RCDs

My home's electricity supply is overhead. Consequently (I think) our main consumer board is fitted with an RCD. Presumably this means that there is really no real need to use an RCD when using say a hedge trimmer or dirty water pump, as the main RCD is going to trip and give the necessary protection anyway in the case of any fault developing. Am I right in making that assumption?
TIA
Keith
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Leaving aside your first assumption which I don't think is true (but someone will say), a whole house RCD may well be 100mA. Individual use circuits that you describe should be protected by a 30mA RCD (more sensitive to faults by a factor of 3), in that case one should still be used.
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In an earlier contribution to this discussion,

No. It depends to some extent on when the house was built and/or electricity supply installed. Overhead mains were used *long* before RCDs were invented - and I suspect that lots of houses still have overhead supplies without RCDs.
Even if you do have an RCD, a whole house unit may well have a higher threshold current needed to trip it than one intended for use with garden tools etc.
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W-e-e-e-e-e-e-e-lll.
the 100/30 mA trip issue has already been raised.
Are you happy for the whole house to lose power because of your hedge trimmer? We have an overhead/PME supply with a master RCD and I still use 30mA RCDs for external supplies.
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Huge wrote:

Worth noting that cascaded RCDs don't usually discriminate - so in the event of a fault one or both will trip - and it is as likely that the upstream one will go before the downstream one. The solution is a upstream RCD with a time delay.
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Keith Dunbar wrote:

If its 30mA you are street legal.However many including myself find that this gives a lot of nusiance tripping,and I ahve uorated to 100mA on teh whole board, which dies trip if something really is wrong, but not otherwise. That means I really OUGHT to get around to putting 30mA trips on the circuits that feed outside...
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On Mon, 25 Feb 2008 11:44:46 +0000, The Natural Philosopher wrote:

AIUI the 100mA trip should be a time delay type to provide discrimination, otherwise if, say, you cut your hedgetrimmer cable and cause a fault to earth both your 30mA circuit RCD and 100mA whole house RCD will probably trip at the same time.
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Does it have a test button? Is there a 'mA/milliamp' rating printed on it?
cheers, Pete.
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wrote:

There are 2 RCDs, one for the main house (probably rewired 40 years ago) and one for the annexe (built about 15 years ago).
The RCD for the main house is only marked: 500V and 60A - made by Crabtree. I can see no mention of its rating. Interestingly, it has two earth wires wired into at the output end.
The RCD for the annexe is marked: 240V and 63A. It is also marked with BS 4293 and n0.03A (is this a 30mA rating?)
Keith
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Keith Dunbar wrote:

Thats an ELCB, not an RCD. Which means you would gain a safety benefit by using an RCD on outdoor tools.
NT
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wrote:

main
it?
and
wires
ELCB's are condemned now, since they can be rendered inoperable by parallel earth paths. They should be replaced with an RCD unit.
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Err, you misunderstand what they do. They are designed to work with parallel earth paths. If you have enough, they cease operating because you have a good enough earth not to need them.
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Andrew Gabriel wrote:

Exactly - a common misunderstanding. Plenty of ELCBs are still in use and doing their job fine. What they do doesnt provide as many safety benfits of RCDs, but they do do the job theyre needed for effectively.
NT
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snipped-for-privacy@care2.com wrote:

This does presume they are installed in the way intended. Most I have seen are not alas...
It is also important to note they offer no protection from direct contact shocks.
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I can't think that I've ever seen one installed incorrectly. I think there was a higher skill level amongst the electricians who would have been installing those 40 years ago than I see in many of today's electricians.

Indeed. Neither should 100mA RCD's be regarded as doing so.
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Keith Dunbar wrote:

Does it look like:
http://wiki.diyfaq.org.uk/index.php?title=RCD#ELCB
If so then it is not a RCD and can not be relied upon to give adequate protection from shock via direct contact with exposed wires etc.

Yes, this is a modern RCD with a 30mA rating.
The main wiki article on RCDs might be worth a read for some background on the implications of your current setup.
http://wiki.diyfaq.org.uk/index.php?title=RCD
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Yes indeed, the "RCD" for the main house is in fact an ELCB identical to the one in the photo at:
http://wiki.diyfaq.org.uk/index.php?title=RCD#ELCB
I appreciate the advice to have it replaced by an RCD. Am I right in assuming that this cannot be done however without bringing the rest of the 40-year-old installation up to the latest modern standards, at what I guess would be huge expense for a 1650s 4 bedroom, 3 reception room house?
Keith
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Not in my view, although some electricians might not do it.
A 40 year old installation, unless it's been badly hacked about, shouldn't be in a dangerous condition, even though not to today's standards. Even if you want to bring it up to todays standards, you can probably retain most of the existing cabling.
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Keith Dunbar wrote:

There is nothing stopping you updating part of the installation. The only thing that really ought to be brought up to modern spec if not present, is the main equipotential bonds with the water and other service pipes.
You have a few options for upgrading, it depends a bit on what you want to achieve and how far you want to go.
The simplest option is to go for the "whole house" RCD sat in a separate enclosure, and basically replacing the ELCB that you have now. This is a deprecated way of doing things, since it gives no discrimination between faults - i.e. you lose all power on any trip. However it would add greatly to to shock protection and would cost under 40 for the bits.
A much better but relatively simple solution would be to replace the ELCB with a time delayed 100mA RCD, and then split the tails to add a second CU after the new RCD. This CU could have a standard 30mA RCD on its incomer. You would then move any power socket circuits to the new CU. Total cost would probably be under 130 in parts and would offer greatly enhanced protection.
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Cheers,

John.

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John Rumm wrote:

the only 'but' to that its not uncommon for old installs to not wont work with an RCD due to minor issues that need to be hunted down and fixed. So if you go for an RCD, budget for further work.
However its just one more example of solving a problem that isnt worthwhile. The number of electrocution deaths per year is very small, and if youre going to spend 50 and a day's work there are far bigger risks you could solve with that.
NT
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