Faulty RCD or coincidence?

[frusc]

I have an 80A 30mA (WRS80/2) RCD protecting the supply to my open plan kitchen/living room area.

Last year it started tripping if I used my main oven in Fan cooking above 200c. Normal cooking worked fine at all temps. As did my Combi Micro/Oven which also has fan cooking mode.

I figured it was a defective element/fan so kept an eye out for a new oven and just used it in normal cooking mode in the meantime. It's getting on a bit, so figured rather then messing about trying to repair, may as well hope for a good deal on a new one. I'd always regretted going Steam Combi instead of Pyro.

Few days ago I picked up a new AEG BPK84272 on a clearance deal and wired it in. Worked fine for two days, including testing the Pyro cooking and Fan modes. But yesterday I switched it on to Fan mode and the RCD tripped. Every time I flicked the RCD on it tripped again within 10 seconds.

Kitchen appliances are all wired via fused spurs so they can be isolated. So I switched them all off and narrowed the fault down to the new Oven. With that spur switched off everything has been working fine, including the Combi oven.

As soon as I switch the Oven spur on, it starts too boot (its one of these new "smart" ovens) and the RCD trips within 10s. I've got AEG coming on Thursday, but am wondering if this is just a big coincidence or there may be an issue with the RCD. Previous oven was also AEG and had been working fine for 9 years using Fan mode.

There's a 13A fuse to the oven and short run of cable. Fuse is fine and oven does start boot process. Given the issue occurs before any cooking elements are fired up, makes me suspect it is a coincidence and there's a problem with the oven. And ofcourse other appliances all work fine on this RCD.

Any thoughts greatly appreciated.

 

[AndyPRK]

consider how many circuits are on the same RCD ?

Turn the MCBs off for these other circuits and pull the netural wires out the CU terminals and see if it still trips.
If it does, could be damp behind cooker/switch ?


If you have 2 RCDs, consider putting a plug on the cooker, and running an extension to a socket circuit on another RCD to rule the cooker out. Only test briefly, don't overload.

Pics of your cu and things help

 

[EFLImpudence]

Try plugging the oven into an extension lead from a circuit on a different RCD and see if that one trips

 

[JohnW2]

.... Kitchen appliances are all wired via fused spurs so they can be isolated. So I switched them all off and narrowed the fault down to the new Oven. With that spur switched off everything has been working fine, including the Combi oven. .... As soon as I switch the Oven spur on, it starts too boot (its one of these new "smart" ovens) and the RCD trips within 10s. I've got AEG coming on Thursday, but am wondering if this is just a big coincidence or there may be an issue with the RCD.

That surely sounds like something wrong with the new oven, doesn't it?

I suppose it's just conceivable that the RCD has become 'trigger happy' and is tripping on the basis of whatever 'normal leakage current' the oven may result in - but, if that were the case, one might expect it to also happen with other appliances, so I think it's very unlikley to be the problem.

Kind Regards, John

 

[frusc]

Thanks for the quick responses

Plugging into a circuit on a different RCD is a good idea. Daughters Bday today, but will give it a shot over the weekend.

I suspect it's a massive coincidence. The Spur is on an inside wall and aside from Fan cooking above 200C the other oven was working fine, pretty much used every day. This one again worked fine for two days. Plus its tripping on boot up, not even getting to heating part. I stuck the combi on fan cooking and RCD didnt trip either.

But figure best to double check, because if it is the RCD/wiring, AEG will very likely slap me with a call out charge

If it wasn't for the issue with the previous oven I wouldn't even suspect anything other then new oven is the issue.

 

[ericmark]

* Oven elements are hydroscopic they attract moisture so if the end seal is damaged then water will get in.
* We should test a RCD circuit and ensure the leakage is no more than a 1/3 of the rating in normal use, i.e. 9 mA maximum for a 30 mA trip, I have never done this, one hopes if it does not trip on the 1/2 trip current then it is within limits, each unit unless special supply should be less than 3.5 mA again I have never tested, it is around 6.6 kΩ and we would normally reject anything under 1 MΩ so should never get near the limit, but impedance is not quite the same as resistance so not really tested. However where one RCD covers many circuits it could well be the cumulative leakage exceeds 15 mA and the normal RCD is designed to trip between 15 mA and 30 mA, there are specials like the X-pole designed to give a warning when near tripping current and trip between 90 and 100% but in the main easier and cheaper to use RCBO's than the special RCD's. But only way to say if RCD or appliance is to test.
* I have had a RCD which using the RCD tester shows as OK, and I have tested the installation with an insulation tester and that also showed OK, but swapping the RCD did cure the problem, all RCD's are not equal, so do seem to trip with spikes, which will not trip others.

