Hi,

In the last few weeks we've had a couple of power cuts caused by the
RCD in the CU tripping but it's not obvious what it causing it. None
of the MCBs have tripped.

It seems that each time the washing machine is on its spin cycle and I
am wondering whether that is responsible. However, we have had the
washing machine on plenty of other times and nothing has happened.

Is there anyway to pin point what is causing these trips? Is a washing
machine a likely culprit?

I suppose a board full of rcbos would make it easy to identify the
problem but I haven't got one of those But didn't I read somewhere
here that rcbos only switch the live not the neutral (makes sense
since the neutrals are connected to the bus bar not the rcbo), so is
an advantage of using rcds that it gives you protection on both poles?

TIA

In message , Fred
writes


I suppose a board full of rcbos would make it easy to identify the
problem but I haven't got one of those But didn't I read somewhere
here that rcbos only switch the live not the neutral (makes sense
since the neutrals are connected to the bus bar not the rcbo), so is
an advantage of using rcds that it gives you protection on both poles?


No, RCBO's have a live and neutral connection. The neutral from the
circuit is connected to the RCBO, and there is a flying neutral
connection on the RCBO which is connected to neutral busbar.

I fitted out the new CU with RCBO's. More expensive yes, but in the
scheme of things the extra cost doesn't seem something to worry about to
much.
--
Chris French

On Nov 1, 11:14*am, Fred wrote:
Hi,

In the last few weeks we've had a couple of power cuts caused by the
RCD in the CU tripping but it's not obvious what it causing it. None
of the MCBs have tripped.

It seems that each time the washing machine is on its spin cycle and I
am wondering whether that is responsible. However, we have had the
washing machine on plenty of other times and nothing has happened.

Is there anyway to pin point what is causing these trips? Is a washing
machine a likely culprit?

I suppose a board full of rcbos would make it easy to identify the
problem but I haven't got one of those But didn't I read somewhere
here that rcbos only switch the live not the neutral (makes sense
since the neutrals are connected to the bus bar not the rcbo), so is
an advantage of using rcds that it gives you protection on both poles?

TIA

It could be caused by anything. Best bet is to begin by insulation
testing all your appliances - its very simple. I'd start by doing it
with a multimeter, which will catch a lot of such faults but not all.
Test resistance on the highest range from L&N to E, you're looking for
any appliance with a low reading.
http://wiki.diyfaq.org.uk/index.php?title=RCD


NT

In message , John
Rumm writes
On 01/11/2010 11:44, chris French wrote:
In message , Fred
writes


I suppose a board full of rcbos would make it easy to identify the
problem but I haven't got one of those But didn't I read somewhere
here that rcbos only switch the live not the neutral (makes sense
since the neutrals are connected to the bus bar not the rcbo), so is
an advantage of using rcds that it gives you protection on both poles?


No, RCBO's have a live and neutral connection. The neutral from the
circuit is connected to the RCBO, and there is a flying neutral
connection on the RCBO which is connected to neutral busbar.

Its worth checking - they will certainly have to have connections for
both poles, but not all switch both poles.


Of course yes, I wasn't really engaging brain in my answer....

checking on a spare single pole MK Sentry one I have here, the little
diagram on it certainly seem to say it switches just the live (as you'd
expect for a single pole one...)

Not that I'd see this as a safety issue

--
Chris French

On Mon, 1 Nov 2010 11:44:22 +0000, chris French
wrote:


No, RCBO's have a live and neutral connection. The neutral from the
circuit is connected to the RCBO, and there is a flying neutral
connection on the RCBO which is connected to neutral busbar.

As you can tell, I've never seen or used an RCBO before. Of course,
when I think about it, it would need live and neutral to work as an
RCD. I'm sure there's some combination of punctuation to make a
blushing face that I should use here!

On Mon, 01 Nov 2010 12:08:45 +0000, John Rumm
wrote:

Its worth checking - they will certainly have to have connections for
both poles, but not all switch both poles.

