Annoying as it may seem at first, I'd be tempted to keep it this way as a feature to let you know when your oven is ready and then you won't miss putting the turkey in.
Better still, move the oven onto a non RCD protected circuit. Devices with heating elements of this type are prone to high earth leakage currents even when working correctly, and hence can cause problems such as you are seeing. Replacing it would not guarentee a fix.
no. The problem is earth leakage in the oven plus inappropriate earth leakage trip in the CU. Earth leakage is a risk on many items, but on fixed appliances like ovens its common and harmless. Hence the real solution is to get the oven off the RCD. Replacing the CU with a split load unit is an option, though its a pain amount of work.
Part of the problem here is you have a so called "whole house RCD" - i.e. once single RCD (or RCBO in this case) protects all circuits. This arrangement is now depricated because of the potential problems it causes with loss of unrelated circuits (especially lighting ones) in the event of a fault elswhere).
The choice of protective gear is interesting since it is kit not often used for domestic installs. Where does your power feed come from?
Well not bypass so much as either replace it or install an additional one. It is common these days to use a "split load" CU that has both RCD protected and unprotected ways for MCBs. That way you can arrange circuits so that only those that benefit from RCD protection get it [1].
Not really. You only have 30mA of total leakage to play with - in fact less since the RCD part of the incomer can trip at 66% of this figure and still be in spec. You are probably just too close to the limit.
[1] note you will need to check wat sort of earthing your supply uses:
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- one type known as TT requires *all* circuits to have RCD protection. However in these cases you use more than one RCD and have a higher trip threshold model on non critical circuits.
So why does the trip always occur when, and only when, the thermostat cuts out? I suspect the problem here is nothing to do with leakage in the metal-clad elements, but is more likely to be due to a faulty oven thermostat. Perhaps when the contacts open they arc badly and the arc propagates until it touches some component in contact with the earthed metal thermostat housing - aided, quite possibly by the thing being greasy and dirty inside. Without the RCD in line the fuse would blow, or the MCB trip.
As a first step I suggest replacing the oven, or at least its thermostat
- which might be an easy repair job. An insulation resistance check on the appliance would also be prudent, if you've got a megger handy.
More likely I tihnk that the arcing of the opening bimetal stat puts hf on the wiring, thus greatly increasing capacitive leakage, and pop goes the rcd.
Another oven will be likely to produce the same result, either immeiately or after not long.
There is also the possibility that the stat's in the neutral line to the element, in which case swapping/correcting l/n feed or replacing the element would work.
But really the oven is only part of the picture, unless you fix the CU youre going to have the same thing occur again. For most houses a split box would be a real improvement, with lighting and fixed appliances on the non-rcd side. If you have TT earthing its not quite as straightforward.
That seems a rather contrived explanation. In any case RCDs are filtered, either mechanically or electronically, to reduce their sensitivity to harmonics and fast transients. Direct arcing or tracking to earth is far more likely in my view.
I think that's nonsense. If the problem it is the thermostat a replacement should provide a complete long-term cure (barring multiple faults. Similarly for a new oven. The earth leakage for such an appliance in good condition shouldn't be more than a mA or two - the IEE Code of Practice for In-service Inspection & Testing (the PAT testing code) states 0.75 mA per kW of loading, with max. of 5 mA.
Reversed polarity did occur to me, but the symptoms would be the same, surely.
In every case of RCD nuisance tripping I've ever investigated (which I'll admit is not all that many) there has been a definite cause - usually a faulty appliance. While I'd be the first to agree that ideally there shouldn't be a whole-house 30 mA RCD, replacing or upgrading the CU in this case seems like gross overkill - and it probably won't cure the underlying problem in any case.
this doesnt make them insensitive to it though, it just helps with this known issue. Thats why the filtering is in RCDs.
How is that going to work though? If the oven's wired properly, the stat is in the live line, and it wont be a 2 way stat, just on/off. How is opening of the contacts going to cause an arc or track to anything else? I would say thats most unlikely. You'd have to have a piece of charcoal positioned very accurately to do that, and it would have to stretch from contact to case, and I'm not clear how it would ever have got there in the first place. I think very unlikely.
The only hard fact is we dont know the cause or causes yet. Either it will or wont recur are both perfectly likely. Given this, replacing the oven or part of it is not a safe solution. It might work, but given that I cant see any realistic scenario for the oven being the prime cause, its hard to see how replacing oven is going to fix it.
A new CU is in the same =A3 region as a used oven, and I'd be more inclined to spend the money there. That would give a 100% sure fix, plus avoid future whole house rcd problems.
