On 23/10/2013 22:50, Bill wrote:

Hi all, especially the knowledgeable electricians on here.

What are your thoughts on me replacing all of the MCBs in my CU with
RCBOs? At present I have 14 MCBs all protected by one 30mA RCD main
switch. Over the last few years I've had tripping problems from a
trapped L to E fault in a metal clad light unit, it too 10 years before
this became a problem! Water blowing into an extractor fan, water in an
outside light and the big issue of too many switch mode PSUs in the
house. While some of the genuine faults were a pain and time
consuming to locate the SMPSUs are something I have to live with and
they are taking my overall leakage too near the 30mA limit on the CU.

This has been brought to a head by a few trips this evening, no one
circuit being the culprit, leave any 2 or 3 MCBs off and it is happy,
just not happy with all on, I'm assuming something is leaking a bit more
than normal and has taken the main RCD to it's limit. I've left one
circuit off that has a number of SMPSUs on it and so far all is stable
again.

Your experience adequately illustrates why the "whole house" RCD
arrangement is deprecated - it was never a great idea, and the march of
time (and things with leaky input filters) has only made the limitations
more apparent.

Yes I will turn everything off and check the trip current of the1 RCD in
case it has become overly sensitive, but NOT tonight!

So my thought was to replace the main switch/RCD with a simple switch
and then all of the MCBs with RCBOs, everything will be protected and in
the event of a real problem I don't lose the whole darn house, just the
one errant circuit.

So is it a practical idea?

Practical - yes certainly. Its the "Rolls Royce" solution that gives the
best possible discrimination in the even of an earth fault. Obviously
its more expensive than other options.

I think so, but would welcome any thoughts,
good or bad about doing it. I know the 17th edition suggests a split
load CU, but that would still leave me with a 50/50% split and a gamble
as to what worked and what didn't.

Split load does not have to mean a 50/50 split. Some 17th edition CUs
may have say 4 RCDs and some unprotected (aka "high integrity") ways as
well (for feeding circuits that meet the requirements for not being RCD
protected). You can also arrange the split in various combinations of
ways as suits your application.

Yes I am competent to do it, just nervous that I may have missed a
fundamental flaw in the idea.

If it matters the CU is a Crabtree Starbreaker with a single 80A
switch/30mA RCD and 14 MCBs and a DIN mount bell transformer.

Without checking I don't know if you can get single module wide RCBOs
for this enclosure (and if so if there is really enough space to wire
them). You may find you need to replace the enclosure as well.

Apart from the cost, am I crazy?

No, although you probably ought to look at some pragmatic options. You
may find a combination of several RCDs covering a two or three circuits
each, plus a smaller number of RCBOs would make for a system that is
equally effective, but also leave more money to spend on improving other
aspects of the system.

--
Cheers,

John.

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