Any help on the following would be much appreciated. We moved into a 10
year old house earlier this year, the RCD has been fine up until last
Friday, having said that it is a 30mA rated RCD for the whole house
(groan). Since last Friday it has gone of 4-5 times a day.

Over the weekend I tried to identify which particular circuit was
causing the problem by turning off circuits (I realise this does not
stop N-E leaks on those circuits from tripping the RCD...). The RCD
tripped with both lighting circuits off and with the central heating
circuit off. Oh and it also tripped with the washing machine completely
disconnected from the supply.

There seems to be no pattern as to which device was being turned on/off
when the trip occurs. My suspicion is that something late last week
started leaking enough current to earth to sensitise the RCD and now
the system trips randomly when 'something' turns on or off.

OK, so the possible culprits:

1. Light in bedroom: We have a 2 month old baby who last week sounded
like he was not happy with the dry atmosphere in our bedroom so (don't
laugh!) I boiled a kettle in our room for a couple of minutes. (No, it
did not end up like a sauna). The RCD didn't trip at that point but I'm
wondering whether moisture got into the light fitting causing an
increase in the overall leakage... note that based on the fact that the
RCD has tripped when the lighting circuits were off, this leakage would
have to be N-E not L-E. The fitting itself is 3 x 40W halogen (I think)
bulbs with an earthed metal body...

2. Lights or sockets in the garage: The garage is attached to the house
and has a couple of sets of sockets which are run off the kitchen
socket circuit and 4 strip lights which I assume are run off the
downstairs lighting circuit. It rained like mad last Friday and over
the weekend... so I was wondering whether dampness had got in
'somewhere'... there are no obvious leaks in the garage. Note we have
had no problem with this before.

3. Emmersion heater on the hot water tank. I noticed quite a bit of
corrosion around the electric 'backup' heating element on the hot water
tank. This has it's own circuit.

So, last night I:

a. Removed the light fitting from our bedroom and took a hairdryer to
the circuitry beneath... hopefully got rid of any moisture.

b. Connected the fridge/freezer to the ring main rather than the
kitchen sockets circuit and turned off the kitchen socket circuit..
(thus 'disconnecting' the sockets in the garage and all kitchen
appliances... I wonder whether the electric igniter on the gas hob has
got 'leaky'?)

c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

So I'm wondering what I do next... I'm hoping that it was the light in
the bedroom as this would be by far the most easy to fix.. however if
this is the problem then I am suprised that it didn't immediately trip?
I'm also worried that I may not have found the cuplrit but simply
reduced the amount of extra 'valid leakage current'?

Any ideas about how to diagnose further where the problem maybe? I'm
not sure I'm not particularly happy about opening up the consumer
unit.... (otherwise I assume one approach would be to attempt to
measure the N-E/L-E impedances on the circuits).

The follow up to this is that once I've got to the bottom of this I'm
intending to improve our consumer unit. Which of the following is best
and roughly how much should I expect to pay to:

1. Change CU to split-load system and use a 100mA RCD. At least place
lights on non-RCD side!

2. Leave current CU in place but replace RCD with 100mA unit and
replace 'required' circuits with their own 30mA RCD/MSB 'thingy'

Should I also think about getting all the garage circuitry onto a
separate circuit... now that could be painful!

Finally (is anyone still reading?) I'm not to keen on replacing the RCD
etc myself... how the hell do I assess an electrician as 'OK' apart
from rejecting anyone with a Stetson hat on! :-)

Sorry for the long post but I wanted to get as much info down as
possible!

Ben

In article .com,
Ben wrote:
c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

That's the most likely culprit IMHO.

--
*Do infants enjoy infancy as much as adults enjoy adultery?

Dave Plowman London SW
To e-mail, change noise into sound.

