Any electricians around please?

The house suddenly lost power a couple of days ago and on checking the
circuit breaker box I found the one controlling the immersion heater
had tripped. But the immersion heater is not switched on (it hardly
ever is), so how I'm puzzled how a leak could be detected by the
circuit breaker?

After resetting that breaker there has been no re-occurrence, but with
a two week holiday imminent I'm nervous that it may happen again. It's
no big deal to return to a house with maybe some of my
electronically-controlled devices switched on (lights, radio, TV,
garden waterfall, etc), as occasionally happens due to storm
lightning, or a widespread power failure, but the fridge and freezer
would be a major issue.

The immersion heater cable from the heater coil on top of the hot
water tank in the airing cupboard goes directly to the switch on the
wall, with no intervening mains plug/socket. So presumably, to get an
ohm reading on this unit to test for leakage, I have to first switch
off at the breaker box, remove the switch panel in the airing
cupboard, unscrew a connection, and work from there?

Any practical advice would be much appreciated please.

--
Terry, West Sussex, UK

Terry Pinnell wrote:

The house suddenly lost power a couple of days ago and on checking the
circuit breaker box I found the one controlling the immersion heater
had tripped. But the immersion heater is not switched on (it hardly
ever is), so how I'm puzzled how a leak could be detected by the
circuit breaker?

Reading between the lines, you have a RCD that acts as the main switch
on you consumer unit, plus a dedicated circuit for the immersion heater
protected by a (probably) 16A MCB. Does that sound about right?

It sounds as if something caused not only the MCB to open (i.e.
overcurrent or fault current) and also the RCD (leakage current).

lightning, or a widespread power failure, but the fridge and freezer
would be a major issue.

Indeed. Immersions are a known cause of RCD sensitisation, and hence
should not ideally be fed from a RCD protected circuit (TT supplies
excepted).

The immersion heater cable from the heater coil on top of the hot
water tank in the airing cupboard goes directly to the switch on the
wall, with no intervening mains plug/socket. So presumably, to get an
ohm reading on this unit to test for leakage, I have to first switch
off at the breaker box, remove the switch panel in the airing
cupboard, unscrew a connection, and work from there?

Any practical advice would be much appreciated please.

Firstly to get some background on how these things work and the possible
causes of trips etc, you can find most of what you need to know he

http://wiki.diyfaq.org.uk/index.php?title=RCD

See the section on nuisance trips in particular.

Your particular fault is a little interesting since the MCB opened as
well. This suggests we are not looking for just a high leakage problem
in isolation (in which case the RCD would have tripped but not the MCB).

It may be you do have high leakage and a sensitised RCD as well, but you
could have a faulty immersion as well. It may be the element in it is
both thinning and drawing excess current.

You immersion switch may only be a single pole device (hence not
preventing RCD trips caused by neutral to earth leakage).

If in doubt disconnect the wires to the immersion heater prior to your
holls!

As a longer term solution look at moving to a split load consumer unit.


--
Cheers,

John.

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

On 8 Jul, 17:39, John Rumm wrote:
Terry Pinnell wrote:
The house suddenly lost power a couple of days ago and on checking the
circuit breaker box I found the one controlling the immersion heater
had tripped. But the immersion heater is not switched on (it hardly
ever is), so how I'm puzzled how a leak could be detected by the
circuit breaker?

Reading between the lines, you have a RCD that acts as the main switch
on you consumer unit, plus a dedicated circuit for the immersion heater
protected by a (probably) 16A MCB. Does that sound about right?

It sounds as if something caused not only the MCB to open (i.e.
overcurrent or fault current) and also the RCD (leakage current).

lightning, or a widespread power failure, but the fridge and freezer
would be a major issue.

Indeed. Immersions are a known cause of RCD sensitisation, and hence
should not ideally be fed from a RCD protected circuit (TT supplies
excepted).

The immersion heater cable from the heater coil on top of the hot
water tank in the airing cupboard goes directly to the switch on the
wall, with no intervening mains plug/socket. So presumably, to get an
ohm reading on this unit to test for leakage, I have to first switch
off at the breaker box, remove the switch panel in the airing
cupboard, unscrew a connection, and work from there?

Any practical advice would be much appreciated please.

Firstly to get some background on how these things work and the possible
causes of trips etc, you can find most of what you need to know he

http://wiki.diyfaq.org.uk/index.php?title=RCD

See the section on nuisance trips in particular.

Your particular fault is a little interesting since the MCB opened as
well. This suggests we are not looking for just a high leakage problem
in isolation (in which case the RCD would have tripped but not the MCB).

It may be you do have high leakage and a sensitised RCD as well, but you
could have a faulty immersion as well. It may be the element in it is
both thinning and drawing excess current.

You immersion switch may only be a single pole device (hence not
preventing RCD trips caused by neutral to earth leakage).

If in doubt disconnect the wires to the immersion heater prior to your
holls!

As a longer term solution look at moving to a split load consumer unit.

--
Cheers,

John.

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

There is always the possibility that someone has "tapped into" the
immersion cable somewhere out of sight. Did anything unexplainedly
stop working?

The 2 events may be unconnected. The MCB on your immersion may have
tripped some time ago - and went unnoticed until some time later when
the whole house lost power and you checked the consumer unit.

Is there any other event that you could associate with your RCD
tripping?

