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