Ben wrote:
1. Light in bedroom: We have a 2 month old baby who last week sounded
like he was not happy with the dry atmosphere in our bedroom so (don't
laugh!) I boiled a kettle in our room for a couple of minutes. (No, it
did not end up like a sauna). The RCD didn't trip at that point but I'm
wondering whether moisture got into the light fitting causing an
increase in the overall leakage... note that based on the fact that the
RCD has tripped when the lighting circuits were off, this leakage would
have to be N-E not L-E. The fitting itself is 3 x 40W halogen (I think)
bulbs with an earthed metal body...

That seems incredibly unlikely, otherwise we'd all have RCDs popping
off whenever we boil kettles.


2. Lights or sockets in the garage: The garage is attached to the house
and has a couple of sets of sockets which are run off the kitchen
socket circuit and 4 strip lights which I assume are run off the
downstairs lighting circuit. It rained like mad last Friday and over
the weekend... so I was wondering whether dampness had got in
'somewhere'... there are no obvious leaks in the garage. Note we have
had no problem with this before.

Somewhat more likely.


3. Emmersion heater on the hot water tank. I noticed quite a bit of
corrosion around the electric 'backup' heating element on the hot water
tank. This has it's own circuit.

Elements of all sorts can go leaky.


intending to improve our consumer unit. Which of the following is best
and roughly how much should I expect to pay to:

1. Change CU to split-load system and use a 100mA RCD. At least place
lights on non-RCD side!

Not allowed, sockets which might be used for outside equipment *must*
have 30mA RCD protection.


2. Leave current CU in place but replace RCD with 100mA unit and
replace 'required' circuits with their own 30mA RCD/MSB 'thingy'

This is better and within the regulations. Do you actually need a
'whole system' RCD though? It's only required if you have a TT
supply, i.e. one with a local earth. This is only likely if you have
an overhead supply.


Should I also think about getting all the garage circuitry onto a
separate circuit... now that could be painful!

Probably a good idea, but could be done in the longer term.

--
Chris Green

Ben wrote:
Any help on the following would be much appreciated. We moved into a 10
year old house earlier this year, the RCD has been fine up until last
Friday, having said that it is a 30mA rated RCD for the whole house
(groan). Since last Friday it has gone of 4-5 times a day.

There seems to be no pattern as to which device was being turned on/off
when the trip occurs.

OK, so the possible culprits:

You cant usually diagnose a fault by guesswork.

I'd sort out the CU arrangement first, not last. Theres a good chance
that will solve your nuisance trips. If not, it will narrow it down to
the one problem circuit. Alternatively you can continue with your
divide and conquer to narrow it down before any work.

A whole house 30mA RD with immersion and garage lectrics on it is truly
poor design, this was just waiting to happen.

The simple way to find the problem is to open the CU (with power off
and all breakers off) and test each circuit L to E resistance with a
multimeter. Before the purists complain, a simple multimeter will pick
up the problem in almost every case.

However if youre not happy about opening the CU, let a sparky do it.
Obviously doing so you'd be working on partly live electrics, so its
not for someone that doesnt fully understand what theyre doing.


NT

wrote:
A whole house 30mA RD with immersion and garage lectrics on it is truly
poor design, this was just waiting to happen.

Oh dear, then mine will give problems... I've only been waiting for
about 12 years, though. Not one "trip" so far.

I thought this was a prime candidate, however... having taken off the
wiring I then measured L-E and N-E resistance at the connections on the
hot water tank and found neither of these had a resistance that my
multimeter could measure (I think it's max resistance is 20 or
50Mohms). Maybe my multimeter's voltage is too low to 'stimulate' the
low resistance... he said clutching at straws!

Ben wrote:
Any help on the following would be much appreciated.

IT would help greatly to know what type of earthing you have - is it
TN-C-S (PME), or TN-S (earthed to sheath of supply cable) or TT (no
mains earth supplied & you rely on your own earth electrode)?

Remainder of reply assumes TN.