(I'm assuming you've got a whole house RCD - is that correct?)

" wrote:

The 2 events may be unconnected. The MCB on your immersion may have
tripped some time ago - and went unnoticed until some time later when
the whole house lost power and you checked the consumer unit.

Is there any other event that you could associate with your RCD
tripping?

(I'm assuming you've got a whole house RCD - is that correct?)

Thanks for those prompt and helpful replies. I'm responding to the
requests for more info:

The unit is a modern Crabtree 'modular' control unit, and carries a
label showing it has a sensitivity of 30mA.

It has two sections, and I made the following summary from the
electrician's hand-written notes when he installed it a year or so
ago:

CIRCUIT BREAKERS:
-----------------
1. Upstairs lights: 2 shaver-sockets in bathrooms; outside security
light (from loft extension)

2. Downstairs lights: hall, kitchen ceiling & under cupboard,
downstairs toilet (and fan), cupboard under stairs (and alarm)

3. Downstairs lights: lounge, dining room

4. Transformer by board (This is an add-on of my own, as part of a
gadget to detect when the front door bell was pressed, to buzz a
garden extension. Not a possible culprit IMO.)

5. Spare

RCD PROTECTED CIRCUITS:
-----------------------

1. Immersion heater. (This is the one that tripped.)

2. Sockets (radial): My office. Too many items to list here.

3. Sockets (ring): Downstairs except kitchen (5); Garage (1); Upstairs
except main bedroom (6)

4. Sockets (ring): Main bedroom including spurs (1): Hall (1); Kitchen
(6), Utility (3), upstairs bedroom

5. Cooker and 1 socket by cooker switch.

I also have two other RCD units
- In garage mains socket, protecting garden and workshop circuits
- In lounge mains socket, protecting garden pond submersed pump

After the failure I thought *all* power was lost. But now I can't be
100% sure whether I had the lights on, and I don't recall checking
them. Certainly things like fridge, microwave, etc were off.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

I haven't unscrewed the immersion heater wall switch yet, so can't
confirm whether it's single or double pole. But doesn't the fact that
this RCD was tripped mean that it *must* be single pole, with a leak?

Hope this helps further diagnosis.

I'm going to have to do some more careful reading, starting with the
link John suggested, as I don't really understand the basics, and
hence some of the advice given. Meanwhile, beginning to regret
switching from my old fuse-based unit a year or so ago! At least I
understood that ;-)

--
Terry, West Sussex, UK

On Mon, 09 Jul 2007 06:28:29 +0100 someone who may be Terry Pinnell
wrote this:-

Thanks for those prompt and helpful replies. I'm responding to the
requests for more info:

The unit is a modern Crabtree 'modular' control unit, and carries a
label showing it has a sensitivity of 30mA. [snip]

RCD PROTECTED CIRCUITS:
-----------------------

1. Immersion heater. (This is the one that tripped.)

This should have been put on the non-RCD section. If you are capable
of doing it properly, remove the MCB and put it in the spare way on
the non-RCD side.

You haven't said whether the heater was (due to be) switched on, or
not, when the RCD operated. As has been said it might just be that
the immersion heater sensitised the RCD and there was another fault
which actually caused it to operate.

It is unlikely that a fault developed on the fixed wiring to the
heater. More likely the heater element has a less than ideal
resistance. I would disconnect the immersion heater cable at the
point where it joins the fixed wiring before going on holiday, if I
had not moved the thing to the non-RCD side.

After the failure I thought *all* power was lost. But now I can't be
100% sure whether I had the lights on, and I don't recall checking
them. Certainly things like fridge, microwave, etc were off.

As all these circuits were off it is likely that the RCD operated.
This would have removed the supply from all the circuits on the RCD
side of the unit, but not those on the non-RCD side.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

The RCD controls electricity to all the MCBs on the RCD side of the
unit. It does not just control the immersion heater.

When you have read up the differences between MCBs and RCDs and how
they are arranged you may be able to give us more precise
information.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:

On Mon, 09 Jul 2007 06:28:29 +0100 someone who may be Terry Pinnell
wrote this:-

Thanks for those prompt and helpful replies. I'm responding to the
requests for more info:

The unit is a modern Crabtree 'modular' control unit, and carries a
label showing it has a sensitivity of 30mA. [snip]

RCD PROTECTED CIRCUITS:
-----------------------

1. Immersion heater. (This is the one that tripped.)

This should have been put on the non-RCD section. If you are capable
of doing it properly, remove the MCB and put it in the spare way on
the non-RCD side.

You haven't said whether the heater was (due to be) switched on, or
not, when the RCD operated. As has been said it might just be that
the immersion heater sensitised the RCD and there was another fault
which actually caused it to operate.

It is unlikely that a fault developed on the fixed wiring to the
heater. More likely the heater element has a less than ideal
resistance. I would disconnect the immersion heater cable at the
point where it joins the fixed wiring before going on holiday, if I
had not moved the thing to the non-RCD side.

After the failure I thought *all* power was lost. But now I can't be
100% sure whether I had the lights on, and I don't recall checking
them. Certainly things like fridge, microwave, etc were off.

As all these circuits were off it is likely that the RCD operated.
This would have removed the supply from all the circuits on the RCD
side of the unit, but not those on the non-RCD side.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

The RCD controls electricity to all the MCBs on the RCD side of the
unit. It does not just control the immersion heater.

