Generator earthing

[JohnW2]

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.

You seem to be postulating an extremely improbable fault (or combination of faults), particularly on a TN-S system (which is what we are talking about). However, if it ever did happen that a TN-S earth rose to 230V above ground, I doubt whether any normal TT earthing system would have much impact on the situation!

In any event, this isn't the hazard which is being discussed. The concern which has been voiced is that the generator supply could somehow (I've yet to really work out how) cause the potential of the local TT earth (hence the DNO's earth conductor on the consumer side of a break) to rise to appreciably above earth potential, thereby representing a hazard to DNO personnel.

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

Is this just a general statement about electrical installations, because I don't see how (in relation to your voiced concern) the existance of the generator makes any difference to anything.

Kind Regards, John

 

[bernardgreen]

You seem to be postulating an extremely improbable fault (or combination of faults), particularly on a TN-S system (which is what we are talking about).

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

With that and a strong wind anything is possible. A broken earth wrapped round a phase is not impossible.

The generator doesn't make any real difference provided it supplies only the TT system in the parlour OR the PME system in the house. If it supplies both at the same time then it has to be arranged that the PME earth is not connected to the TT earth. It would work supplying both if the generator has its own earth rods but then a live earth fault in the parlour ( TT ) would create a voltage gradient between the TT earth system and the generator's earth. That could be fatal to animals in the area of the gradient

 

[JohnW2]

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

With that and a strong wind anything is possible. A broken earth wrapped round a phase is not impossible.

Agreed, not impossible, but one hopes that some DNO protective device would then operate. However, as I said, in the case of any such low impedance connection between the supply's L and E which didn't operate a protective device, I really don't see that any likely TT earthing system would have an appreciable impact on the situation!

The generator doesn't make any real difference provided it supplies only the TT system in the parlour...

My understanding is that the parlour's TT system is isolated from everything else.

OR the PME system in the house.

As I said, it's TN-S.

If it supplies both at the same time then it has to be arranged that the PME earth is not connected to the TT earth.

See above.

It would work supplying both if the generator has its own earth rods but then a live earth fault in the parlour ( TT ) would create a voltage gradient between the TT earth system and the generator's earth.

As above, my understanding was that the parlour's earthing system was totally isolated from everything else. Indeed, were that not the case, then RF would presumably not be worrying about the resistance of his rods, since the generator would already be enjoying the 2Ω earth of the parlour's mesh.

Kind Regards, John

 

[RF Lighting]

Yes that is correct the parlour's earthing system is completely isolated and covers the parlour only.

In the parlour, the cows are stood on wet ground, with machinery attached to their udders. The cows can feel even a few mV and whilst it may not be harmful to them, they will not give their milk, so it is essential that there is no PD at all in a milking parlour.
Keeping the earthing system TT and more or less bonding every thing to everything else is the best way of acheiving this. It doesn't matter what happens outside of the parlour as you effectively create a good old Faraday cage.

My main concern is ensuring that any earth fault on either the domestic or especially the TT parlour installation will have a low enough impedence back to the generator star point to ensure ADS.

 

[JohnW2]

Yes that is correct the parlour's earthing system is completely isolated and covers the parlour only.

Thanks for confirming - that's what I understood you to be saying, and it makes total sense, for the reasons you go on to give.

My main concern is ensuring that any earth fault on either the domestic or especially the TT parlour installation will have a low enough impedence back to the generator star point to ensure ADS.

As I said, from that point of view, I don't see the situation as being any different from any other TT installation. Unless you went to ridiculous lengths with the earthing system, you'd obviously not get ADS from OPDs, so you would be dependent upon RCD protection. As you know (with a 30mA RCD), for that 1667&#937; is theoretically adequate, but we always aim for <200&#937;, preferably <100&#937;.

However, westie has introduced a different concern. Although, as I've said, I don't really understand the postulated mechanism, he seems concerned that your genny could somehow result in the potential of (your side of) the broken end of a DNO earth conductor rising to a dangerous level above true earth, thereby endangering DNO personnel, and seems to be suggesting that you need a TT earth with a resistance less than 20&#937; (or 21&#937 to minimise that risk. Since I don't really understand any of this, I can't really comment!

You might want to ascertain the resistance (or EFLI) of the farm's earthing system to earth with the TN-S earth disconnected. As I've suggested, you might well find that it is already low, because of bonded extraneous-c-ps. Whilst you might well not feel that you can 'rely' on that, the knowledge of its existance might well increase your (and maybe even westie's) confidence that you don't need to fall over too far backwards in designing/installing your TT earth for the genny.

Kind Regards, John

 

[RF Lighting]

As I understand it, the generator star point needs to be effectively tied to true earth for two reasons.