[snip details]

So, last night I:

a. Removed the light fitting from our bedroom and took a hairdryer to
the circuitry beneath... hopefully got rid of any moisture.

b. Connected the fridge/freezer to the ring main rather than the
kitchen sockets circuit and turned off the kitchen socket circuit..
(thus 'disconnecting' the sockets in the garage and all kitchen
appliances... I wonder whether the electric igniter on the gas hob has
got 'leaky'?)

c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

It's better to change only one thing at a time. Try restoring these
items one by one.

If you can venture inside the CU and disconnect the relevant neutral(s)
for each circuit you isolate, that would help a lot. In any case I
suspect there's a definite fault that can be pinned down. My money
would be on it being either either a neutral-earth short in the wiring
somewhere, or else the immersion heater (or similar) is on its way out.

So I'm wondering what I do next...

You need to do some insulation resistance tests with a proper 'Megger'
(500 V insulation tester). You won't get very far without getting
inside the CU, so maybe it's time to call a sparks.

Which of the following is best
1. Change CU to split-load system and use a 100mA RCD.

Not permitted (unless you have separate RCD protection for sockets, and
then unorthodox).

At least place lights on non-RCD side!

Unless TT-earthed, in which case a minimum of two RCDs is needed.

2. Leave current CU in place but replace RCD with 100mA unit and
replace 'required' circuits with their own 30mA RCD/MSB 'thingy'

Possible, but not "best" - and the 100 mA RCD would need to be a
time-delayed (Type S) item, to "afford discriminative operation" as the
jargon has it.

Better (still assuming TN) would be a new split load unit with a 30 mA
RCD and everything except socket circuits on the non-RCD side.
(Individual RCBOs might be "best" be the cost might put you off.)

And bear in mind that it might only be a new immersion heater that you
need, or that the problem might only be a neutral wire trapped under a
screw in a wiring box.

Should I also think about getting all the garage circuitry onto a
separate circuit... now that could be painful!

Difficult to comment without knowing (a) how it's wired now and (b) what
goes on in your garage...

how the hell do I assess an electrician as 'OK'

Strikes me you know enough of the principles to detect blatant bull****.
Get hold of copies of the IEE On-Site Guide and/or the newer
Electrician's Guide to the Building Regulations and study...

--
Andy

Chris Bacon wrote:
wrote:
A whole house 30mA RD with immersion and garage lectrics on it is truly
poor design, this was just waiting to happen.

Oh dear, then mine will give problems... I've only been waiting for
about 12 years, though. Not one "trip" so far.

Faulty RCD? ;-)

Mathew Newton wrote:
Chris Bacon wrote:
wrote:
A whole house 30mA RD with immersion and garage lectrics on it is truly
poor design, this was just waiting to happen.

Oh dear, then mine will give problems... I've only been waiting for
about 12 years, though. Not one "trip" so far.

Faulty RCD? ;-)

Bah. It works if I press "test", also it has tripped when I
had my fingers inside a socket to screw some wires in (the
downstairs ring). Bang! it threw "off", just over my head.

"Dave Plowman (News)" wrote in message
...
In article .com,
Ben wrote:
c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

That's the most likely culprit IMHO.

And not a lot of effort is needed to remove it from the circuit. As there
will be a double pole switch operating it there is no need to remove any
wires as the N-E would be seperated when the switch is turned off.

Adam

c. Completely removed the wiring from the emersion heating element

Since then... no trips.. hoorah!

That's the most likely culprit IMHO.

And not a lot of effort is needed to remove it from the circuit. As there
will be a double pole switch operating it there is no need to remove any
wires as the N-E would be seperated when the switch is turned off.

Life is never easy :-) Got home and turned on the kitchen socket
circuit. Couple of hours later, RCD trips... heyho.