When you have read up the differences between MCBs and RCDs and how
they are arranged you may be able to give us more precise
information.

OK, thanks, I'll do some more studying and come back. Meanwhile I've
taken a photo and labeled it as per my previous post.
http://www.terrypin.dial.pipex.com/Images/CU-1.jpg

(BTW, MCB #5 on the RCD side is disconnected.)

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

--
Terry, West Sussex, UK

On Mon, 09 Jul 2007 08:32:40 +0100, Terry Pinnell
mused:


As all these circuits were off it is likely that the RCD operated.
This would have removed the supply from all the circuits on the RCD
side of the unit, but not those on the non-RCD side.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

The RCD controls electricity to all the MCBs on the RCD side of the
unit. It does not just control the immersion heater.

When you have read up the differences between MCBs and RCDs and how
they are arranged you may be able to give us more precise
information.

OK, thanks, I'll do some more studying and come back. Meanwhile I've
taken a photo and labeled it as per my previous post.
http://www.terrypin.dial.pipex.com/Images/CU-1.jpg

(BTW, MCB #5 on the RCD side is disconnected.)

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

Still thing there's some confusion here but if you only reset the
'RCD' controlling the immersion heater then something is wrong
internally to the CU. If you meant the main RCD had tripped then it
could have been anything that tripped it. Was the immersion heater MCB
definitely on immediately prior to the tripping of the RCD and
erverythign losing power? Were you doing anythign at the time?
--
Regards,
Stuart.

Terry Pinnell wrote:

The unit is a modern Crabtree 'modular' control unit, and carries a
label showing it has a sensitivity of 30mA.

It has two sections, and I made the following summary from the
electrician's hand-written notes when he installed it a year or so
ago:

CIRCUIT BREAKERS:
-----------------
1. Upstairs lights: 2 shaver-sockets in bathrooms; outside security
light (from loft extension)

2. Downstairs lights: hall, kitchen ceiling & under cupboard,
downstairs toilet (and fan), cupboard under stairs (and alarm)

3. Downstairs lights: lounge, dining room

4. Transformer by board (This is an add-on of my own, as part of a
gadget to detect when the front door bell was pressed, to buzz a
garden extension. Not a possible culprit IMO.)

5. Spare

RCD PROTECTED CIRCUITS:
-----------------------

1. Immersion heater. (This is the one that tripped.)

Just this one, or the RCD as well? If you lost power elsewhere then it
must have been the RCD as well.

2. Sockets (radial): My office. Too many items to list here.

3. Sockets (ring): Downstairs except kitchen (5); Garage (1); Upstairs
except main bedroom (6)

4. Sockets (ring): Main bedroom including spurs (1): Hall (1); Kitchen
(6), Utility (3), upstairs bedroom

5. Cooker and 1 socket by cooker switch.

I also have two other RCD units
- In garage mains socket, protecting garden and workshop circuits
- In lounge mains socket, protecting garden pond submersed pump

As long as these are not fed from an exiting RCD protected circuit then
you can eliminate these from consideration for this problem.

After the failure I thought *all* power was lost. But now I can't be
100% sure whether I had the lights on, and I don't recall checking
them. Certainly things like fridge, microwave, etc were off.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

This bit does not make sense.... You said above the MCB for the
immersion opened. But your description suggests the RCD should have
tripped as well. However you only reset the RCD - presumably leaving the
immersion off at the MCB. Is this right?

I haven't unscrewed the immersion heater wall switch yet, so can't
confirm whether it's single or double pole. But doesn't the fact that
this RCD was tripped mean that it *must* be single pole, with a leak?

Not necessarily. If you read the bits on RCD sensitisation in the link I
gave before, you can see that big transient loads switching in and out
can trip a RCD if it is already close to its tripping point.

I'm going to have to do some more careful reading, starting with the
link John suggested, as I don't really understand the basics, and
hence some of the advice given. Meanwhile, beginning to regret
switching from my old fuse-based unit a year or so ago! At least I
understood that ;-)

Well on the plus side, this one has a better chance of keeping you alive
when something goes wrong in a big way ;-)

Shifting the immersion over to the non RCD side would also be a good
move. It may not cure your problem, but it will stop the immersion
contributing to it.


--
Cheers,

John.

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

On 9 Jul, 14:46, John Rumm wrote:


SNIP

when something goes wrong in a big way ;-)

Shifting the immersion over to the non RCD side would also be a good
move. It may not cure your problem, but it will stop the immersion
contributing to it.

If you follow this route make certain that you also move the immersion
heater cable neutral across to the non RCD protected neutral rail as
well or you WILL get tripping when you try to use the immersion heater

Lurch wrote:

On Mon, 09 Jul 2007 08:32:40 +0100, Terry Pinnell
mused:


As all these circuits were off it is likely that the RCD operated.
This would have removed the supply from all the circuits on the RCD
side of the unit, but not those on the non-RCD side.

The only switch I reset in the Crabtree unit was the RCD one
controlling the immersion heater.

The RCD controls electricity to all the MCBs on the RCD side of the
unit. It does not just control the immersion heater.

When you have read up the differences between MCBs and RCDs and how
they are arranged you may be able to give us more precise
information.

OK, thanks, I'll do some more studying and come back. Meanwhile I've
taken a photo and labeled it as per my previous post.
http://www.terrypin.dial.pipex.com/Images/CU-1.jpg

(BTW, MCB #5 on the RCD side is disconnected.)