I would say most likely seal on oven element damaged, and only real cure is new element, but can only guess.

 

[JohnW2]

...I would say most likely seal on oven element damaged, and only real cure is new element, but can only guess.

I'm not so sure about that ....

As soon as I switch the Oven spur on, it starts too boot (its one of these new "smart" ovens) and the RCD trips within 10s.

.... its tripping on boot up, not even getting to heating part.

Kind Regards, John

 

[AndyPRK]

N to earth fault on element, fan.

but we are talking about a new oven here

 

[JohnW2]

N to earth fault on element, fan.

Yes, possibly, or maybe just iff components (e.g. the filter capacitors) on the input to the PSU for the electronics.

.... but we are talking about a new oven here

Indeed - but 'failure curves' for most manufactured products (and many other things, including human beings and other animals) are usually 'bathtub' shaped, so perhaps not a massive surprise.

Kind Regards, John

 

[frusc]

Indeed - but 'failure curves' for most manufactured products (and many other things, including human beings and other animals) are usually 'bathtub' shaped, so perhaps not a massive surprise.

Kind Regards, John

Yep, especially for electronics (and probably bathtubs as well). Fail early or several years down the line. In between (unless built on the cheap, sub standard/under rated Caps are a classic example) failure rates are usually far lower.

As I said, if it weren't for previous oven tripping RCD on Fan cooking (which I'd marked down to element, given it was almost a decade old) I'd not have suspected anything other then the oven. I'm still leaning towards it being the new oven and a coincidence. But best to check as I'm sure the call out charge wont be cheap if i'm wrong

Plugging it into another circuit seems to be the best next step. Hopefully it still trips the RCD or i'll have to do some more digging.

 

[JohnW2]

Yep, especially for electronics (and probably bathtubs as well). Fail early or several years down the line. In between (unless built on the cheap, sub standard/under rated Caps are a classic example) failure rates are usually far lower.

Indeed - and, as you say, those 'curves' seem to work in reality, as well as in theory. Thinking back over the decades, I can't think of all that many things, particularly electronic things, that have failed in 'mid-life' - but, in addition the the 'expected' late failures, I have a good few very early ones (including "didn't work out of the box").

As I said, if it weren't for previous oven tripping RCD on Fan cooking (which I'd marked down to element, given it was almost a decade old) I'd not have suspected anything other then the oven. I'm still leaning towards it being the new oven and a coincidence.

I understand your concern, but it would probably need more than one simultaneous co-incidence (even less probable) were the problem with the RCD but without it showing itself with other appliances.

But best to check as I'm sure the call out charge wont be cheap if i'm wrong

Were you to find yourself in that unfortunate position, you could at least try arguing - if a brand new (and allegedly 'non-faulty') product which created enough leakage to trip an RCD (even an out-of-spec RCD) which other kitchen appliances didn't trip, one could try arguing that it was 'not fit for purpose' or 'not of merchantable quality'!

Plugging it into another circuit seems to be the best next step. Hopefully it still trips the RCD or i'll have to do some more digging.

That experiment will certainly be pretty decisive. Let us know the result! If you are unlucky and it doesn't trip a different RCD, let us know and some other ideas for investigation will undoubtedly appear!

Kind Regards, John

 

[frusc]

Turning into one of those weeks Drain was smelling so pulled up the cover and it's full to the top. Sent the rod down ~7m and still no joy, so had to call Thames water.

Anyway that put a delay in my diagnostics. But as expected it tripped the upstairs RCD as well (same rating). So must be bad oven. Hopefully they fix it when they turn up Thursday. Everything seems to be falling apart around me right now

 

[JohnW2]

Turning into one of those weeks Drain was smelling so pulled up the cover and it's full to the top. Sent the rod down ~7m and still no joy, so had to call Thames water. ... Everything seems to be falling apart around me right now

Sorry about that - but beware - they say that 'things come in threes'!

Anyway that put a delay in my diagnostics. But as expected it tripped the upstairs RCD as well (same rating). So must be bad oven. Hopefully they fix it when they turn up Thursday.