At least I now know which circuit is at fault.... all I have to do now
is determine what part of it! I assume I'm looking for a L-E (it didn't
trip when the circuit was switched off) and that the most likely
culprits are the garage circuitry or maybe the gas ignition. I think
the only appliance actually plugged in when it tripped tonight was the
hob extractor fan. Will do a quick check of that plug with the
multimeter, assuming that turns up a blank it'll have to be a call out
to a sparky.

Ben wrote:

At least I now know which circuit is at fault.... all I have to do now
is determine what part of it! I assume I'm looking for a L-E (it didn't
trip when the circuit was switched off) and that the most likely

Alas not certain - a N-E fault could show up because the N may (quite
normally) rise a few volts when the circuit is loaded (depending on how
far you are along the circuit from the CU, and also on how "stiff" your
supply impedance is).

culprits are the garage circuitry or maybe the gas ignition. I think
the only appliance actually plugged in when it tripped tonight was the
hob extractor fan. Will do a quick check of that plug with the
multimeter, assuming that turns up a blank it'll have to be a call out
to a sparky.

Things with heating elements and things involving water in close
proximity to power are always likely contenders. Needless to say the
average kitchen is often full of things that match those descriptions.

You could always carry on playing the stepwise refinement game a little
more if required. You may find for example breaking the ring at a socket
half way round the circuit, disconnecting one end of the circuit at the
CU and running a temporary lash up cable from the CU to the break in the
circuit to make a new ring using the new cable plus half of the old
circuit.

Also if you can't find anything with a DMM, is there any chance you
could borrow a megger from someone?

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Ben wrote:

At least I now know which circuit is at fault.... all I have to do now
is determine what part of it! I assume I'm looking for a L-E (it didn't
trip when the circuit was switched off) and that the most likely

Alas not certain - a N-E fault could show up because the N may (quite
normally) rise a few volts when the circuit is loaded (depending on how
far you are along the circuit from the CU, and also on how "stiff" your
supply impedance is).

Point taken....

Things with heating elements and things involving water in close
proximity to power are always likely contenders. Needless to say the
average kitchen is often full of things that match those descriptions.

Well I've now unplugged everything from the sockets, the only thing
left 'powered' on is the gas ignition (as it looked quite a challenge
to get 'into' the gas hob last night).

You may find for example breaking the ring at a socket
half way round the circuit, disconnecting one end of the circuit at the
CU and running a temporary lash up cable from the CU to the break in the
circuit to make a new ring using the new cable plus half of the old
circuit.

Makes sense.

I don't know whether this is good or bad 'wiring practice' but looking
in the garage I found that it was a spur from the kitchen socket
circuit. The spur had a two socket block from which another spur leads
round the garage to a set of two 'two socket blocks' (i.e. the garage
has 3 sets of 2 socket blocks).

I thought that I may as well check that the the latter wiring was not
at fault so simply disconnected the wires connecting the first '2
socket block' to the two 'two socket blocks'. Thus the only 'live'
wiring in the garage is appox 1.5m length to the first 2 way block.
Does this 'spur' arrangement sound OK?

I'm sure that the fault will end up being behind the most difficult to
get at tiled part of the kitchen :-)

Not wishing to start a 'ruck' but can I perform anything useful testing
wise using a DMM? Presumably I'm reliant on the voltage being generated
by the DMM being large enough to 'stimulate' the fault? I assume the
approach would be to disconnect each part of the ring circuit in turn
measuring L-E and N-E on each isolated section?

Also if you can't find anything with a DMM, is there any chance you
could borrow a megger from someone?

Hmm... good question... unless the isolated garage wiring is at fault I
fear getting hold of a megger or, more likely, getting hold of a man
with a megger will be the next step! If I do manage to get hold of a
megger is it relatively straight forward using them?

Thanks for all the advice thus far.... I'm very impressed with
uk.d.i.y!

Ben

In article .com,
Ben wrote:


Not wishing to start a 'ruck' but can I perform anything useful
testing wise using a DMM? Presumably I'm reliant on the voltage
being generated by the DMM being large enough to 'stimulate' the
fault? I assume the approach would be to disconnect each part of
the ring circuit in turn measuring L-E and N-E on each isolated
section?