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

Still thing there's some confusion here but if you only reset the
'RCD' controlling the immersion heater then something is wrong
internally to the CU.

Can you amplify on that please? Why would it imply that?

If you meant the main RCD had tripped

No, just the immersion switch on the RCD side, the one I labeled #1 in
the photo I posted.

then it
could have been anything that tripped it. Was the immersion heater MCB
definitely on immediately prior to the tripping of the RCD
and everything losing power?

Yes

Were you doing anything at the time?

Authoring a DVD on my PC ;-(

--
Terry, West Sussex, UK

Terry Pinnell wrote:

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

Still thing there's some confusion here but if you only reset the
'RCD' controlling the immersion heater then something is wrong
internally to the CU.

Can you amplify on that please? Why would it imply that?

If you meant the main RCD had tripped

No, just the immersion switch on the RCD side, the one I labeled #1 in
the photo I posted.

So, let's get this straight:

The MCB in position 1 on your CU (just that and nothing else) tripped.
In doing so it cut power not only to the immersion heater, but also to
your computer and some other appliances?

To restore normal operation you reset that single MCB for the immersion
heater, and did not need to touch the RCD itself?

If this is the case then you have a serious wiring error that needs
immediate attention.


--
Cheers,

John.

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

John Rumm wrote:

Terry Pinnell wrote:

Also, I realise that "the RCD controls electricity to all the MCBs on
the RCD side of the unit. It does not just control the immersion
heater." (That of course is why everything I mentioned lost power.)
But think I was correct in my recollection that "The only switch I
reset in the Crabtree unit was the RCD one."

Still thing there's some confusion here but if you only reset the
'RCD' controlling the immersion heater then something is wrong
internally to the CU.

Can you amplify on that please? Why would it imply that?

If you meant the main RCD had tripped

No, just the immersion switch on the RCD side, the one I labeled #1 in
the photo I posted.

So, let's get this straight:

The MCB in position 1 on your CU (just that and nothing else) tripped.
In doing so it cut power not only to the immersion heater, but also to
your computer and some other appliances?

To restore normal operation you reset that single MCB for the immersion
heater, and did not need to touch the RCD itself?

If this is the case then you have a serious wiring error that needs
immediate attention.

Although that's exactly what I recall happened, I'm now beginning to
doubt it even myself! I could swear I didn't have to reset the RCD
switch itself as well. But, as you and others have said, that doesn't
seem to make sense.

The following may be unrelated, but I'd appreciate it if you and
others would consider the following additional information to see if
it offers any new clues.

Earlier this morning I decided to do some methodical leakage tests. My
intention was to introduce a leakage from the live connection of the
circuit under test to a convenient earth, and observe what tripped.
That would at least confirm that I understood what behaviour I was
*supposed* to get from my Crabtree StarBreaker CU. Maybe it would also
reproduce that bizarre behaviour I described. I used a couple of
robust 15k resistors in parallel, theoretically giving me a leakage
around 32 mA (in my UK 240 V circuits).

I decided to start with a circuit which also had an extra RCD unit
over and above the main CU. This is a double socket RCD in the garage,
fed from #3 on the Main Switch (non-RCD) side of the CU. These power
all sorts of stuff in my garage and shed/workshop. As expected,
momentarily connecting this leakage current immediately tripped the
garage RCD. However, to my surprise, maybe 1-2 seconds later (with the
leakage removed), the *main* switch on the CU was tripped. So my PC
and various household devices and clocks went down anyway, despite my
intention to avoid that ;-)

I'd not have expected this to happen, so could someone offer a
possible explanation please? Does it offer more insight into the odd
behaviour already reported? Or totally unrelated?

In case it helps, the electrician's scrawled installation notes record
trip times of 18/19 ms for all the RCD circuits, and 7 ms for the
external garage RCD unit. There are various other columns like 'Loop
Impedance', 'CPC-CPC ohms', 'Ph-Ph ohms', 'R1+R2 or R2 ohms'; they
mean nothing to me but please let me know if they could help the
diagnosis.

---------

Pondering this over a coffee just before hitting Send, I'm wondering
if perhaps some sort of inductive effect might be responsible? Amongst
other things, I have an ancient large DC power supply in my
shed/workshop (30A), permanently connected to provide automatic garden
lighting at dusk. Could the sudden removal of power have had the
effect observed? If so, my starting choice was poor, and I'll try
another circuit. Or maybe disconnect the two plugs from the garage RCD
unit.

--
Terry, West Sussex, UK

Terry Pinnell wrote:

So, let's get this straight:

The MCB in position 1 on your CU (just that and nothing else) tripped.
In doing so it cut power not only to the immersion heater, but also to
your computer and some other appliances?

To restore normal operation you reset that single MCB for the immersion
heater, and did not need to touch the RCD itself?

If this is the case then you have a serious wiring error that needs
immediate attention.

Although that's exactly what I recall happened, I'm now beginning to
doubt it even myself! I could swear I didn't have to reset the RCD
switch itself as well. But, as you and others have said, that doesn't
seem to make sense.

Indeed, and its about to even more strange!

The following may be unrelated, but I'd appreciate it if you and
others would consider the following additional information to see if
it offers any new clues.

