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Topic Title: "give PME the boot"
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Created On: 17 July 2011 02:14 PM
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 18 July 2011 11:19 AM
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whjohnson

Posts: 750
Joined: 24 January 2009

What if the Type S is faulty too?

After all, we all know that every single RCD in the UK is regularly tested as prescribed by both the manufacturers instructions and other regulations don't we.

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Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
 18 July 2011 11:24 AM
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ericmark

Posts: 319
Joined: 12 February 2008

Where a TN-C-S supply feeds loads of houses all with their gas and water bonded and in metal there is not really a problem. But where the gas and water arrive in plastic then should the neutral be lost very high currents can flow to a solitary earthed item in the estate. I saw the results where a radio ham had sunk earth rods in the four corners of his garden and liked them with bare copper as a ground for his radio. The electrician installing the supply had bonded this with 4mm cable which melted when neutral was lost.

I would think this is very rare. And the poor radio ham was pulling out his hair. He had seen it get hot and switched off his whole supply to no avail it continued to heat up and finally melt. Good point he unlike rest of street did not lose any equipment.

Before that I had never considered an earth could be too good. But after I noted the huge resistor installed between the gas plants earth which I worked on and the suppliers earth. I would assume it was to stop what happened with the radio ham.

But as to fitting earth rods as well as TN-C-S then if all houses have the rods that would work. But not if only a selected few. So the only way it could be done is for the DNO to install the rods. My understanding that is what PME is?

The second problem is where properties are close to each other then extraneous-conductive-parts on one may be within reach of extraneous-conductive-parts of another. So it is important for the earthing system to be the same. With a TN system there is no real problem but with a TT system then one house with a fault could be up to 400 volt different to next house also with a fault on a different phase. Although all TT supplies should use RCD's these could be well over the 30ma considered as safe limit. So it is down to a risk assessment as to which is safer system and it would in the main depend on things like earthed metal waist pipes and the like.

My dad's house was TT and the connection to the rod was lost. Now TN-C-S but walk around the house and every metal drain has been eaten away which I put down to loss of the earth.

With boats we put diodes in the earth cable to stop this but not seen that with houses. Maybe we should fit diodes between the extraneous-conductive-part and the supply earth?
 18 July 2011 11:25 AM
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postman

Posts: 56
Joined: 03 July 2011

Originally posted by: whjohnson

What if the Type S is faulty too?



After all, we all know that every single RCD in the UK is regularly tested as prescribed by both the manufacturers instructions and other regulations don't we.




True but there is only so much you can do to make an installation safe.

I mean if all the RCD's are tested and the installation is handed over to the client with full instructions on how to test these RCD's and they don't bother I mean where do you draw the line?

My point is with a TT you are the one who has control over faulty equipment whereas with PME you have no control, you are signing your protection over to big companies who say they do regular checks on lost neutrals.

At least with a TT system you are more in control.

You have no risk of your metal work becoming live due to a network fault and all you have to do is test your RCD's by pressing the test button every so often.

Personally I prefer being in control.
 18 July 2011 12:14 PM
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ericmark

Posts: 319
Joined: 12 February 2008

I would say it depends how close your extraneous-conductive-parts are to your neighbours extraneous-conductive-parts. From what I am told it is not our call. The DNO has to decide what earthing system we should use not us. And that is because of the problem of neighbours earth being a different voltage to your earth. I have used the fact to my advantage. With a house that had a TT supply before 2008 when only TT houses had all the supply on RCD I would phone up the DNO and ask what the earthing system was. Often they would have to send some one out to look. When they arrived I would tell then "I'm sure next door is a TN-C-S and 9 times out of 10 they would then give me a TN-C-S supply without any extra charge.

So where you live in a house surrounded in your own grounds so no one can build close to your house then to use TT may be an option. But where we live in little boxes all the same then we just have to do as we are told.

Although I would agree that even the authorities see the problems with TN-C-S and have banned it's use with Petrol stations, Caravans, and boats. Unless you are going to use an isolation transformer (As used with boats) then you often have no option.
 18 July 2011 12:17 PM
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postman

Posts: 56
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Originally posted by: ericmark
Unless you are going to use an isolation transformer (As used with boats) then you often have no option.


Are we talking big bucks for this sort of equipment?
For a domestic installation.
IET » Wiring and the regulations » "give PME the boot"

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