Domestic Earthing and RCDs

Postby davembk » Sat Aug 17, 2013 5:44 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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Postby kbrownie » Wed Aug 28, 2013 9:40 am

What you have is an RCD that detects an imbalance between L & N on the load side of the RCD.
So if you had leakage to earth (which is continuous to the supply side) Then you would have an imbalance.
The toroid that detects this imbalance is on the load side not supply, so the supply does not effect L-N values.
Hope this makes senses!
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Postby ericmark » Fri Aug 30, 2013 9:36 am

TN-C-S means
TN neutral bonded to earth.
C combined.
S separated.

So underground supply to your house some where the neutral and earth are combined but before it arrives at your consumer unit or fuse box they are separated. This actually causes a problem as since the combining can be anywhere along the route near impossible for any one looking at the head to be able to say it's a TN-S supply unless it's written on it.

Since the ELI allowed with TN-C-S is 0.35 and with TN-S the ELI allowed is 0.8 (I think) many electricians use that to work out which supply. But in the main it does not matter. Only where a supply to charge a car, supply a caravan or boat it required do we need to know. With a petrol station the DNO would not supply a TN-C-S system in the first place.

Some times you can see it is a TN-C-S supply but not if it's a TN-S supply. The problem arises when we look at out buildings and we have to decide if to export the TN supply or use a TT supply. Much depends on what is near the out building and what is beyond. 150 meters down garden with fields beyond then TT and 1 meter from house with exposed metal drain pipes and then use the TN but at which point one changes is where it gets hard.

Fault finding with a RCD fault is some times easy but also with neutral earth faults it can get complex. If we look at a kettle and a toaster as an example. The kettle uses more power than the toaster so with a neutral earth fault switching kettle on can trip the RCD and the toaster can be used without problem even though it is the toaster at fault.

What causes the problem is the more current that is drawn the larger the difference between neutral and earth voltage so for a given resistance (bit of bread in toaster) the current neutral to earth increases with load.

Regulation 314.1 says:-
Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

(iv) states "Unwanted" so where there is a fault like bit of bread stuck in toaster there is no breach of regulations. So only (i) is a problem in complying and as a result it has been considered with domestic two 30ma RCD's are enough. However with a TN supply using all RCBO's would greatly reduce the problems in fault finding. (Since with TT it needs to be all pole switching really does have to be just two RCD's)

There are other methods the X-Pole RCD has a monitor built in so you know when you are approaching the leakage likely to trip it and there are even RCD's which auto reset after checking the fault has cleared. But at over £350 each these are not aimed at the domestic market.

Step one with fault finding is un-plug anything not being used. With class II equipment where there is no earth connection then not such a problem so leaving a phone charger plugged in likely will not be a problem but a faulty EMC filter can cause havoc specially with low current draw so a PC power supply or even a filtered extension lead can have one running around in circles.

Filtered fixed sockets are a real pain as often impossible to isolate. The other problem is timed processes like washing machine and freezer the latter in particular where the frost free switches on a heater once or twice a day and that is causing the fault. Extension leads and plugging into the other RCD and seeing if fault swaps to other RCD is often the only way to prove if something is OK.

Good look in finding your fault. An account of what it was and how you found it may help others in the future.
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