Earlier this morning I decided to do some methodical leakage tests. My
intention was to introduce a leakage from the live connection of the
circuit under test to a convenient earth, and observe what tripped.
That would at least confirm that I understood what behaviour I was
*supposed* to get from my Crabtree StarBreaker CU. Maybe it would also
reproduce that bizarre behaviour I described. I used a couple of
robust 15k resistors in parallel, theoretically giving me a leakage
around 32 mA (in my UK 240 V circuits).

I decided to start with a circuit which also had an extra RCD unit
over and above the main CU. This is a double socket RCD in the garage,
fed from #3 on the Main Switch (non-RCD) side of the CU. These power

You have two #3's on your photo, one is on the main switch and is
labelled "lighting" and has a 6A MCB (i.e. #3 on the left), and the
second one is on the right hand side of the CU and is labelled sockets.

I trust that the socket you were testing is not fed from the lighting
circuit, and so is in fact powered from the right hand #3?

If this is the case, then that socket *is* on the RCD controlled side of
the CU and not the non RCD side.

(Sorry if this sounds like I am teaching grandma to suck eggs, but I
need to make sure we are using the same terms): The main switch it the
big two module wide thing on the extreme left, with a red switch marked
cat No 100/MI2. This switches power to the whole CU. The five MCBs
adjacent to it are the non RCD protected ones. The three module wide
thing in the middle with the test button is the RCD, and this protects
the next five MCBs to its right.

all sorts of stuff in my garage and shed/workshop. As expected,
momentarily connecting this leakage current immediately tripped the
garage RCD. However, to my surprise, maybe 1-2 seconds later (with the
leakage removed), the *main* switch on the CU was tripped. So my PC
and various household devices and clocks went down anyway, despite my
intention to avoid that ;-)

The main switch (i.e. red thing) has no capability to trip. It is just a
switch. The only way for it to open (assuming it is not faulty) is for
you to open it.

I'd not have expected this to happen, so could someone offer a
possible explanation please? Does it offer more insight into the odd
behaviour already reported? Or totally unrelated?

The behaviour is exactly (well mostly) as expected. I would expect that
either one or both of your RCDs should have opened on the test. You have
an undesirable bit of wiring here, in that the RCD socket in the garage
is itself powered from a RCD protected feed. Unfortunately when you have
cascaded RCDs like this there is no "discrimination" between them, a
leakage current large enough to open a RCD can open either or both, and
you don't know which one will get there first (it will also usually be
inconsistent possibly giving different results each time you test)[1].

Hence the garage socket with RCD should be powered from the non RCD side
of the CU. (As it stands you could remove the RCD socket in the garage
and replace with a normal one, since its own RCD is in effect redundant)

In case it helps, the electrician's scrawled installation notes record
trip times of 18/19 ms for all the RCD circuits, and 7 ms for the
external garage RCD unit. There are various other columns like 'Loop

When commissioning a new install there are various tests that one does
on RCD circuits to check they open fast enough under fault conditions.
The limit for a normal RCD is 40ms on this test. So both yours were ok
when tested. You may not find that you get the same results every time,
but so long as they are fast enough this is ok.


Impedance', 'CPC-CPC ohms', 'Ph-Ph ohms', 'R1+R2 or R2 ohms'; they
mean nothing to me but please let me know if they could help the
diagnosis.

Probably not going to tell us much here, but FYI:

Loop impedance (i.e. resistance), is a measure of the total round trip
resistance between live and earth at (typically) the furthest socket on
a circuit. This tells you how much current will flow if a short were to
occur between live and earth. The lower this value, the higher the fault
current. The higher the fault current the faster the protective device
(breaker or fuse) will open. So a very low earth fault loop impedance is
a "good thing".

CPC-CPC Ohms (Circuit protective conductor - or Earth Wire to its
friends), is a measure of the resistance between both ends of the earth
wire in a ring circuit. A|gain it should be low. The actual values
dictated by the total length of cable assuming all joints are made
correctly. It is one of a number of continuity tests that demonstrates
the "correctness" of a new install.

Ph-Ph phase to phase (i.e. live to live) is the same thing for the live
wire (normally slightly lower than the CPC to CPC since the live wire is
a bit thicker). Test done for the same reason as above.

If you were to refer to the wire resistance table he

http://wiki.diyfaq.org.uk/index.php?... istance_table

you could probably work out the circuit length from the numbers.

R1 + R2 is a resistance measurement of the phase and neutral wires - If
you know the impedance of the earth connection provided by your
electricity supplier, it can be used to calculate the earth fault loop
impedance. (alternatively electricians will have a test meter that can
measure the loop impedance automatically)

Pondering this over a coffee just before hitting Send, I'm wondering
if perhaps some sort of inductive effect might be responsible? Amongst
other things, I have an ancient large DC power supply in my
shed/workshop (30A), permanently connected to provide automatic garden
lighting at dusk. Could the sudden removal of power have had the
effect observed?

Well it might give you a big back voltage on disconnection, but that
would be after the RCD has opened.

If so, my starting choice was poor, and I'll try
another circuit. Or maybe disconnect the two plugs from the garage RCD
unit.

Looking at the bigger picture, if when your immersion MCB tripped, it
was for example the result of an insulation breakdown that allowed a
live to earth short to occur, then it would be entirely possible for
both the MCB and the RCD to open. The same could result even if the
cause of the overcurrent was not a short to earth, but there was also
at the same time a largish leakage to earth (either from that source or
others) that meant the RCD was close to its tripping point anyway.