This means little in practice. Reality is ovens often leak, as do most types of fixed heating appliance. Putting them on a whole house rcd is just asking for trouble.
Yes, I'm suggesting it as a possible cause of this problem. If the stat is in the N line its easier to fix: correct polarity and replace element.
Its certain to cure it. How can it not? Put the oven on the non-rcd side and I guarantee you it wont trip the rcd. Leaky ovens are perfectly safe, and very common.
I'd agree it seems heavy handed, but really it is the one sure solution. The OP could muck about with this and that and maybe get it going again, maybe not, but the same deal is always waiting to recur. Whats it all going to cost over the years in time and parts, and a lost freezer load of food? Its just not worth it.
A new CU isnt that expensive, and its a guaranteed cure. If its not done, no definite cure is in place, whatever else is done.
It's the sort of thing that does happen though. These things get dirty inside, dust and grease build up and reduce creepage and clearance distances until you eventually get a fault to the earthed casing. If it is tracking (creepage) it will get slightly worse every time as more carbon gets deposited.
What else could it be? We know that the trip only occurs as the 'stat contacts open, so simple resistive leakage in the element can be ruled out. In any case that only tends to occur when the element is damp, e.g. with an appliance that hasn't been used for a long period, particularly if it's been stored in an unheated garage or shed, or with immersion heater and washing m/c elements when a pinhole develops. That leaves something associated with the 'stat itself, or possibly breakdown in the element or the wiring due to the transient voltage spikes which occur as the contacts open. The latter will of course be accentuated if the contacts open slowly (again due to the 'stat mechanism being dirty or 'gummed up') causing prolonged arcing.
I accept that the cause is not known for certain, but your latter assertion seems to be a huge illogical leap. Both options are _possible_, but they're certainly not equally likely. There are probably a few million installations with whole-house 30 mA RCDs or ELCBs and most these do not have problems with frequent tripping. In my workshop building I have two storage heaters (containing similar elements, and thermostats) on a 30 mA RCD circuit group (because it's TT-earthed). The number of nuisance trips in the twelve years since installation has been precisely zero.
I completely disagree with you here. An oven, or any other domestic appliance, in fair condition simply should not leak enough to trip a 30 mA RCD.
That would be a very cheap CU (fitted and properly tested and certified) or a rather expensive oven. There is no evidence here about whether or not the CU needs replacing, although the 30 mA whole-house RCD situation certainly "requires improvement" (PIR code 2). Replacing perfectly good CUs is something of a racket at the moment. Removal of the RCD and provision of local RCD-sockets (to meet 471-16-01) might be all that is required (unless it's TT of course).
Nor can you say it "would give a 100% sure fix." If the fault is in the oven it's entirely possible - quite likely, in fact - that it will continue to deteriorate until it operates the OPD and/or self-destructs, or perhaps just goes o/c and stops working.
Reality is that such things simply do not leak significantly unless faulty. The RCD provides early warning of something being wrong.
You'll have to explain your reasoning behind that, because I'm completely baffled.
No it isn't.
See above.
Can't argue with that :-)
No and no. (Although they doubtless exist and are probably more common than is desirable.
The oven can go months without use. The bin however sees constant use from takeaway cartons.
And the kitchen is an add-on and albeit brick, does resemble a cold damp draughty shed.
Do they tend to build more expensive ovens from better components? The oven in there now was from B&Q and was under £200. The oven it replaced was a cheapo, skipped because where the wiring attached to the grill element the insulation had crumbled away and the wire/spade connectors had drooped to the point where the bare wire was in contact with the casing (obviously tripping the supply). I bent then away a few times but the problem reoccured.
If anyone can recommend a good brand of oven that would be great, I would like to cook a meal without my head in the meter cupboard half a dozen times.
Not dissagreeing with the general thrust of what Andy is saying here...
Indeed, although I have seen setups where there was enough cumulative leakage from all circuitsd combined that a slightly leaky oven was the last straw that pushed the RCD to its trip threshold - at which point step changes in load alone were enough to generate a trip.
Well, I could sit here all day disagreeing with some of the things you say, I'll just pick one or 2.
No, see further down
But in this case we DO have a nuisance trip.
this just isnt the case... but its a long thread on its own.
With the stat switching the N line, the element sees either of 2 conditions while the oven is on: L one end N the other, or L both ends. Add leakage at the usually N end, and youve got an element thats ok when on, but trips the rcd when its switched off.
they are not a cause of shocks or fires, and that equals safe. Same deal as immersion heaters, which are also often conducting through the water. And no, I'm not starting a long thread on it. :)
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