The first being to provide ADS especially with a TT system as we have discussed at length, and the second being to prevent ground potential rising to that of one of the phases (or at some point of that potential) due to a fault. A second fault could cause a much more severe shock than if the ground is at star potential.

There are some bonded extraneous parts on the TN system, but I would much prefer a dedicated electrode personally.

Also there is a 100mA protected submains in the parlour, so I'm going to need a theoretical maiximum of 500&#937; rather than 1667&#937;.

I'm now aiming for 20&#937; to play it safe.

 

[Spark123]

I think Westie said before, you need to have an earth electrode with <21 &#937; to earth, you need to use a 3 or 4 wire electrode resistance tester for this.
Your generator will provide a TN-S system which shouldn't be a problem connected to the existing earthing system, as long as the new electrode is <21 &#937; in isolation. The new electrode provides the means of earthing of the system i.e. what puts the T in Tt, Tn-s etc. Double check with the DNO though. If they won't allow it then one option springs to mind, TT the existing installation.

The fact the milking parlour is TT relies on the new electrode as part of the fault path but this one can be <200&#937; measured with a loop tester. Just don't test with the cows in there as you may end up with them dancing.

 

[JohnD]

when i was working in the electricity supply (and generation and distribution) industry, I had to read the Electricity Act from time to time. It wasn't my concern, but I remember it specified that independent supplies must be isolated so that they cannot backfeed into the network, for example when the standby generators cut in after a digger incident.

There must be some rules and model specifications somewhere that would define what must be done for a compliant installation. Probably the supplier would have a standard document.

 

[Spark123]

The change over switch provides isolation of all live conductors, it wont be that easy to achieve full isolation including protective conductors keeping the current TN-S arrangement. As long as it complies with BS7671 and isn't run in parallel back feeding the network then this will apply:

Reg 21 of ESQCR:

Switched alternative sources of energy
21. Where a person operates a source of energy as a switched alternative to a distributor’s
network, he shall ensure that that source of energy cannot operate in parallel with that network
and where the source of energy is part of a low voltage consumer’s installation, that installation
shall comply with British Standard Requirements.

 

[JohnW2]

As I understand it, the generator star point needs to be effectively tied to true earth for two reasons. The first being to provide ADS especially with a TT system as we have discussed at length, ...

Indeed. I primarily responded in terms of that because, in your last post, you said that was your 'main concern'.

...and the second being to prevent ground potential rising to that of one of the phases (or at some point of that potential) due to a fault. A second fault could cause a much more severe shock than if the ground is at star potential.

All true, but would that not be equally true in the absence of the generator (and its earthing system), in the scenario we seem to be contemplating of the DNO earth being broken?

There are some bonded extraneous parts on the TN system, but I would much prefer a dedicated electrode personally.

As I said, that's what I'd expect, but knowledge that there was a low impedance 'backup' path to earth via extraneous-c-ps would provide some extra 'reassurance'

Also there is a 100mA protected submains in the parlour, so I'm going to need a theoretical maiximum of 500&#937; rather than 1667&#937;.

Fair enough - but, as I said, one would normally be aiming for <100&#937;, anyway. When you say "submains in the parlour" to you mean 'submains to the parlour'? I would have thought that (essentially by definition) any potential fault path in the parlour woud be to its own 2&#937; earth.

I'm now aiming for 20&#937; to play it safe.

Fair enough - can't do any harm, even if its not (and I'm not saying that it's not!) strictly necessary.

Kind Regards, John

 

[securespark]

you may end up with them dancing.

http://www.youtube.com/watch?feature=player_embedded&v=5KaltD0Z6ww

 

[SimonH2]

Looks like a herd just let out of the building, or out of a field with no grass, into a new one with fresh grass. Most people think of cows as slowly ambling about, but when excited they can certainly get a shift on - as the video shows.
Of course, to see they really shift you need a few gadflies about.

Cows are actually quite intelligent, and certainly quite inquisitive.

 

[RF Lighting]

They're a bloody nusiance!

 

[securespark]

They are if they get in your way...

 

[ericmark]

I worked on one job where we installed our own earth mats and rods with large standby generators seem to remember around 2 mega watt each and the DNO earth and the site earth was connected through a huge resistor.

As to why I am not really sure. I would guess so that under fault conditions one would not get enough voltage to cause sparks as this was a gas terminal.

But it could also be to limit current between both systems. I know I sank around 200 rods all with readings of less than 8 ohms and the mat had four rods at less than 8 ohms joined by copper tape so once back filled would be looking at likely less than one ohm.

The farm will likely also have a very low reading with all the re-bar used in the cow shed concrete being bonded to the earth. Again likely less than one ohm. Under fault conditions massive currents can flow between the two systems. So it is possible a resistor will be required.