This is very similar to a Saga (with a capital-S) that I
had, and posted here for help a year or so ago.

Random RCD trips started just after the incoming supply
cable was replaced and I spent days, tracing red herring
after red herring. I realised early that it was an N-E
short, but took days to consider the possibilty that it
could be in the cabling, rather than an appliance.

So I powered-down the consumer unit, opened all MCB's,
and lifted all Neutrals off the commoning-block in the
CU. Then used a DMM to ohm each separate Neutral-Earth.

Sure enough one N-E showed continuity, the T&E going to
the kitchen ring. Unplugging everything off that ring
made no difference, so it *was* in the cabling.

Fortunately I had a low-resistance ohmeter as well, and
was able to target the lowest N-E resistance to one
particular socket in the kitchen. Took the socket off,
no problems in there.

Then I looked up at a row of new nails in the wall and
idly tapped one probe of the ohmeter across the heads.
Found a continuity to one of them, 4ft above the socket
and about 6 inches to the left. Pulled the nail back
and the N-E continuity disappeared.

--
Tony Williams.

So the latest in this particular Saga is that the RCD has tripped and
hence it's not in the bit of garage wiring I managed to isolate last
night.

So I powered-down the consumer unit, opened all MCB's,
and lifted all Neutrals off the commoning-block in the
CU. Then used a DMM to ohm each separate Neutral-Earth.

I assume to do this I:

1. Power down the CU, open all MCBs, lift the kitchen socket neutral
off the common block. Test for low resistance on N-E or L-E... then to
isolate:
2. Determine the order of the sockets in the ring and open each in
turn, 'break the ring', test N-E and L-E and do this round the loop
until I get a low resistance ...

Then I know it's in the last section. Is this correct?

Then I looked up at a row of new nails in the wall and
idly tapped one probe of the ohmeter across the heads.
Found a continuity to one of them, 4ft above the socket
and about 6 inches to the left. Pulled the nail back
and the N-E continuity disappeared.


And that was what killed that woman a few years ago which was what led
to part P....
--
Tony Sayer

They could have just legislated that everyone entering a kitchen has to
wear rubber wellies and have one hand tied behind their back....would
have made more sense.

In article .com, Phil
writes
They could have just legislated that everyone entering a kitchen has to
wear rubber wellies and have one hand tied behind their back....would
have made more sense.


Thought that was for bedrooms......
--
Tony Sayer

Ben wrote:

I don't know whether this is good or bad 'wiring practice' but looking
in the garage I found that it was a spur from the kitchen socket
circuit. The spur had a two socket block from which another spur leads
round the garage to a set of two 'two socket blocks' (i.e. the garage
has 3 sets of 2 socket blocks).

The first spur should be taken as a fused connection in this case.
Assuming there is a fuse for the spur (since you can take a spur from a
spur without protection from overload for the cablw) and we are talking
about an attached or integral garrage then we are on the borderlines of
good practice - depends on what the garrage power is used for.

If it is a detached garrage then it is a pretty naff way of doing the
wiring to it.

What about lights in the garrage? Are there any and how are they powered?

I thought that I may as well check that the the latter wiring was not
at fault so simply disconnected the wires connecting the first '2
socket block' to the two 'two socket blocks'. Thus the only 'live'
wiring in the garage is appox 1.5m length to the first 2 way block.
Does this 'spur' arrangement sound OK?

Yup good move. Garrages and outbuildings in general not being heated etc
are more likely to suffer problems with dampness getting into fittings
and are hence are a typical place for the origin of these types of faults...

