Generator earthing

[RF Lighting]

I've got a stand by generator to install soon. It's 80kVA tractor driven gen set.

The generator is to be stored in a barn, and brought out when required, so it will be fitted with a 125A ceeform socket, to plug into an inlet on the building.

This feeds a changeover switch, as does the DNO supply which can both supply the farm.

The DNO feed is TN-S, which is used for the house and some of the farm buildings, and then in the milking parlour the supply is converted to TT with a Z reading of about 2 ohms provided by 20 x 3 meters of steel mesh.

I realise I need to provide my own own earth, and I'm thinking that some 5/8" rods should be sufficient, but is there a recommended reading for the electrode?

Also, would it connect to the generator or the farm side of the plug and socket? I'm thinking probably the farm side so it can't be forgotten about, but is it ok to connect private rods to a TN-S supply?

I'm going to take a 5 core flex from the genny to the installation, and connect earth and neutral together at the star point in the genny. Is that correct?

 

[securespark]

I've never done this sort of thing before, so I'm not 100% on this. Although BGB harps on about 1667 (which actually goes over the magic 50V figure), I would aim for no more than 100, as per standard TT.

What's the ground like?

As for connecting it, I would say on the genny side. Having said that, there is a thread knocking about (can't remember the title) by John talking about connecting TT rods to a PME supply. Can't see this as any different, but whether it is the correct thing to do, I'm not sure. But as I say, I've never done this before!

 

[JohnW2]

As for connecting it, I would say on the genny side. Having said that, there is a thread knocking about (can't remember the title) by John talking about connecting TT rods to a PME supply. Can't see this as any different, but whether it is the correct thing to do, I'm not sure.

With TN-C-S it's a no-brainer, because it's already PMEd, attaching further TT rods simply adds numerically to the 'M' of PME. TN-S is perhaps slightly less obvious but, let's face it, thanks to Main Bonding Conductors, TN-S earths are invariably connected to an number of service supply pipes with Re figures far less than any TT rod - so it's very hard to see that there could be any problem adding a few TT rods.

Kind Regards, John

 

[westie101]

What I think you need to ensure is that the earth value is low enough, assuming the total loss of the DNO earth, to keep the generator volts stable and not give rise to dangerous voltages on the DNO system (earth sheath between the customer and the point of breakage/disconnection) in those conditions.

 

[JohnW2]

I realise I need to provide my own own earth, and I'm thinking that some 5/8" rods should be sufficient, but is there a recommended reading for the electrode?

As securespark has implied, I don't think what you are talking about is any different from any other TT installation, is it? If so, as he says, presumably the standard "<200&#937;, ideally <100&#937;" (and ignore the '1667'!).

Also, would it connect to the generator or the farm side of the plug and socket? I'm thinking probably the farm side so it can't be forgotten about, but is it ok to connect private rods to a TN-S supply?

As I've just written, given what's normally connected to a TN-S earth, it's hard to see that there's a problem. FWIW, your TT earth would presumably necessarily be connected to the supply TN-S earth when the genny was plugged in, since you can't have a switch in the earthing system.

Kind Regards, John

 

[RF Lighting]

The supply is by 5 wire overhead, so total loss is quite possible.

Do you see any issue with me connecting my earthing system to yours?

I've got 16 feet of rods ready for this job. Hopefully that will be sufficient, but I can always put more in if needs be. If I aim for sub 100&#937; when loop tested off the DNO supply, hopefully that should be good enough?

 

[westie101]

I can't remember the actual requirements or look them up as I'm on leave until the 28th.
Though as a comparison we would be looking for below 20ohm for the neutral/star point on a transformer on our system, so would assume the same for a generator

 

[JohnW2]

What I think you need to ensure is that the earth value is low enough, assuming the total loss of the DNO earth, to keep the generator volts stable and not give rise to dangerous voltages on the DNO system (earth sheath between the customer and the point of breakage/disconnection) in those conditions.

I must be missing something, because there's something I don't quite understand here. In the absence of any other earth reference (e.g. a distant rod connected to the L of the genny!), under what circumstances could the generator result in the potential of the genny's N-connected earth rod(s) (hence the DNOs earth 'sheath') rising to a significant potential above true earth, no matter what the resistance of the rod?

As an additional point, in practice, will not RF's rods be effectively supplemented by bonded extraneous-conductive-parts - which I imagine could be quite extensive (hence low resistance) around a farm.

Kind Regards, John

 

[securespark]

If you're talking about TT, on "your" side Westie, yeah, the regs list a max of 21.

As you say, assuming the same for a genny.

 

[bernardgreen]

The generator will be used when the local network is in a fault or failure mode so the incoming "earth" from the network could be comprised. It may in worse case be at phase potential.

Hence when designing the earthing of the generator for worse case network faults it may be considered sensible to have an earthing system on the generator that can handle the currents if the incoming network "earth" is at 230 volts above ground.

My suggestion would be to have the entire site on TT and to exclude the network earth from the system.

 

[Lectrician]

21 ohm is what sticks in my mind.

Why is the parlour TT? Nothing wrong with it being on the TN-S earth. With the mesh in that floor too, you would have a great earth, and they can be used as a rod.

 

[bernardgreen]

Why is the parlour TT? Nothing wrong with it being on the TN-S earth.

Ask the cows. A volt or two difference between the electrical earth and true ground will give them a tingle in the udders. A few transients will have them dancing.

 

[securespark]

What next? I suppose it'll be dancing ponies?

 

[bernardgreen]

What next? I suppose it'll be dancing ponies?

Didn't know one milked ponies. But seriously the problems of earthing on farms require care in the design.

Voltage gradients in the ground surrounding the rods and voltage differences between machinery metal work and ground affect animals far more than they affect humans

http://www.installeronline.co.uk/wp-content/uploads/2013/05/Down-on-the-farm.pdf

 

[Lectrician]

Why is the parlour TT? Nothing wrong with it being on the TN-S earth.

Ask the cows. A volt or two difference between the electrical earth and true ground will give them a tingle in the udders. A few transients will have them dancing.

And thats why you have the steel mesh in the floor...