I suppose that's really what you wanted to find - so hopefully they'll either fix it on Thursday or (I would have thought at least as likely) replace it.

Kind Regards, John

 

[frusc]

Yep, that’s what I was hoping the result would be. Saves me further diagnostics. Fortunately we have a combi Micro/oven as well. Bit of pain to use in oven mode because it’s stops when you open the door and you have to press start again. Which ofcourse you forget in the middle of cooking. So hoping for quick resolution.

Yes I’m expecting another mishap somewhere. Or perhaps one that’s occurred already but I haven’t registered. Odd how those bucket curves seem to line up in three’s

Will pop back to update what the fault is with the oven. Might help someone else that comes across this with a similar Aeg oven (BPK84272).

 

[JohnW2]

Yep, that’s what I was hoping the result would be. Saves me further diagnostics. Fortunately we have a combi Micro/oven as well. Bit of pain to use in oven mode because it’s stops when you open the door and you have to press start again. Which ofcourse you forget in the middle of cooking. So hoping for quick resolution.

I share your hope!

Yes I’m expecting another mishap somewhere. Or perhaps one that’s occurred already but I haven’t registered. Odd how those bucket curves seem to line up in three’s

Well, albeit rather different, "lining up in threes" (when they "shouldn't") always seems to have worked for buses

Now that your issue is resolved, this gives me an excuse to go wildly off-topic ....

More seriously (than "buses in threes"), given that these happenings obviously are virtually always just 'coincidences' ('random'), there are at least three relevant things to remember - one 'psychological' and the others statistical in nature...

'Psychologically', there is the issue of 'recall bias'. Occasions on which single things (in the broadest of senses) have 'gone wrong' do not generally stick in our minds very much, unless they are very major 'things'. On the other hand, if two or three (or more!) things 'go wrong' in very quick succession, then we are very much more likely to remember that, even though such an occurrence is far far less common than single 'events'.

Moving to the statistical, the 'failures' (or whatever) will nearly always be independent of one another. During the very low failure period in the middle of the curves, the probability of two or more things failing within a specified time window in the future is incredibly small (the product of the probability of each of those failures within the time window. However, given the independence, if one item fails today the probability of another failing tomorrow is no less than in would have been had the first failure not occurred ... and the same for the probability of the third after the second has occurred .. and so on.

The other statistical point is more of a 'paradox', because it is very counter-intuitive and hence much harder to get one's head around (a variant of "The Birthday Paradox"). The point here is that we are not talking about, say, three failures within a specific (pre-specified) small time window, but, rather of three failures within any 'small time window' in the course of, say, the next few years - and the probability of that is very much higher than most people's intuition would leave them to believe....

For those not aware of it, the 'Birthday Paradox" is about the probability of two people within a ('random') group having the same birthday. Again, if one was talking about the probability of two people having the same specific birthday (say 2nd June), then the probability would be extremely low - given a probability of about 1 in 365.25 (about 0.274%) that a person would have that birthday, the probability of two randomly-picked people having that birthday would be 1 in 365.25² (i.e. about 1 in 133.408, ~0.0075%) - one would therefore need a group of countless thousands of people before there was any appreciable chance that two would have a birthday of, say, 2nd June.

However, when the question changes to 'the same (any) birthday', rather than 'both birthdays = 2nd June", that's when it becomes very counter-intuitive. The truth is that one then needs a group of only about 23 people for there to be about a 50% probability of two having the same birthday, and by the time the group size gets to about 70, it is approaching a certainty (99.9%+ probability) that at least two will have the same birthday.

... and so it would be with equipment failures. If one assumes the middle part of the bathtub curve is 'flat', with a constant, but very small, probability of failure within any week in, say, the next 5 years, then the probability of two or three failing within the same week (any week) within that period is very much greater than most people would expect.

Will pop back to update what the fault is with the oven. Might help someone else that comes across this with a similar Aeg oven (BPK84272).

Yes, please do. I suspect that it may not be easy to detect the fault, and I also suspect that the engineer won't spent too much time on-site hunting for it [might try replacing the electronics boards(s) etc.], but will merely 'take it away' - but whether or not he will have a replacement in his van is a different question! I have to say that the last time I had a new appliance (a washing machine) fail within a few days of delivery, they simply took my word for it and didn't send an engineer, but merely sent a delivery guy to replace it with a new one.

Kind Regards, John