Nuisance tripping RCD - down to oven I think?

Am having a hell of a time with nuisance tripping at the moment. It happens intermittently, and when it does, you can just immediately flip the breaker back and it works again. So it's very hard to localise. It's definitely in the kitchen; and I'm fairly sure it happens when the oven's on; but seems it may be more likely when the microwave's on as well?
When it does trip, it's the main RCD which goes; however sometimes an individual MCB flips as well. I hadn't particularly noticed which one, but I this morning I spotted that it was the main ringmain for all sockets in the house *other* than the kitchen! (the oven has a dedicated radial, and the kitchen sockets - including microwave - are on another MCB).
First question - do the above observations add up at all? The MCB thing makes no sense to me at all; I wonder if today's observation was just a one-off oddity today?
The first time this happened I thought (well, assumed, as I'd experienced this before) the problem was a duff element in the oven and just ordered a new one; when it arrived though (during which time there'd been no more trips) I checked the old and new with a multimeter, found identical resistance across terminals (20 ohms IIRC) and to earth (open circuit) and decided I'd been wrong, so returned the new element unused. Now wondering if I was incorrect in doing so. Could there be undetected earth leakage (which I don't have kit to detect? Could it be somewhere else in the oven? Or the house wiring? Oven is a built-in one about 10 years old; main element was replaced about 9 months ago.
Right now I'm really confused - I don't know whether I should be looking at appliances (which!?) or wiring. I'd happily buy a new bloody oven if that would definitely solve it, but I am concerned it might not!
Problem's really made worse by being intermittent, and because it seems to involve appliances we really can't do without.
Any advice on how to proceed most welcome!
--
David

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Lobster wrote:

It is quite likely that you have a number of devices each with small leaks to earth, none of which will cause a trip in its own right but in combination can add up to enough to cause a trip. The only real way is to measure the differential current in the likely candidates and fix them 1 by one. Immersion heaters are another favourite problem source. Even when not used, can give an earth to neutral leak. Disconnect completely if you are not using it whilst testing.
Some electrical leaks will only be present when a voltage is present and perhaps say a heating element is hot.
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This never happened with fuses. I'm often suspicious that these detectors are now more electronic than mechanical and as such prone to all sorts of interference causing them to trip for an invalid reason. Can you not try some alternative trips just in case its fooling you? Brian
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Lobster wrote:

Check that the labels actually do match the MCB circuits! And make sure who ever resets the MCB when it trips notes which one it is.
Any outside circuits on the RCD? I am thinking wet weather and NOT the oven may be the cause. If it was the oven I would (in most cases) expect it to happen every time the oven was used.
Good luck.
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Adam



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s

ho

en

also if a multi-element oven (fan, conventional top/bottom, grill etc) it may be only when a certain (failing) element is in use...
Jim K
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Yes, I have a combination oven which is OK on microwave only, but trips the RCD on grill or convection settings. Almost certainly a duff heating element, and as it's an old Comet CombiChef spares may be a problem.
Elsewhere I have a socket (which I haven't yet checked because it's in a rather inaccessible place) which probably has an N to E fault as discussed before on this NG. The symptoms are that if a vacuum cleaner (which is OK on other circuits and shows no fault on a multimeter check) is plugged in there, random trips occur, once when something heavy fell over causing floor vibration, once when the vacuum was turned on, and once when a light switch was turned on *elsewhere*. But nothing except the RCD is affected. None of this happens after the plug has been removed from that socket.
I'm expecting to find bad neutral or earth insulation behind the faceplate.
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My son had the oven occasionally powering down in the middle of cooking his tea. We discovered that the oven RCD also fed the "plant room" at the other side of the garden and the lights up his path. The plant room had another set of breakers feeding (we think) a faulty light on the garden wall, a summerhouse (with leaking roof that we still haven't fixed) and on from there to the pump in the fish pond via a weatherproof box.
It took an awful lot of walking before we isolated the problem to the weatherproof box. He rewired this and he is now able to rely on hot meals again.
I may be asking about pond pumps later, because the took the opportunity to replace the impeller/rotor assembly in the pump. The new one looks identical to the old, but not worn, turns freely on its shaft and seems to have equally strong magnets in the same places on the rotor, but the pump doesn't run when we install it. The old one is now back in there and the fish seem happy again.
The house has some other rather eccentric wiring.
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Bill

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On 18/03/2013 15:20, Lobster wrote:

It suggests that your problem may be deeper than just excess earth leakage. A MCB trip can only be the result of wither overload or very brief (and very high) current surges on the circuit it is protecting. For an overload on a 32A MCB you would need to be pulling a significant long term current (it should handle 40A indefinitely for example)
However the combination of high ish earth leakage (i.e. not enough to trip the RCD on its own - but "sensitising" it - moving it close to tripping, combined with a large transient (i.e. load switching on or off) can be enough to trip the RCD.

Using an insulation resistance meter would be the way to be sure. If the oven is a plug in type, then withdrawing the plug, bridging L & N together, and then measuring the resistance between L+N to E on a 500V range would eliminate that from your enquires.
(last time I went looking for an oven tripping problem, it eventually came down to the lady of the house being a touch over enthusiastic while wiping down some tiles near the oven, and allowing water to drip into the plug! Needless to say I had the whole oven apart testing the element, fan, lamp etc right back until I had nothing but a plug on the end of a bit of flex that was still reading low!)

You may be barking up the wrong tree.

