I have found lots of info about the tripping current ratings of RCDs
but not much about the actual current rating.
Specfically I have a 100A main fuse and a split load CU with a 100A
isolator and a 63A/30mA RCD.
What I would like to know is if I can apply diversity to the circuits
on the RCD side. For example can I have 3 ring mains on the RCD side
(which adds up to 96Amps).
Strictly speaking, the RCD doesn't do anything for short-circuit protection
of the busbar and tails. It provides *only* earth-leakage (i.e. current
imbalance) protection for the final circuits (yes, OK, and the busbars
on the RCD side). Short-circuit and overload protection for the tails
and busbars is provided by the service fuse. The "big current" rating on
an RCD for a split-load board (typically 63A, 80A, or 100A) is a "maximum
sustained load" rating (and yes, you can and do apply diversity to it),
but there's nothing in the RCD which will make it trip when you pull more
than the rated current. That's an RCBO's job.
HTH - Stefek
On 5 Feb 2004 05:45:39 -0800, firstname.lastname@example.org (Jon M)
If you have a 100A main fuse you should really have a 100A rated RCD.
The RCD is protected by the main fuse so should be suitably rated, as
you would with final circuits. You wouldn't put a 20A switch on a 32A
radial circuit feeding a shower would you?
You can apply diversity to all circuits in the consumer unit, that's
what you're meant to do otherwise most houses end up at 250A if the
full capacity of MCB's are used for calculations!
No, that doesn't follow. The RCD only needs to be rated to carry the
expected maximum demand (MD) connected downstream of it. This is a
split-load board with the RCD only feeding three 32A ring circuits. To
answer the original question: yes, diversity can be allowed. The
traditional rule for the standard socket circuits in domestic installations
is to take the max. demand as 100% of the rating of the largest circuit plus
40% of each of the remaining circuits. (This is in Table 1B of Appendix 1 of
Hence we have an MD of 32+0.4*(32+32)A = 57.6A, so the 63A rated RCD will be
fine. This diversity rule is in fact very generous. The probability of
having more than ~60A of appliance load plugged into house sockets and
switched on at once is obviously vanishingly small.
No, because a shower's going to take more than 20A. But there's nothing
wrong with using 6A rated switches on a 10A or 16A lighting circuit,
provided that each switch is controlling no more than 6A worth of load (&
derated for discharge lighting if necessary).
Even if you do apply the Table 1B diversity rules you can still end up with
well over 100A of assessed MD. This little detail just seems to get glossed
over, and we all know that 100A is more than enough[*] for a house - which
again amounts to saying that the rules are a bit over-cautious.
[*] Except for houses with comprehensive storage heater installations which
might need 3x100A.
Although that follows, I get told things by the gods at NICEIC, one
of the things that has been mentioned is that ALL CU components should
be rated to at least that of the service fuse.
Again, technically correct, but you're not meant to do it like that.
As I have said in previous posts, there's the logical way of doing it,
then there's the NICEIC\IEE way of doing it. I have to go the
NICEIC\IEE route. Anyone doing diy can do what they want really.
Well blow me down with a feather. A trade body suggesting a more
restrictive interpretation/misapplication of the definitive regulations,
which just happens to result in more work for its members in refitting
existing safe installations? Surely not! I mean, if I were to believe such
a thing, I might believe that CORGI would post ambiguous stuff about
d-i-y gaswork being 'likely to be illegal' on their website, that
long-serving judges with previous records of serving in non-jury special
courts in Northern Ireland might have a tendency to close ranks around a
government and its intelligence apparatus, and all sorts of idiotic
Mmm. In matters of applied engineering, 'technically correct' is in fact
the *only* relevant standard; and this particular issue of sizing conductors
and switchgear according to design load rather than circuit protective
device when the nature of the load makes an overload implausible is the
exact one thrashed half to death a while back in the context of
split-hob-n-oven wiring. If the NICEIC are incapable of understanding
the IEE regs, it's their problem, not ours; and is a matter for their
well-informed and competent members to beat up whichever noodles get to
lay down country-wide or local NICEIC inspection lore. Hmm, and it's NICEIC
members with time on their hands (as opposed to the good ones who have
work booked out until 2006 ;-) who are going to have an effective monoploy
on issuing 'inspection certs' on minor-works modifications in domestic
premises soon? Oh joy...
 Me, I'd not touch my gas installation for d-i-y: never spent time
developing pipe soldering skills. But I'd rather see an honest statement
along the lines of "gas leaks can accumulate for a long time and explode
with fatal consequences; if you undertake d-i-y gas work you *must* be
competent to make sound connections and follow accepted working and testing
procedures" than the 'likely to be illegal' claim and the even more
blatant lie 'Unregistered installers and DIYers are not only breaking the
law...' - lumping in DIYers with unregistered installers - which has now
appeared on the Corgi website (under 'Your Rights & Responsibilities'
in the 'Don't DIY' section).
 Not that the IEE Regs are utterly without their own minor illogicalities,
but as a reflection of current understanding of good engineering practice,
I'll take their careful deliberative practices over some off-the-cuff
pronouncement from an NICEIC jobsworth any day.
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