The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.


[1] There is a way round this using a special type of time delayed RCD
as the upstream device. Often used on TT systems (see the RCD link I
posted before for more on this)

--
Cheers,

John.

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

John Rumm wrote:

Terry Pinnell wrote:

So, let's get this straight:

The MCB in position 1 on your CU (just that and nothing else) tripped.
In doing so it cut power not only to the immersion heater, but also to
your computer and some other appliances?

To restore normal operation you reset that single MCB for the immersion
heater, and did not need to touch the RCD itself?

If this is the case then you have a serious wiring error that needs
immediate attention.

Although that's exactly what I recall happened, I'm now beginning to
doubt it even myself! I could swear I didn't have to reset the RCD
switch itself as well. But, as you and others have said, that doesn't
seem to make sense.

Indeed, and its about to even more strange!

The following may be unrelated, but I'd appreciate it if you and
others would consider the following additional information to see if
it offers any new clues.

Earlier this morning I decided to do some methodical leakage tests. My
intention was to introduce a leakage from the live connection of the
circuit under test to a convenient earth, and observe what tripped.
That would at least confirm that I understood what behaviour I was
*supposed* to get from my Crabtree StarBreaker CU. Maybe it would also
reproduce that bizarre behaviour I described. I used a couple of
robust 15k resistors in parallel, theoretically giving me a leakage
around 32 mA (in my UK 240 V circuits).

I decided to start with a circuit which also had an extra RCD unit
over and above the main CU. This is a double socket RCD in the garage,
fed from #3 on the Main Switch (non-RCD) side of the CU. These power

You have two #3's on your photo,
one is on the main switch and is
labelled "lighting" and has a 6A MCB (i.e. #3 on the left), and the
second one is on the right hand side of the CU and is labelled sockets.

I trust that the socket you were testing is not fed from the lighting
circuit, and so is in fact powered from the right hand #3?

Yes, sorry, you're right, my mistake - and I'm glad you're being more
careful with your wording than I am! The garage is fed from #3 on the
*RCD* side. [ As per my earlier full listing yesterday, which I got
right that time ;-) ]

If this is the case, then that socket *is* on the RCD controlled side of
the CU and not the non RCD side.

(Sorry if this sounds like I am teaching grandma to suck eggs, but I
need to make sure we are using the same terms): The main switch it the
big two module wide thing on the extreme left, with a red switch marked
cat No 100/MI2. This switches power to the whole CU. The five MCBs
adjacent to it are the non RCD protected ones. The three module wide
thing in the middle with the test button is the RCD, and this protects
the next five MCBs to its right.

all sorts of stuff in my garage and shed/workshop. As expected,
momentarily connecting this leakage current immediately tripped the
garage RCD. However, to my surprise, maybe 1-2 seconds later (with the
leakage removed), the *main* switch on the CU was tripped. So my PC
and various household devices and clocks went down anyway, despite my
intention to avoid that ;-)

The main switch (i.e. red thing) has no capability to trip. It is just a
switch. The only way for it to open (assuming it is not faulty) is for
you to open it.

I'd not have expected this to happen, so could someone offer a
possible explanation please? Does it offer more insight into the odd
behaviour already reported? Or totally unrelated?

The behaviour is exactly (well mostly) as expected. I would expect that
either one or both of your RCDs should have opened on the test.

OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB and the main LH switch.)

2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.

My assumption: nothing obvious happens.

3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.

My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the
individual MCB for that circuit would open, so I could identify it,
but power would remain on for the rest of the non-RCD circuits.

4. A device on the RH side (RCD) develops a short.

My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.

You have
an undesirable bit of wiring here, in that the RCD socket in the garage
is itself powered from a RCD protected feed. Unfortunately when you have
cascaded RCDs like this there is no "discrimination" between them, a
leakage current large enough to open a RCD can open either or both, and
you don't know which one will get there first (it will also usually be
inconsistent possibly giving different results each time you test)[1].

Hence the garage socket with RCD should be powered from the non RCD side
of the CU. (As it stands you could remove the RCD socket in the garage
and replace with a normal one, since its own RCD is in effect redundant)

Thanks for that advice, I'll ask my electrician for a quote. The
garage RCD was presumably installed when the house was built, about 18
years ago, before I bought it some 6 years later. Until only a year or
so ago we still had the old fuse-type CU, so presumably that was OK.

In case it helps, the electrician's scrawled installation notes record
trip times of 18/19 ms for all the RCD circuits, and 7 ms for the
external garage RCD unit. There are various other columns like 'Loop

When commissioning a new install there are various tests that one does
on RCD circuits to check they open fast enough under fault conditions.
The limit for a normal RCD is 40ms on this test. So both yours were ok
when tested. You may not find that you get the same results every time,
but so long as they are fast enough this is ok.


Impedance', 'CPC-CPC ohms', 'Ph-Ph ohms', 'R1+R2 or R2 ohms'; they
mean nothing to me but please let me know if they could help the
diagnosis.

Probably not going to tell us much here, but FYI:

Loop impedance (i.e. resistance), is a measure of the total round trip
resistance between live and earth at (typically) the furthest socket on
a circuit. This tells you how much current will flow if a short were to
occur between live and earth. The lower this value, the higher the fault
current. The higher the fault current the faster the protective device
(breaker or fuse) will open. So a very low earth fault loop impedance is
a "good thing".

