RCD Consumer Unit inconsistent about resetting after cutting


Postby jvector » Sun Feb 13, 2011 6:43 pm

Hi, first post but I have tried reading up about this a bit.

We have a consumer unit with a single RCD covering all circuits in the house. I realise this is now outside the new (Edition 17?) regulations and we have always cussed the fact that whenever a single light bulb blows it shuts down the entire house. I guess it's ten plus years old.

The RCD has now tripped out a few times, often before we are awake in the morning (now that can really spoil your day ;-) Odd thing is, that when I go to reset it it will often trip out again but not always at the same MCB.

E.g.
Starting state
RCD off, 8 MCBs on.
Switch off all MCBs.
Switch on RCD.
Switch on in order MCBs 1..2..3..4..5..6..7
Switch on MCB 8 -> RCD turns off.
--

Switch off all MCBs.
Switch on RCD.
Switch on in order MCBs 7..6..5..1..2..3
Switch on MCB 4 -> RCD turns off.
--

Switch off all MCBs.
Switch on RCD.
Switch on in order MCBs 1..2..5..6..4
Switch on MCB 7 -> RCD turns off.
--
after a few minutes of cussing, and bleeping noises from the phone waking up etc., eventually I get a sequence that stays up. Sometimes I resort to e.g. leaving off the first-floor lights MCB, or the kitchen+den lights MCB, and trying to put them on later.

I've done a rough survey and I don't think any of the circuits is likely to be overloading the 6/16/32A limits (although I am guesstimating the consumption of some items), similarly I think the whole house is not pushing the 80A limit of the main RCD even if you turned on everything at once. But there are a number of computers around.

I am imagining it's time to say goodbye to this consumer unit (although it appears like the unit itself is quite a 'low-tech' piece of kit) and the MCBs (can MCB's get unreliable with age? I guess so) and also whilst about it making it conform to the new regs by not having all circuits under one RCD. Or is this behaviour indicative of a more general wiring problem in the house? I've not done any major electrical works in the house myself, they've been done by electricians (including e.g. creating en-suite bathroom in roof space) although I have replaced old sockets with new ones, and put in downlighters in place of existing lighting when we took a ceiling down.

Thanks!
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Postby ericmark » Mon Feb 14, 2011 9:02 am

Any electronic equipment will likely have a filter to stop spikes entering the equipment. Each one of these allows a little current to leak to earth. Each one likely less than the 3ma limit but as a whole building up to over the 15ma at which a 30ma trip will start to operate.

All RCD's are not the same quality with some giving a warning before they trip and some even auto reset (Although over £350 each) but depending on the consumer unit you may be able to fit some RBCO's and then not have those circuits through the main RCD. The single RCD units were designed to have a 100ma RCD to protect from fire and do not protect people for use with 30ma RCD's the consumer units have 2 or more RCD's with a TN supply you can have RCBO's which means a separate RCD on every circuit which in the main stops the tripping for no reason.

However not really a DIY job.
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