PME changes

[RickH]

Information question.
My house was on a local earth rid as on temporary 30A supply.
Had it changed to permanent supply and PME.

From the house, I have an RCD type A protected armoured cable feeding an outbuilding, and this was changed so the house earth is exported to that building (was originally on a local rod)

There is an onward fed 32A feed to a Hot Tub, this does not export house earth, having it’s own local rod
All tested and Part P signed off.

During the change to PME, it was found that the outbuilding rod had too much resistance, didn’t matter as earth now being exported … electrician advised not uncommon due to corrosion.

Is it a consideration to export an earth to the Hot Tub, as well as leave local rod connected ?

Obviously a concern if the local rod started building up resistance, I’d lose protection of local earth, and be unaware of it?

 

[ericmark]

There are a number of earthing systems, TN, TT, and IT. And the TN is further split into TN-S and TN-C-S and the user may well be unaware of which system he/she has. The major problem is loss of PEN with the TN-C-S system, and with that in mind, bonding becomes important. The bird can land on 3.3 kilovolt without harm, so our aim is not so much to have everything earthed, but to have all which can be touched at the same time, at the same voltage.

So a bath of any type, in a room where all is bonded, is reasonably safe, even if all at 230 volts to true ground, there is not really a problem, called equal potential bonding.

Where the problem lies, is where there are two systems within touching distance, be this a tin shed, or hot tub does not really matter, if I am standing on earth, and I grab a handle of a metal shed, caravan or any similar structure which is at say 100 volts different to the earth I am standing on, I will get a shock.

I suppose no electric system is safe, but we have to assess the risks, we seem, for some reason, to assume sheds are made from non-conducting material the same as a house, bricks don't conduct electric very well.

First point is an RCD does not help, as the earth is not switched by the RCD, and neither does hammering in earth rods willy-nilly, as we have no idea how close they are to earthed underground services.

But once on site, we have a reasonably good idea where services likely run, in my home, I am reasonable sure there are no services in my back garden, and it is reasonably large, so a TT supply to the hot tub is unlikely to be a problem. But my front garden is a completely different kettle of fish, there is likely water supply, and electric supply under the ground, my deeds may show the planned route, but not idea if that is followed, so likely need to hire a CAT to find out where it is safe to place an earth rod.

Point is, only someone at your home can assess the risk, this is not something which can be worked out by answers on a forum.

 

[RickH]

The earth rod was installed on bare land … no risk of hitting services.
Impedance was tested by the electrician before the connection was made.
The install was completed & tested.
So all the decisions regarding where & if an earth rod can be installed have been made.

The question I was looking for information on, is it a consideration to export the PME earth, in the event that the local earth could corrode and go high impedance and I would not know.

 

[AndyPRK]

An idea.

Stick a 60w bulkhead light on the wall by tub.

connect between TT and PMe earths.

This will allow them to be connected via resistance and hopefully keep things safe.

If the light starts glowing, you know you have a PD.
Light off = no rod connection
Light on = big imbalance

Love to hear ppls thoughts!!!

My guess is it would glow a lot.
Should have 2 bulbs for redundancy really, but interesting to test with 1 to start with

 

[ericmark]

The 10 year EICR will pick up any faults in time. Do not connect a TN-C-S earth to the hot tub in any way. Leave it at TT that is far safer.

 

[jj4091]

The 10 year EICR will pick up any faults in time. Do not connect a TN-C-S earth to the hot tub in any way. Leave it at TT that is far safer.

Not many people have them done Eric.

 

[AndyPRK]

one risk (with pme) is with a broken pen conductor.

One leg could be in the hot tub at 240v
Your other leg on the ground at 0v

And you would get mains voltage across u

 

[ericmark]

I have put the link to broken pen it seems the annual numbers are increasing. And there is concern that it may go some time, before the fault is found, I know I have TN-C-S from the DNO and also have an earth rod for the TN-S supply should the DNO supply fail.

If every house has an earth rod, then will the loss of PEN detectors work? My solar panel inverter will disconnect if the voltage is out of range, and there is no reason why my smart meter should not also tell the DNO, but how fast this would happen, and if in time to stop any damage to equipment or personal is not so cut and dried.

I saw it in Algeria, but in the UK only once, and it was quite dramatic. The earth wire to friends radio shack was reduced to short lengths of copper on the floor.

One reason for loss of PEN is copper theft, and it seems reports of fires as a result miss out the fact that if TN-C-S had not been used, the problem would not exist.