What test equipment do you have access to?
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On 18/03/2013 18:05, John Rumm wrote:

Hmm, I've just been advised there's been a bit of domestic confusion about what's causing the trip; apparently our temporarily resident daughter has been trying to switch to grill mode without realising[1], when using the oven which (a) may have damaged something and (b) accounts for some of the muddle. In the interim no more trips so hopefully it's the grill element. But need to investigate further when I have a bit of time.

Just an ordinary multimeter. And a lump hammer...
[1] to be fair the icons on the oven dial are completely bizarre and incomprehensible - I just use it at the "4 o'clock" position without paying any attention to the icons)
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a multimeter almost certainly won't show leakage from the element of the grill. You will need a "megger" or PAT tester which uses at least 250v
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On Monday, March 18, 2013 3:20:48 PM UTC, Lobster wrote:

You need to troubleshoot, or your efforts will mostly be in vain. I'd insulation test every appliance first.
NT
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On Mar 18, 8:28 pm, snipped-for-privacy@care2.com wrote:

ulation test every appliance first.

I seem to have done my usual of writing to the thread and then failing to Send it ---dohhh!
About a year or so ago a neighbour was having bother with his barn lighting. It was an excuse for me to buy one of the Ebay ~£30 insulation testers. Inevitably in the time it took to arrive, the problem was found and I've never found a further excuse to use it. Has anyone had any experience of these - do they work - are they to be relied on?
If there is a favourable answer, then that David, is your solution !!
Rob
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On 19/03/2013 22:24, robgraham wrote:

This type of thing I presume?: http://www.ebay.co.uk/itm/321021300857
I'm quite interested... (if only to keep in a cupboard as a guarantee of no further occurrences, like you!)
Are they straightforward to use - are the basically like using a multimeter on a very high-range resistance setting?
Pitfalls for the unwary, etc?
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David

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Lobster wrote:

Well I always get a new apprentice to hold the cables at the other end to the tester when I am doing a 500V test:-)
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On 2013-03-20, ARW wrote:

And the survivors go on to the next rite of passage?
http://oglaf.com/riteofpassage/
(NSFW cartoon)
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Adam Funk wrote:

:-) I have not killed one yet.
Although I nearly did today when he wiped the memory from a door entry system that rendered all the working key fobs redundant.
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Adam



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On 19/03/2013 22:24, robgraham wrote:

Even a second hand real megger should not set you back more than £50 ish...
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On Monday, March 18, 2013 3:20:48 PM UTC, Lobster wrote:

ns intermittently, and when it does, you can just immediately flip the brea ker back and it works again. So it's very hard to localise. It's definitely in the kitchen; and I'm fairly sure it happens when the oven's on; but see ms it may be more likely when the microwave's on as well? When it does trip , it's the main RCD which goes; however sometimes an individual MCB flips a s well. I hadn't particularly noticed which one, but I this morning I spott ed that it was the main ringmain for all sockets in the house *other* than the kitchen! (the oven has a dedicated radial, and the kitchen sockets - in cluding microwave - are on another MCB). First question - do the above obse rvations add up at all? The MCB thing makes no sense to me at all; I wonder if today's observation was just a one-off oddity today? The first time thi s happened I thought (well, assumed, as I'd experienced this before) the pr oblem was a duff element in the oven and just ordered a new one; when it ar rived though (during which time there'd been no more trips) I checked the o ld and new with a multimeter, found identical resistance across terminals ( 20 ohms IIRC) and to earth (open circuit) and decided I'd been wrong, so re turned the new element unused. Now wondering if I was incorrect in doing so . Could there be undetected earth leakage (which I don't have kit to detect ? Could it be somewhere else in the oven? Or the house wiring? Oven is a bu ilt-in one about 10 years old; main element was replaced about 9 months ago . Right now I'm really confused - I don't know whether I should be looking at appliances (which!?) or wiring. I'd happily buy a new bloody oven if tha t would definitely solve it, but I am concerned it might not! Problem's rea lly made worse by being intermittent, and because it seems to involve appli ances we really can't do without. Any advice on how to proceed most welcome ! -- David
How about one of the heaters. It sounds like that something is expanding (g etting hot) then tripping the RCD, like a dry joint in a curcuit board
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We had this problem with our light circuit, it kept randomly tripping but then would sometimes go straight back on. It went on for weeks. We ended up testing all the lights on the circuit but still could not isolate the fault. Our friendly local electrician and his mate visited and were in the house for four hours before they finally diagnosed the fault.....a mouse had nibbled through one of the cables in the crawl space between the first and second storey. There were burn marks around the nibbled area. Cable was replaced and all is well now. This also happened to a friend, and her house burnt down. We got off lightly.
"zaax" wrote in message
On Monday, March 18, 2013 3:20:48 PM UTC, Lobster wrote:

How about one of the heaters. It sounds like that something is expanding (getting hot) then tripping the RCD, like a dry joint in a curcuit board
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I posted here a few months ago about nuisance tripping with my oven:
https://groups.google.com/forum /#!topic/uk.d-i-y/v3KUpUAUHIg
In the end, I decided the problem had to be down to the grill element, so I bought and fitted a new one, and the issue went away.
However yesterday, about 2 months on, we switched on the oven (*not* the grill element) and shortly afterwards, 'pop' the CU tripped out. FFS! Examining the CU I discovered that it was the MCB for the sockets (along with the main RCD) which had tripped, and *not* the MCB which feeds the dedicated oven radial.
Reset everything, turned the oven back on and it instantly tripped again. This time no MCBs had gone, it was just the main RCD.
I think it's time I bought myself one of those megger testers off ebay! But can anyone hazard a guess WTF is going on from the above symptoms? In particular why did the MCB for the sockets trip? Red herring?
Thanks
--
David

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