Garden pump on PME
- Thread starter Mecca
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Yes, it's fine.
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"
I'm talking about going "outside the equipotential zone"
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Metal that is "earthed" by coinnection to a PME "earth" can be at a potential several volts different to the potential if the ground. THe PME "Earth" is the Neutral of the incoming supply.In the event of a fault in the lcal supply network the potential on the incoming Neutral can become very high relative to the ground.
Inside the equipotential zone of the house there is no true ground ( there are exceptions ) so the "Earth" potential inside the house being different to the potential of the ground is not a hazard as a person cannot ( in theory ) touch "Earth" and ground at the same time.
Taking the "Earth" outside the equipotential zone of the house allows a person to be in contact with both "Earth" and ground at the same time. The difference in potential should be small enough to only provide at the most a mild tingle to a person in contact with "Earthed" metal. If there is a fault in the supply networj then the potential is there for more than just a tingle and possibly a shock resulting in injury or death.
A pond pump with "Earthed" metal in contact with the water will put the water at a different potential to the ground. A person kneeling on the ground with fingers in the water may experience mild tingles If the water is also in contact with ground then there will be electric current flowing in the water which will be uncomfortable for any fish in the pond.
Hence my opinion is that the best option for a pond pump ( and a car being charged ) is a TT suppy with an effective ground rod to earth the pump and an RCD on the supply.
Inside the equipotential zone of the house there is no true ground ( there are exceptions ) so the "Earth" potential inside the house being different to the potential of the ground is not a hazard as a person cannot ( in theory ) touch "Earth" and ground at the same time.
Taking the "Earth" outside the equipotential zone of the house allows a person to be in contact with both "Earth" and ground at the same time. The difference in potential should be small enough to only provide at the most a mild tingle to a person in contact with "Earthed" metal. If there is a fault in the supply networj then the potential is there for more than just a tingle and possibly a shock resulting in injury or death.
A pond pump with "Earthed" metal in contact with the water will put the water at a different potential to the ground. A person kneeling on the ground with fingers in the water may experience mild tingles If the water is also in contact with ground then there will be electric current flowing in the water which will be uncomfortable for any fish in the pond.
Hence my opinion is that the best option for a pond pump ( and a car being charged ) is a TT suppy with an effective ground rod to earth the pump and an RCD on the supply.
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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.
I have not installed a vehicle charging point, but I understand at home it does not need to be TT, it depends where parked.
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, 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.
I have seen the result of having a radio shack in a garden with to be frank an OTT earth for the radios 4 rods connected together with buried earth tape, connected to the house supply using a 4 mm single tie wrapped to the SWA for the earth connection to the house. Road works caused the neutral to be broken on the TN-C-S supply and when I got there the 4 mm cables was copper globules on the floor. How much current ran through that earth wire is any ones guess, I personally think the electrician should have never taken to house earth to the shack (shed) the poor guy was just standing there watching it happen, he had turned off the main isolators on his consumer unit to no avail the earth wire was still melting. He called me on 2 meter and when I got there and started to measure voltages on the incomer to isolator it was swinging from zero to 400 volt. I rung up the DNO and they said I was mistaken and this could not happen. But the power then went off and they worked on the supply and compensated all the houses in the street for burn out equipment, except for his earth cable which they said could not be caused by the fault. His whole house was made TT after that, may be it should not have been, it is down to the DNO as to what earth system is provided, but after what happened no one was going to push the issue.
In most cases an earth rod would not be good enough to take that current, where you have a real danger is where the earthing is through a gas pipe, if every house in street has gas then shared between all houses and current limited, today I believe there is an insulator in the gas supply to stop the gas pipe acting as an earth.
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. Only when the out building is 1000 yards into your private woodland do you really need to consider using a TT supply, or of course when you are transmitting at 400 watt on 80 meter band.
Gas meter readers on the ground touching an external meter that is bonded to the PME "earthed" CPC inside the house are at risk of a tingle. One meter reader I knew wore gloves to avoid the tingles ( or worse ) between ground (or damp wall) and meter,
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 )
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.
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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 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 circuit
I've always meant to ask you, but don't think I ever have ....
... given your concern about TN-C-S 'earths', and particularly given the complication of an adjoining TT installation, why have you chosen to use the TN-C-S earth?
Kind Regards, John
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.
I am "concerned" about TN-C-S supplies due the the experiences at my previous house. From the substation there was 600 yards of ancient underground cable supplying the High Street and then a further 100 yards of ancient overhead to a pole at the end of our drive which fed 6 houses. 50 yards along the overhead was a drop to the village sewage pumping station. Neutral bounce was significant most of the time and two ( or was it three ) cable faults under the High Street gave major problems. Got better when the sewage pumping station got it's own 11kV supply and transformer
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?
Kind Regards, John
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.
Fair enough.
It obviously depends upon how your prioritise any concerns you have about the TN-C-S earth in relation to other tasks. Given that you have let the situation persist for 6 years, I presume that (as hinted in your previous post) you do not really have any appreciable concerns.
Kind Regards, John
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.
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