From what's been said, it sounds as though single width ones only
switch the live. If you have to use double width ones to switch both
poles, you'll quickly use up CU space. Is there any disadvantage to
only switching one pole?

On Mon, 01 Nov 2010 12:15:55 +0000, John Rumm
wrote:

Sounds like you have a "whole house" RCD?

If so, it worth remembering that while these can work fine for some
folks for years, they can also cause no end of trouble for others!

Yes it is. Until now we've never had any nuisance trips in the years
we've lived here, so I've not seen the need to update it. My worry
would be would the wires be long enough to reach if we replaced with a
new CU or would I have to run new cables to the first and last socket
of each ring, each light, etc? Also, in order to do it myself, I'd
need to buy an expensive meter to measure the resistances; at the
moment I've only got a "basic" multimeter.

Is a split CU any better though? If you have a nuisance trip rather
than the whole house going off, half the house does and surely that's
just as annoying/inconvenient?

We had another trip yesterday, again when the washing machine was on
its spin cycle, so I am sure that is the culprit but we had three
washes and it only tripped on the second wash.

I read the faq you linked to and it begins by saying something along
the lines that nuisance trips are very difficult to pin point and that
is proving to be very true.

Thanks.

On Mon, 1 Nov 2010 05:12:07 -0700 (PDT), Tabby
wrote:

It could be caused by anything. Best bet is to begin by insulation
testing all your appliances - its very simple. I'd start by doing it
with a multimeter, which will catch a lot of such faults but not all.
Test resistance on the highest range from L&N to E, you're looking for
any appliance with a low reading.

I've tried this but I'm not finding anything suspicious. Since the
trip occurs only occasionally, could it be intermittent? Thanks.

On Wed, 10 Nov 2010 11:28:39 +0000, John Rumm
wrote:

You can hire multifunction testers if needs be, or there may be someone
here local enough to lend you one.

Thanks. Are all the required tests in the latest edition of the OSG
and/or wiki? I'd like to see what I would have to do to determine
whether they are within my capability first. Is it just measurements
of L, N, and E resistances and a test of the RCD?

What sort of CU have you got at the moment? Are RCBOs available for it?

It's a crabtree one with one rcd covering 8-10 (will check later how
many) MCBs. I guess that makes it 16th edition? It uses the
starbreaker mcbs available from edmundsons, screwfix, etc. There are
RCBOS. Are you suggesting I fit one to the washing machine ring and
see what happens?

We've done some more washing since and no trips, so like you say
perhaps lots of devices are contributing and the washing machine
pushes it over the edge on certain days?

On Fri, 12 Nov 2010 15:29:45 +0000, John Rumm
wrote:

If 8-10 MCBs are all the ways and the RCD protects all the circuits,
then that is more of an early 15th edition style arrangement.

It's 10 mcbs and they are as you describe, so it must be a 15th ed
install.

Yup, if you have a reasonable hunch that its that circuit causing the
problem, and you can move it to a non RCD protected section of the CU,

I don't have a split section so unfortunately I am stumped.

Its possible certainly. I have had a similar problem with a tumble drier
in the past.

Things like heater elements in washing machines are known for these
problems. They can get tiny cracks or pinholes in the metal case of the
element. Water seeps into the hygroscopic mineral insulation and they
start leaking to earth.

Thanks. I had read about washing machine elements but didn't quite
understand what it all meant until you explained it. How would a
tumble dryer leak since it shouldn;t contain water? I thought their
heaters were just wires like a fan heater/hair dryer.

TIA

On Mon, 15 Nov 2010 12:09:31 +0000, John Rumm
wrote:

10+ circuits on one RCD is asking for trouble anyway in a modern
household. There will be folks who get away with it, but its not
something you can count on.

Thanks. So possibly each circuit has a "leak" of 3mA and together they
are adding up to 30mA, whereas if I had two groups of 5 MCBs, each
split would only have a leak of 15mA and not trip? I think I'll post
asking about split CUs in a couple of days time