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Domestic Earthing and RCDs

Postby davembk » Sat Aug 17, 2013 6:17 pm

I'm not an electrician, just curious and I know I'm just missing something in my understanding..

I've had an rcd tripping and I'm doing all the usual fault finding to eventually sort it out.
So I'm not worried nor asking for help with this.

But I've been doing some reading about rcd's and domestic earthing systems and have got stuck trying to understand a particular point...

My earthing system looks like it's a TN-C-S and think I've proved it as I can buzz between Earth and Neutral ie they are connected together. Looks like the incoming cable sheath is both the neutral and the electricity company earth.

This is where I come unstuck...I've also read that an rcd senses a current imbalance between Live and Neutral, under a fault situation, when some current leaks from live to earth so that current in the live line becomes different to the current in neutral line, less I assume since some has gone to earth.

I can see that happening if the earth is separate from the neutral, but I can't understand how this works if the neutral is also the earth, because any load. whether an iron, or a person, putting current back to neutral would put it back to the earth as well.

So for someone standing in barefoot on a wet floor, how could they trigger a live neutral imbalance in the rcd, as any current going through that person to earth, albeit the ground and not the sheath of the TN-C-S cable, would also be going to the neutral, and there would be no imbalance to detect.

Hope this makes some sense,

Thanks, Dave.
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Simply Build It

Postby ericmark » Sun Aug 18, 2013 3:30 pm

OK to start with I will explain the letters.
TN means the earth is supplied through a cable.
TT means the earth is supplied through the ground.
The TN is further divided and
S means separated so a separate cable carried the earth.
C means combined so it uses the same cable.
So TN-C-S means at some points the earth and neutral use the same cable but before it reaches you it is separated again. Often this is the same as PME which means the neutral and earth are combined and loads of earth rods are used through out its length so should the combined neutral earth be broken there is a limit to how much the voltage can fluctuate.

So if you switch off you main isolator then check between earth and neutral you will find they are not connected.

It is near impossible to tell if a TN-S or a TN-C-S at the DNO head with some supplies and the only way is to phone the DNO and ask them. Since the earth loop impedance is limited to 0.35 ohms with TN-C-S and 0.6 ohms with TN-S using an earth loop impedance meter often one has a good idea which it is. In the main it does not matter. There are a few things where TN-C-S is not permitted.
Caravan Supply.
Petrol Station Supply.
Boat Supply.
Electric car supply if not in a garage.
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