CPC-CPC Ohms (Circuit protective conductor - or Earth Wire to its
friends), is a measure of the resistance between both ends of the earth
wire in a ring circuit. A|gain it should be low. The actual values
dictated by the total length of cable assuming all joints are made
correctly. It is one of a number of continuity tests that demonstrates
the "correctness" of a new install.

Ph-Ph phase to phase (i.e. live to live) is the same thing for the live
wire (normally slightly lower than the CPC to CPC since the live wire is
a bit thicker). Test done for the same reason as above.

If you were to refer to the wire resistance table he

http://wiki.diyfaq.org.uk/index.php?... istance_table

you could probably work out the circuit length from the numbers.

R1 + R2 is a resistance measurement of the phase and neutral wires - If
you know the impedance of the earth connection provided by your
electricity supplier, it can be used to calculate the earth fault loop
impedance. (alternatively electricians will have a test meter that can
measure the loop impedance automatically)

Pondering this over a coffee just before hitting Send, I'm wondering
if perhaps some sort of inductive effect might be responsible? Amongst
other things, I have an ancient large DC power supply in my
shed/workshop (30A), permanently connected to provide automatic garden
lighting at dusk. Could the sudden removal of power have had the
effect observed?

Well it might give you a big back voltage on disconnection, but that
would be after the RCD has opened.

If so, my starting choice was poor, and I'll try
another circuit. Or maybe disconnect the two plugs from the garage RCD
unit.

Looking at the bigger picture, if when your immersion MCB tripped, it
was for example the result of an insulation breakdown that allowed a
live to earth short to occur, then it would be entirely possible for
both the MCB and the RCD to open. The same could result even if the
cause of the overcurrent was not a short to earth, but there was also
at the same time a largish leakage to earth (either from that source or
others) that meant the RCD was close to its tripping point anyway.

The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.

OK, that will be done next.


[1] There is a way round this using a special type of time delayed RCD
as the upstream device. Often used on TT systems (see the RCD link I
posted before for more on this)

Thanks a bunch for that, I'm learning a heck of a lot from this
thread!

--
Terry, West Sussex, UK

Terry Pinnell wrote:
Hasty correction!

---------
OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB, but power remains for all other LH
and RH (RCD) circuits; and it probably blows a plug fuse too.

(The 'main LH switch' is MANUAL only, as you pointed out.)

--
Terry, West Sussex, UK

Terry Pinnell wrote:

OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB and the main LH switch.)

2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.

My assumption: nothing obvious happens.

3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.

My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the
individual MCB for that circuit would open, so I could identify it,
but power would remain on for the rest of the non-RCD circuits.

4. A device on the RH side (RCD) develops a short.

My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.


Not always. The various voltage spikes and surges associated with that
OFTEN trip the RCD AS WELL. Or sometimes they don't. They may alos trip
other non associated RCD's


There is always *some* earth leakage, often of a capacitative nature (if
nothing more than a ****load of T&E running together)and my experience
is that a nasty spike on the live caused first by a short, and then by a
trip opening up, often trips an RCD. ANY RCD.




You have
an undesirable bit of wiring here, in that the RCD socket in the garage
is itself powered from a RCD protected feed. Unfortunately when you have
cascaded RCDs like this there is no "discrimination" between them, a
leakage current large enough to open a RCD can open either or both, and
you don't know which one will get there first (it will also usually be
inconsistent possibly giving different results each time you test)[1].

Hence the garage socket with RCD should be powered from the non RCD side
of the CU. (As it stands you could remove the RCD socket in the garage
and replace with a normal one, since its own RCD is in effect redundant)

Thanks for that advice, I'll ask my electrician for a quote. The
garage RCD was presumably installed when the house was built, about 18
years ago, before I bought it some 6 years later. Until only a year or
so ago we still had the old fuse-type CU, so presumably that was OK.


You may want to consider e.g. a whole house 100mA RCD and 30mA RCD's for
where there is a genuine possibility of shock.




The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.

OK, that will be done next.


Good advice.

But don't discount a chewed through cable either.

Terry Pinnell wrote:
Terry Pinnell wrote:
Hasty correction!

---------
OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB, but power remains for all other LH
and RH (RCD) circuits; and it probably blows a plug fuse too.

(The 'main LH switch' is MANUAL only, as you pointed out.)


Not necessarily. See previous post. RCD's may trip as well.

The Natural Philosopher wrote:

Terry Pinnell wrote:

OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB and the main LH switch.)

2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.

My assumption: nothing obvious happens.

3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.

My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the
individual MCB for that circuit would open, so I could identify it,
but power would remain on for the rest of the non-RCD circuits.

4. A device on the RH side (RCD) develops a short.

My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.


Not always. The various voltage spikes and surges associated with that
OFTEN trip the RCD AS WELL. Or sometimes they don't.

Thanks. But how does that differ from what I said "...it opens that
MCB and the main RCD switch" ?
^^^^^^^^^^^^^^^^^^^^^^^

They may alos trip other non associated RCD's

So am I right in concluding that's a disadvantage compared to my old
fuse-based CU? If say my electric drill in the garage (an RCD circuit)
burns out, at least with a fuse system the garage lights wouldn't have
gone out, leaving me fumbling in the dark. You're saying that can
often happen with a modern CU?