I'm sure that the fault will end up being behind the most difficult to
get at tiled part of the kitchen :-)

Safe bet ;-)

Not wishing to start a 'ruck' but can I perform anything useful testing
wise using a DMM? Presumably I'm reliant on the voltage being generated

You can perform plenty of useful testing with a DMM. It is only the
cases where that fails to show up anything conclusive that the megger
comes in handy in that it will allow you to eliminate some catagories of
fault that you can't eliminate with the DMM.

by the DMM being large enough to 'stimulate' the fault? I assume the
approach would be to disconnect each part of the ring circuit in turn
measuring L-E and N-E on each isolated section?

Yup that would do it for starters...

Also if you can't find anything with a DMM, is there any chance you
could borrow a megger from someone?

Hmm... good question... unless the isolated garage wiring is at fault I
fear getting hold of a megger or, more likely, getting hold of a man
with a megger will be the next step! If I do manage to get hold of a
megger is it relatively straight forward using them?

Much the same as you would use a DMM on resitance range with the obvious
provisio that you are now playing with 500V and not 10V. Unplug
everything, and disconnect (or "wire out") any vulnerable electronics
that are permenently connected to the circuit that may not appreciate
the test voltage.


Thanks for all the advice thus far.... I'm very impressed with
uk.d.i.y!

Ben



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Ben wrote:

Hmm... good question... unless the isolated garage wiring is at fault I
fear getting hold of a megger or, more likely, getting hold of a man
with a megger will be the next step! If I do manage to get hold of a
megger is it relatively straight forward using them?

Much the same as you would use a DMM on resitance range with the obvious
provisio that you are now playing with 500V and not 10V. Unplug
everything, and disconnect (or "wire out") any vulnerable electronics
that are permenently connected to the circuit that may not appreciate
the test voltage.

be aware its all too easy to unintentioanlly export dangerous voltages
to metalwork around the house with one. Meggers are something best not
used by the untrained imho. A multimeter will find the problem in
almost every case.

NT

In article .com,
Ben wrote:

I assume to do this I:
1. Power down the CU, open all MCBs, lift the kitchen socket
neutral off the common block. Test for low resistance on N-E or
L-E... then to isolate:
2. Determine the order of the sockets in the ring and open each
in turn, 'break the ring', test N-E and L-E and do this round the
loop until I get a low resistance ...
Then I know it's in the last section. Is this correct?

Not really. It's a matter of testing all N-E in the CU to
find out which ring is causing trouble (if it is an N-E).
Mine was the kitchen ring, yours may be a different ring.

If you don't mind me saying so, your question bothers me
a little. It suggests that you may not be safe in doing
this. eg, You may not put things back properly safe.

Think about calling in a leckie maybe?

--
Tony Williams.

Tony Williams wrote:
If you don't mind me saying so, your question bothers me
a little. It suggests that you may not be safe in doing
this. eg, You may not put things back properly safe.

Think about calling in a leckie maybe?

I have replaced everything as it was before... however I agree entirely
with you in that it's time to call in someone who knows what he's
doing, I have a degree in electronic engineering which means I'm happy
with all volages between 0 and 5 :-)

A recommended leckie is heading round this afternoon to try and see
where the problem is.

As an aside can anyone hazard a guess about how much I should expect to
pay for someone to install a replacement split-load CU?

Cheers,

Ben

So we had a competent (I believe) electrician round, got out his
megger, tested the kitchen sockets circuit L-E and N-E and found no low
resistances at all. Having found nothing he then tested all the other
circuits and again found nothing.

(Bear in mind that the RCD only tripped, every 4-8 hours, when the
kitchen sockets were on)

He then tested the RCD and found that it was tripping at 20mA not 30mA
(although from comments I've seen in this forum I think I'm right in
saying that an 'in-spec' 30mA RCD could trip anywhere between 15 and
30mA?).

Given we wanted the CU replaced with a split load unit anyway I got him
to do that putting just the sockets circuit and kitchen sockets circuit
on the RCD side. That was 2 days ago... since then, so far (fingers
crossed), we've had no trips at all! The test certificate indicated
that all circuits were OK.

