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.
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).
Indeed. Immersions are a known cause of RCD sensitisation, and hence should not ideally be fed from a RCD protected circuit (TT supplies excepted).
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 here:
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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.
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?)
Downstairs lights: hall, kitchen ceiling & under cupboard, downstairs toilet (and fan), cupboard under stairs (and alarm)
Downstairs lights: lounge, dining room
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.)
Spare
RCD PROTECTED CIRCUITS:
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Immersion heater. (This is the one that tripped.)
Sockets (radial): My office. Too many items to list here.
Sockets (ring): Main bedroom including spurs (1): Hall (1); Kitchen (6), Utility (3), upstairs bedroom
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 ;-)
On Mon, 09 Jul 2007 06:28:29 +0100 someone who may be Terry Pinnell wrote this:-
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.
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 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.
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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."
On Mon, 09 Jul 2007 08:32:40 +0100, Terry Pinnell mused:
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?
Just this one, or the RCD as well? If you lost power elsewhere then it must have been the RCD as well.
As long as these are not fed from an exiting RCD protected circuit then you can eliminate these from consideration for this problem.
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?
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.
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.
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
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.
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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.
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.
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.
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)
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.
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 here:
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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)
Well it might give you a big back voltage on disconnection, but that would be after the RCD has opened.
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)
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 ;-) ]
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:
A device on the LH side (non RCD) develops a short.
My assumption: it opens that MCB and the main LH switch.)
Leakage to earth (say 30 mA) occurs in one of those non-RCD circuits.
My assumption: nothing obvious happens.
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.
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.
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.
OK, that will be done next.
Thanks a bunch for that, I'm learning a heck of a lot from this thread!
--------- 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:
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 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 shitload 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 may want to consider e.g. a whole house 100mA RCD and 30mA RCD's for where there is a genuine possibility of shock.
Thanks. But how does that differ from what I said "...it opens that MCB and the main RCD switch" ? ^^^^^^^^^^^^^^^^^^^^^^^
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?
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