming mains side when the switch is in the generator position.

If the gennie cable is left plugged in all the time does its earth wire just stay connected to the meter box earth cables? Or does the gennie cable earth need isolating from the meter box earth?

I have had a few swerves from local electricians when I have mentioned installing the changeover stuff fpr a 20 kva gennie, or asked similar questions, so want to learn all I can myself to make sure it's being done correctly and safely. Thanks!

 

[James]

The earth cable SHOULD NOT BE SWITCHED.
all bonding and earthing cables should be permanentry connected.

the only way it should be possible to disconnect would be to unplug the generator and thus disconnecting all live cables as well as the earth. (This is also why earth pins on most plugs are longer than live or N) it ensures the earth is the last thing to disconnect.

 

[Chris Wilson]

The earth cable SHOULD NOT BE SWITCHED.
all bonding and earthing cables should be permanentry connected.

the only way it should be possible to disconnect would be to unplug the generator and thus disconnecting all live cables as well as the earth. (This is also why earth pins on most plugs are longer than live or N) it ensures the earth is the last thing to disconnect.

So whilst the supply cable from the generator to the meter box is plugged, even if the changeover switch is on the mains setting, I am now adding another separate earth to the incoming mains supply, and that's OK? Thanks James.

 

[James]

In my opinion it is fine, it is no different to bonding some structural steelwork that is bolted to foundations or buried in the ground. Or a copper water pipe going into the ground.

 

[davesparks]

I am now adding another separate earth to the incoming mains supply, and that's OK?

OK? It's better than OK, it's an ideal situation in my opinion. If you can get a good, low resistance, earth electrode properly connected to your installation then it can help protect you from certain types of network faults.

The regulations are slowly creeping towards a point where they require earth electrodes connected to every incoming supply.

 

[pc1966]

As above, adding a good low impedance local earth is a good thing in practically every way.

If you ask practically any electrician you will get the same reaction as @James since an open CPC/earth is often a factor in many fatal accidents over the years. Most of the regulations about earthing are at pains to make sure it is designed and implemented not to disconnected by accident, or by the effects of corrosion, etc. Having said that it is allowed to disconnect a generator earth, but anything doing that must meet various aspects, and no electrician is going to sign up to that unless it is a pre-built box already certified to meet the specifics (regs 537.1.5 cited by 543.3.3.101).

TL;DR Generally a bad idea.