So great minds out there... what could have been causing the original
problem? Can a CU or MCB become 'faulty' in such away as to cause such
a problem?

All very confusing....

Ben

On 16 Nov 2005 01:38:00 -0800, Ben wrote:

So we had a competent (I believe) electrician round, got out his
megger, tested the kitchen sockets circuit L-E and N-E and found no low
resistances at all. Having found nothing he then tested all the other
circuits and again found nothing.

(Bear in mind that the RCD only tripped, every 4-8 hours, when the
kitchen sockets were on)

He then tested the RCD and found that it was tripping at 20mA not 30mA
(although from comments I've seen in this forum I think I'm right in
saying that an 'in-spec' 30mA RCD could trip anywhere between 15 and
30mA?).

Given we wanted the CU replaced with a split load unit anyway I got him
to do that putting just the sockets circuit and kitchen sockets circuit
on the RCD side. That was 2 days ago... since then, so far (fingers
crossed), we've had no trips at all! The test certificate indicated
that all circuits were OK.

So great minds out there... what could have been causing the original
problem? Can a CU or MCB become 'faulty' in such away as to cause such
a problem?

All very confusing....

Ben

In my house I had orecisely teh same problem. Never tracked it down to
anything partucular - just general leakage from many items of electronics
plugged in seemed to be enough to set the trip 'on the edge' and then
almost any surge would take it over.

Now equipped with 100mA RCD overall..mmust split the bus sometime...and
put 30mA on teh critical bits.

However its still sensitive enough to pop under various faults - had a
leaky kettle..two in fact. Now down to zero electric kettles from three
originally. All showed a minor leak to reath from the elements. All now
binned.

Ben wrote:

He then tested the RCD and found that it was tripping at 20mA not 30mA
(although from comments I've seen in this forum I think I'm right in
saying that an 'in-spec' 30mA RCD could trip anywhere between 15 and
30mA?).

That's right, and ~20 mA is a perfectly normal trip current for a 30 mA
RCD. I would expect an electrician to have known that.

So great minds out there... what could have been causing the original
problem? Can a CU or MCB become 'faulty' in such away as to cause such
a problem?

No, I suspect the intermittent problem is still lurking somewhere in a
circuit now on the non-RCD side of the new board.

--
Andy

Ben wrote:

So we had a competent (I believe) electrician round, got out his
megger, tested the kitchen sockets circuit L-E and N-E and found no low
resistances at all. Having found nothing he then tested all the other
circuits and again found nothing.

(Bear in mind that the RCD only tripped, every 4-8 hours, when the
kitchen sockets were on)

He then tested the RCD and found that it was tripping at 20mA not 30mA

which is right

(although from comments I've seen in this forum I think I'm right in
saying that an 'in-spec' 30mA RCD could trip anywhere between 15 and
30mA?).

Given we wanted the CU replaced with a split load unit anyway I got him
to do that putting just the sockets circuit and kitchen sockets circuit
on the RCD side. That was 2 days ago... since then, so far (fingers
crossed), we've had no trips at all! The test certificate indicated
that all circuits were OK.

reckon you did the right thing there.

So great minds out there... what could have been causing the original
problem? Can a CU or MCB become 'faulty' in such away as to cause such
a problem?

All very confusing....

Ben

Possibles:
an appliance fault
combined or variable normal healthy leakages (immersions are famous for
this, ditto other water heating elements)

Did you test all your appliances? If not, check L&N to E resistance
with the multimeter, that will pick up leak faults in almost all cases.
Dont do the classic mistake of not switching the appliance switches on
first


Re your last question, a faulty mcb cant cause an RCD trip, as there is
no earth path for fault current. Faulty CU? Theyve been known, pretty
unlikely though. Far more likely to be an appliance of some sort, or an
occasional intermittent fault in the wiring. Megers usually pick up the
latter though, since they test at higher V than mains. But no absolute
100% guarantee there.


NT