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.

 

[pc1966]

There are other aspects of the care and feeding of a generator you need to consider. 20kVA is quite a bit of current (87A) so while you can get sockets for that, they would be the 125A 'commando' sort and really are a beast to wrangle, and not cheap either! For example (socket for supply end = generator mounting, should be plug for transfer switch end, so no live pins if anything unplugged and running!):

125A 2 Pin + Earth 230V Wall Mounted Socket IP67

IEC 60309 specification

www.cef.co.uk

As such a generator is anything but portable, you would be better having it hard-wired in SWA cable.

Also they need to be run regularly, say 15-20 minutes every week, to keep the engine in good condition and (if diesel) occasionally run on a high load to keep the valves and exhaust from oiling up ("wet stacking" in USA parlance).

That also helps keep the battery charged, as if that runs down you will find it won't start when you really need it! Our generator admittedly has ATS (automatic transfer switch) and a USB-network adaptor for remote monitoring, in addition to its own control panel, and that lots takes about 0.27A so in just over 6 days its 44Ah battery would be flat. Realistically we have maybe 3 days before reliable cranking in cold is going to be tough.

Usually ATS have a trickle charger to keep the battery full, even if you don't got for automatic operation, having and DC power and control lines run in to the property (separate SWA or similar) for a trickle charger and means to start it from indoors if blowing a storm outside are worth considering.

Finally fuel has limited life. You might already go through a lot of diesel and so can keep the generator tanks not much more than 6 months old, otherwise you need to think what to do. HVO or Shell GTL both have around 10 years shelf life (claimed!) but only available in bulk, and more expensive. LPG is another option but few generators available to use it, but bulk tanks are at least tricky to steal from!

 

[Chris Wilson]

There are other aspects of the care and feeding of a generator you need to consider. 20kVA is quite a bit of current (87A) so while you can get sockets for that, they would be the 125A 'commando' sort and really are a beast to wrangle, and not cheap either! For example (socket for supply end = generator mounting, should be plug for transfer switch end, so no live pins if anything unplugged and running!):

125A 2 Pin + Earth 230V Wall Mounted Socket IP67

IEC 60309 specification

www.cef.co.uk

As such a generator is anything but portable, you would be better having it hard-wired in SWA cable.

Also they need to be run regularly, say 15-20 minutes every week, to keep the engine in good condition and (if diesel) occasionally run on a high load to keep the valves and exhaust from oiling up ("wet stacking" in USA parlance).

That also helps keep the battery charged, as if that runs down you will find it won't start when you really need it! Our generator admittedly has ATS (automatic transfer switch) and a USB-network adaptor for remote monitoring, in addition to its own control panel, and that lots takes about 0.27A so in just over 6 days its 44Ah battery would be flat. Realistically we have maybe 3 days before reliable cranking in cold is going to be tough.

Usually ATS have a trickle charger to keep the battery full, even if you don't got for automatic operation, having and DC power and control lines run in to the property (separate SWA or similar) for a trickle charger and means to start it from indoors if blowing a storm outside are worth considering.

Finally fuel has limited life. You might already go through a lot of diesel and so can keep the generator tanks not much more than 6 months old, otherwise you need to think what to do. HVO or Shell GTL both have around 10 years shelf life (claimed!) but only available in bulk, and more expensive. LPG is another option but few generators available to use it, but bulk tanks are at least tricky to steal from!

Some great answers and I appreciate all the time and trouble.

This generator has what is to me a slightly unusual control panel by a very helpful company called Blandon. It constantly looks for a load on its output, and on sensing one (probably has to be resistive of maybe 100W minimum?), it self starts and after a settable warm up time energizes its output contactor and supplies power. If all loads are removed it goes through a settable cool down period and then switches off to await another load being presented.

My aim is the manual transfer switch in the MAINS position will of course present zero load, but when the mains goes down and I switch to GENERATOR, a load is seen and the gennie starts. Time will tell...

Yes, 125 Amp connectors weren't cheap! I did make sure it was impossible for a flying lead to have live male pins, but thanks for the warning, as you say, it's a LOT of current.

I intend to use a battery charger that won't overcharge, maybe a solar powered one? It's a bit too big for my needs, but came at the right price with a blown head gasket due to someone leaving a bolt out of the new water pump that goes through into the coolant jacket... Being in the race engine game it was a novelty to rebuild a slow revving, hugely heavy diesel! Perkins sure don't think of the weight or skimp on the cast iron Thanks again everyone.

 

[pc1966]

Some great answers and I appreciate all the time and trouble.

This generator has what is to me a slightly unusual control panel by a very helpful company called Blandon. It constantly looks for a load on its output, and on sensing one (probably has to be resistive of maybe 100W minimum?), it self starts and after a settable warm up time energizes its output contactor and supplies power. If all loads are removed it goes through a settable cool down period and then switches off to await another load being presented.

I have seen some generators that have the "intelligence" built in, while our one has that in the ATS panel and assumes a dumb generator.

Cool-down is especial important if its a turbocharged engine to avoid trashing its spindle bearings, but should be done anyway to avoid coolant boiling up, etc.

My aim is the manual transfer switch in the MAINS position will of course present zero load, but when the mains goes down and I switch to GENERATOR, a load is seen and the gennie starts. Time will tell...

Yes, 125 Amp connectors weren't cheap! I did make sure it was impossible for a flying lead to have live male pins, but thanks for the warning, as you say, it's a LOT of current.

I intend to use a battery charger that won't overcharge, maybe a solar powered one? It's a bit too big for my needs, but

Here in Scotland the problems is when you really need a generator in winter you can have very little average sunlight, not just low solar elevation but also lots of cloud cover. You might be best to look at a charger that comes from the same company:

The automatic generator battery charger always keeps your generators ready to start - Blandon Group

Our automatic generator battery charger trickle charges your generator's battery from the mains so that it is always full and your generator is always ready to start. It features a charger fail alarm and LED battery charge state indicators.

www.blandongroup.com

But check the generato's details to see how it is normally done. it is possible that is built in if it has some mains-sensing arrangement.

came at the right price with a blown head gasket due to someone leaving a bolt out of the new water pump that goes through into the coolant jacket... Being in the race engine game it was a novelty to rebuild a slow revving, hugely heavy diesel! Perkins sure don't think of the weight or skimp on the cast iron Thanks again everyone.

At least you can service it then!

That is another thing, our has first oil & filter change at 50 hours, then most stuff at 200 hour intervals. Typical for small engine (1.6L here) as if a car averaging 30-45mph then you are looking at equivalent of 1500-2250 miles first, then 6,000-9,000 after that. but if prolonged outage its only 2 and 8 days!