There is always *some* earth leakage, often of a capacitative nature (if
nothing more than a ****load of T&E running together)and my experience
is that a nasty spike on the live caused first by a short, and then by a
trip opening up, often trips an RCD. ANY RCD.




You have
an undesirable bit of wiring here, in that the RCD socket in the garage
is itself powered from a RCD protected feed. Unfortunately when you have
cascaded RCDs like this there is no "discrimination" between them, a
leakage current large enough to open a RCD can open either or both, and
you don't know which one will get there first (it will also usually be
inconsistent possibly giving different results each time you test)[1].

Hence the garage socket with RCD should be powered from the non RCD side
of the CU. (As it stands you could remove the RCD socket in the garage
and replace with a normal one, since its own RCD is in effect redundant)

Thanks for that advice, I'll ask my electrician for a quote. The
garage RCD was presumably installed when the house was built, about 18
years ago, before I bought it some 6 years later. Until only a year or
so ago we still had the old fuse-type CU, so presumably that was OK.


You may want to consider e.g. a whole house 100mA RCD and 30mA RCD's for
where there is a genuine possibility of shock.




The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.

OK, that will be done next.


Good advice.

But don't discount a chewed through cable either.

Thanks, appreciate your help.

--
Terry, West Sussex, UK

The Natural Philosopher wrote:

Terry Pinnell wrote:
Terry Pinnell wrote:
Hasty correction!

---------
OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB, but power remains for all other LH
and RH (RCD) circuits; and it probably blows a plug fuse too.

(The 'main LH switch' is MANUAL only, as you pointed out.)


Not necessarily. See previous post. RCD's may trip as well.

So that could be a possible explanation for the behaviour described in
my opening post?

--
Terry, West Sussex, UK

Terry Pinnell wrote:
The Natural Philosopher wrote:

Terry Pinnell wrote:
Terry Pinnell wrote:
Hasty correction!

---------
OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB, but power remains for all other LH
and RH (RCD) circuits; and it probably blows a plug fuse too.

(The 'main LH switch' is MANUAL only, as you pointed out.)

Not necessarily. See previous post. RCD's may trip as well.

So that could be a possible explanation for the behaviour described in
my opening post?

I have to say I was too busy to read it properly then.

But if you had an MCB and an UNRELATED RCD trip, then yes.

ANY spike on ANY line can trip an RCD if there is enough capacitance
around, and with a few computers and so on, around, there generally is..

For some reason my newsfeed is being slow today, so this post will be
threaded in the wrong place as I have just grabbed the text from google
and pasted a reply somewhere close to the right place)

Terry Pinnell wrote:

I trust that the socket you were testing is not fed from the lighting
circuit, and so is in fact powered from the right hand #3?

Yes, sorry, you're right, my mistake - and I'm glad you're being more
careful with your wording than I am! The garage is fed from #3 on the
*RCD* side. [ As per my earlier full listing yesterday, which I got
right that time ;-) ]

Good, on the right tracks then.

OK, but this is where I'm still obviously being dense. Given a normal,
properly working setup, can you please summarise what *should* happen
and which switches in the main CU should open for each of the
following events:

1. A device on the LH side (non RCD) develops a short.

My assumption: it opens that MCB and the main LH switch.)

As per your follow up post, the MCB would open, the main switch will
stay put.

2. Leakage to earth (say 30 mA) occurs in one of those non-RCD
circuits.

My assumption: nothing obvious happens.

Correct.

3. Leakage to earth occurs (say 30 mA) in an RCD circuit, such as if I
accidentally touched a live wire while standing on damp ground.

My assumption: I'd jump a bit but within a few ms the main RCD would
trip, so all power to *all* RCD protected circuits would be lost, the

Correct.

individual MCB for that circuit would open, so I could identify it,

Alas no. A MCB will only open on an overcurrent. This needs to be either
a "fault current" or an "overload current".

See for an explanation:

http://wiki.diyfaq.org.uk/index.php?...ault_Cu rrent

but power would remain on for the rest of the non-RCD circuits.

Correct. The big advantage here si you are not plunged into darkness
should you get a trip. Historically they used to use a whole house RCD
which would result in just this effect. What they found was it reduced
electric shock injury, but then *caused* more tripping and falling injuries.

4. A device on the RH side (RCD) develops a short.

My assumption: it opens that MCB and the main RCD switch, but power
would remain on for the rest of the non-RCD circuits.

Depends. If the short is Live to Neutral then the MCB will open, but not
the RCD. If it was live to earth then chances are one or both would open.

The fact that the MCB opened would suggest that the immersion is faulty
or there is a wiring error / damage on its circuit. These things don't
last forever so a faulty element would be my first guess. As a first
step I would suggest you need to isolate it from its supply and test it.

OK, that will be done next.

A simple multimeter test will be a good start. Stick it on its highest
ohms range and check it looks like an open circuit when you measure
between live and earth and neutral and earth. Stick it on its lowest
ohms range and check it is about right for the power of the element. So
a 3kW element at 240V should have a resistance of 240^2/3000 = 19.2
ohms. If it is significantly less then that is your problem. Note that
it is possible that it may look ok on a multimeter, but then give poor
readings at mains voltage - you would need more sophisticated test
equipment to test this however.


--
Cheers,

John.

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