Generator earthing

[kellys_eye]

What's the word on this? Earthing (rod) at the genset AND the property (consumer unit) or consumer unit only? Assuming genset (25m from property) is cabled straight to the property and fed to the cu via a change-over switch.

[ericmark]

It would depend on the other supply, if a TT supply to start with then the same earth rod is used, if a TN supply then an earth rod can be too good, so needs some care. At around 60 ohms there is no problem being connected with both supplies, at one ohm likely an earth resistor is required to limit current to earth rod.

Sinking an earth rod can also be a problem, don't want to hit other services, so often at the generator mainly as sure no other services near by.

[Nos]

The genny will be earthed via the house as you are connecting the three house wires to the genny, with all the gennys I have had all of them have been earthed never a problem apart from the last one . I use a metal tent peg as the earthing point so only in 4/5 inches, not deep enough for services, but my biggest genny's were 10 kwt so bigger ones may need a different approach.

[ericmark]

With a TN supply often the user has no idea if TN-C-S or TN-S and even with TN-C-S which was called PME the protective multiple earthing, may not have an earth close to the home, so with a power cut the user has no idea if there is still an earth.

Although we have RCD protection today, so as long as the earth is less than 200 ohms it is considered good enough, in the main earth is not switched, we are tending to switch the earth in some TN-C-S supplies for the feed to EV charging points, as if the PEN is lost, the earth connection can become live, but in the home like the bird sitting on the power cables this is not a problem as everything is bonded so all is at same voltage even if that voltage is not ground potential, where the problem lies is when outside and there is a voltage between true ground and the earth bonded metalwork, so you can get a shock from the car.

So what is being done is the voltage is being monitored, and if between 207 and 253 it is considered OK, if it goes out of that range it is assumed the PEN may have failed and first the lives (line and neutral) are switched off and then the earth.

What needs to be remembered is likely a generator is used when the DNO supply fails, so there could be a broken PEN, so it may be required to disconnect the DNO earth.

The PEN is a combined earth and neutral, for the heavy cables used for the supply to your house it is permitted to use a combined earth and neutral, but not in your home, however copper theft and road works has from time to time caused the PEN to be lost, since our supplies normally come from either three phase or a split phase supply, the neutral/earth wire the PEN can get to the voltage of any phase. So in theory you can get 460 volts between line and neutral if your drawing no power and the other phase of a split phase is drawing power.

In the UK we don't have many split phase supplies, most are three phase, and it is unlikely one phase has no power being drawn, so unlikely we would ever get the full 400 volt, but can get over 50 volt which below 50 volt is deemed safe (70 volt with EV charging).

However we need to be aware of the danger and to connect the DNO earth to a generator some 25 meters from the main house unless bonding is done correctly could mean when going to start the generator during a power failure you could get a nasty shock.

If the supply is TT then there is no problem, and TT is very common in France but not so much in Oban, so we have to consider how the home is now earthed before we talk about extra earths.

This IET PDF may give you further understanding of the problem, it is not really a DIY job, the regulations use words like "consideration" in other word you use ones knowledge and experience and don't blindly follow the rules.

[kellys_eye]

We have a weird wiring arrangement here..... we're fed by an overhead that used to be concentric cable but was changed to two-twisted cores that still connect (on the pole) to the concentric coming into the premises! So, AFAIK it's a TN-C-S arrangement (no earth rod on the premises).

The generator is 25m away, remote from anything and especially any services so an earth rod could be used at the genset if necessary. The genset feeds to the house (not yet connected internally) via a 3-core SWA cable and is intended to go via a 125A change-over switch to the c.u. Note - the whole house is being assessed for a full rewire so anything that needs to be done needs to be planned in advance hence the queries. I'm doing a 'list' of what I want done.

[ericmark]

Since you say full rewire, then need to consider EV charging in the future, and any other near by property. 10 years ago I would say use DNO earth where provided, today not so sure, the EV scene has changed that.

There is a problem with TN-C-S in that on the rare time the PEN (name for the boards combined earth) is lost unless some thing disconnects the earth connection anything earthed can become live. This was not considered a problem like the birds on a live wire, in the house you will not get a shock, only outside where there is a true earth, and most garden stuff is class II so the chance of anyone getting a shock was slim, it is reckoned around 400 PEN faults per year and 10% cause shocks.

But with EV cars the problem is one car is on charge for an extended time, and two most charged out doors, and the methods used with a TN_C_S supply should limit shock to 70 volt but is expensive, so with possibility of EV in future may be better to go for TT.

But there is also a danger if TT and TN-C-S systems are close together, so in the main the DNO stipulate what earthing type, so two houses right next to each other don't have different earthing system. So it depends on what is around you as to if you can have TT. So really need to talk to guy doing the re-wire.

The problem will TT is you are relying on the RCD, but risk assessment with a stand by generator think I would lean towards TT.

[kellys_eye]

No chance of any EV's happening here. The supply to the premises is limited to 6.5kW as it stands. We don't plan on leaving and we don't want (or can afford) and EV anyway. I wouldn't buy one even if I could - I'd go for a Hybrid before anything else.

There is no house near us (not within a 1/4 mile one way and a mile the other) so neighbouring properties don't count either. Looks like a TT solution to me so a rod at the genset then.

[ericmark]

Yes TT makes it easy.