Re: Domestic RCCB tripping

Jul 03, 2003 2 Replies
U


Had a similar problem recently where there was nuisance tripping about 3


> times a week. Had trouble putting the RCD back online. Checked the
> Earth-neutral and found a lot of leakage on a SWA exterior connection (using
> a standard multimeter). Disconnected this feeling very good and no trips for
> a week, then occasional trips started again. Bit the bullet and changed the
> RCD - bingo, no trips now for over two months.So it can be just a faulty > RCD. >

It could simply be that the old RCD was near maximum sensitivity and the new one near minimum. A 30mA RCD *must* trip at or below 30mA leakage but may trip all the way down to 15mA and still be within specification.


A large installation (as ours was before I replaced consumer units and installed multiple RCDs) may well cause nuisance tripping of a single


30mA RCD simply because of its size. Even when all insulation resistance etc. is within specification if there's enough little bits of leakage they will add up to something that trips the RCD, especially on wet days and/or when a number of PCs are turned on.
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A

Each Class I (earthed) piece of IT equipment is allowed to leak 0.75mA. When designing a circuit with RCD protection, you should design for a maximum leakage of 25% of the RCD rating, 7.5mA in this case. That makes a maximum of 10 class I (earthed) appliances, i.e. 5 PCs and 5 monitors. (Most other separately powered PC peripherals are Class II appliances and don't count).

A 30mA RCD will trip at a leakage current somewhere above 15mA but less than or equal to 30mA. That gives you your 100% headroom in the case of the most sensitive RCD sample within spec.

He is correct. The RFI suppression components cause leakage to earth. That's not contracdicting what I said -- it's actually explaining it.

Depends on the way the UPS works. By the way, hope your UPS earthing meets the 16th edition wiring regs (which is rather non-trivial)...

Also, there are additional requirements on circuits designed for high earth leakage, such as numbers of PCs.

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D

Never heard of that before. I can have as many as 5 computers running, occasionally more. Mine, wife's, hobby room, one on test and the last one, being a test bed for cross checking computer cards for the one on test.

I have a 30 mA RCD and all the house sockets are on it..

Dave

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