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RCD protection on all circuits???

Postby besb421 » Tue Apr 22, 2008 5:19 pm

I've just returned from living in the south of Spain where all the circuits in the consumer units are protected by one isolator and one RCD. I think this is because all of the supplies are overhead and the supplier doesn't supply a earth. Each supply goes to a metal pylon or similar on your property and that itself is earthed as an electrode into the ground. I think (but not attall sure) that this is called a TT system in the UK? Does this type of total protection apply here in the UK if no earth is supplied or can a split load be installed, with some not on RCD protection? I ask because i found that in a small nuisance fault situation everything was disconnected losing the lights and defrosting the freezer if i was away from home.
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Postby ericmark » Tue Apr 22, 2008 10:02 pm

There are two answers to your question. What is now and what will be in 2 months time. So first what is.
In order to ensure should a fault occur where the current to earth would not be high enough to cause the system to trip an RCD may be used and selected so the system will trip in the prescribed time.
Also you need an RCD at no more than 30ma to protect sockets that may be used outside and some items used in a bathroom or where a shower is fitted.
From the 1st July all cables buried in a wall at less than 50mm and not protected by a metallic shield will need protecting with a 30ma RCD. Also now all sockets under 20amp and all items in a bathroom. Also it have been made clear that the requirement to split the supply into circuits also applies to the RCD's so in the future at least two RCD's will need fitting and it is likely three neutral bars will be provided in consumer units allowing the use of two RCD's plus some RCBO's the latter combines the RCD and MCB together but does not switch the neutral. It will not be allowed to supply the lights and sockets in the same area from the same RCD but it has been interrupted that supplying lights in one area and sockets in another area from the same RCD will but this is still being debated by many in the trade.
There are basically three systems in use in the UK.
TT is where the consumer has his own earth rod.
TN-S is where the supplier brings in a separate earth not very common.
TN-C-S also called PME where the supplier earths the neutral and brings two wires into the premises but as soon as in the premises it splits (before the meter) into earth and neutral and is never joined again.
There are other systems like IT but these are not used in UK domestic premises.
There are ways that a freezer can still be feed with its dedicated supply without RCD protection by using surface cable or a cable with metallic covering like flexishield and it must be marked for freezer only. Although with a dedicated supply on its own RCBO it is very unlikely to fail. There are also different types of RCD's which are less likely to fail and even an automatic resetting type although at £250 plus for a single RCD they are expensive. Most RCD's with be 30ma for personal protections, 300ma for fire protection and 500ma for socket protection over 32 amp on building sites. Where personal protection is not required on a TT supply the size or RCD is selected by reference to a table or calculation. There are also active RCD's which also trip when power is lost so things will not auto restart once power is returned. But most are passive.

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