Earthing.


Postby happyhacker » Tue Jun 21, 2011 6:26 am

I have a TN-C-S (I think!) and the earth is connected to the mains water pipe where the plastic water mains enters the building. I have just cleaned this "strap", retightened and check with an ohm meter that there is conductivity to one of the power socket earth terminals. I see also that the oil boiler has an earth connected back to the supply earth strip. I think these leads are 4 & 6mm. There is another earth going from here to the sink in the kitchen (4mm). The property is 1970s and only had the distribution box changed where the up/down power ccts are RCCB protected and the others (cooker, tank, lighting & door bell) MCB (split load I think) on the main box switch.

Is this earthing arrangement now correct?
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Postby ericmark » Tue Jun 21, 2011 9:00 pm

1) There is no need to up-grade to current regulations. When a system is tested it is noted where it no longer complies to warn the owner if there are to be additions it may require up-grading but unless something is added then it can remain.

2) To test an earth some current must flow. The norm is to use an earth loop impedance meter, but the regulations only require a minimum of 200 ma to flow but that will not be the case with a multimeter set to ohm scale.

3) The RCD has two main functions. Personal Protection and Fire Protection for the former it needs to be 30 ma and trip in 40 ms with 5 times 30 ma and for fire 300 ma but can be delayed for tripping (S Type) normally with split board it will be 30 ma type.

4) Today all circuits in bathrooms including lights, all out door circuits, and most sockets under 20 A are RCD protected. (Sockets marked fridge for example do not need to be RCD protected) Also cables less than 50 mm below the surface unless special cable is used also need RCD protection.

As a result it is normal for all circuits now to have RCD protection. As a result the earthing requirements have been reduced especially in bathrooms. But this only applies when RCD's have been fitted.

5) 4 mm is smallest earth cable allowed if not part of the cable 543.1.1 the next demarcation is 6 mm and any non insulated cable must be larger than 6 mm and main earth with TT system min is 6 mm. With a TN system it moves to 10 mm for main earth and where not protected against mechanical damage 16 mm min. So what you say would comply.

When some one asks about earth requirements the first question is why are you asking. There are so many regulations on earth systems it would be very easy to miss one or read it wrongly in the regulations. But the Low ohms ohm meter and the loop impedance meter are both expensive. So in real terms it is not worth DIY for these tests.

Nothing that you say would make me think anything is wrong. But the fact you ask makes me wonder why.

The earth system is so easy to damage without realising so I would always test if I though anything could be wrong.
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Postby happyhacker » Fri Jun 24, 2011 7:17 pm

Thanks for the comprehensive reply. I query because I am keen on my system which hasn't been touched for 40 years, to be safe. I have one of those plug in mains testers which gives an OK signal but of course that does not test the system earthing - just the "local" earth in the socket. I was thinking of getting the Electric suplier to come and do a test but I am not sure if they will go to the trouble of applying these current tests to the system as they will have to find all the earth connections.

I have the latest regs. and am studying them now.
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Postby ericmark » Sat Jun 25, 2011 3:27 pm

There are many plug in testers. Some just test that there is an earth and the lives are correct way around. Others do a much more comprehensive test and it seems also test the earth loop impedance. The better ones like the SOK36 from Socket&see cost around £35 and test it seems earth loop impedance and RCD. I have never used one so can't say how good.

Since the pass level for ELI varies according to the size of the supply MCB/fuse can't quite see how they work. I will guess designed for 32A ring main.
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