Are you sure on that? My understanding is cars and caravans inside a building can have a TN-C-S supply, it is only when supplied outside that there is a requirement for TN-S or TT supply, even then although on a commercial caravan site it may need to be TN-S or TT supply, when being stored at home caravans can and in some cases should have a TN-C-S supply because there is not enough distance between it and the house to ensure a save gradient between the two types of earthing.If I want to fit a car charging point in the garage, I'm going to have to fit a rod out there to comply with the regulations on earthing, but I'm struggling to see the difference between the two.??
I'm talking about going "outside the equipotential zone"
If you have a TT supply to a caravan parked in a drive 3 foot from the house, which has the gas and electric meter cupboards built into the wall it is very possible for some one bending down to read meters to be touching the meters which are earthed to house and caravan metal body at the same time. So the earth system needs to be the same,
Or have assured separation of the two CPCs and any bonded metal work in the two properties. ( as in my cottage (PME) and the adjacent retail unit TT )with a semi-detached house the two houses again need to have same earth system, so in a housing estate again all houses should in theory have the same earthing system.
So in a housing estate where the gardens are surrounded by houses then having the TN-C-S supply to ponds, garages and sheds is not really a problem.
For houses like my mothers still with cast iron waste pipes I can see how a few volts matter causing erosion of the metal work, but as far as safety goes we consider 50 volt as the point where automatic disconnection is required.That depend on how the local network is configured, Back to back houses are likely to be supplied from different cables possibly following different routes to the substation. Their Neutrals and hence the CPCs in the houses they supply are likely to be different by a few volts.
That is in terms of electric shock hazard quite acceptable. But the fire hazard from over-heated / burning CPCs and/or bonding cables starts at much lower voltage differences. 1 volt across 0.1 ohms is 10 amps. 25 volts across 0.1 ohms is 250 amps and there are no fuses in that circuitbut as far as safety goes we consider 50 volt as the point where automatic disconnection is required.
I've always meant to ask you, but don't think I ever have ....... as in my cottage (PME) and the adjacent retail unit TT ...
why have you chosen to use the TN-C-S earth?
That may have been true at the time you had the supply installed, but as soon as you had the supply you could have disconnected the TN-C-S earth from your MET and replaced it with a connection to your own earth rod(s), couldn't you?Six years ago it was the quickest option to get the power connected to the cottage. The cable from the substation is ( according to the DNO ) not very old and low risk of being damaged so I may stay as TN-C-S. Convincing the DNO that my ground rods were adequate could have been a problem to delay to installation.
MET and replaced it with a connection to your own earth rod(s), couldn't you?
Fair enough.Yes but there were more pressing things to sort out, like floors and the plumbing. The othe concern was not knowing just where were the water supply pipes that would need to be avoided. maybe one day I will get round to it.
The only concern of real importance is that the retail unit's water supply still comes from my cottage, ( they were once both owned by the butcher ) and there has to be a metre of "isolation" ( plastic pipe ) between my copper pipe work ( which is bonded to my PME MET ) and their copper pipework which I assume is bonded to their ground rod. When I bought the cottage the electricy supply was from the retail unit's CU, about 30 metres of 4 core pyro feeding two CUs in the cottage.I presume that (as hinted in your previous post) you do not really have any appreciable concerns.
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