RCD (sometimes) trips on switching OFF an ELO

[telectrix]

there's always the alternative




SALAD

 

[topquark]

there's always the alternative
SALAD

That one of your cryptic mneumonics for a sequence of tests Tel?

 

[pushrod]

there's always the alternative




SALAD

or ours would be TAKEAWAY

 

[Des 56]

there's always the alternative

SALAD

An excellent suggestion but it has its flaws

The Salad solution may only be a partial solution

Who is to say the meat eaters in the family will accept this
A deep roast in a suitable oven exceeds the taste of cold chicken portions I'm sure

 

[charlhu]

So you have a hob and oven connected in a junction box , mmm that always rings alarm bells to me especially if its a modern oven that can be plugged into a socket why i hear you ask ,well if and i say if the oven is the type i have mentioned then it is supplied with its own cable and normally is 1.5mm , connected to a cooker circuit protected by a 32 amp MCB ,doing this would provide no overload protection to the oven cable , this wiring should be protected by a FCU easily done behind the cooker circuit or should be on its own circuit mean while back to the problem , it could be the start of the RCD becomming fault and you may find that it will start to trip occasionally when other items are switched on or off, this should be checked , claire is right in some ways , its could well be a switch problem where the switch is sticking closed when its switched off before opening thus not making a clean break of the connection causing an imbalance /slight arch always a possability , if its an RCD fault then the only real way to find out would be to replace the RCD , a Ramp test will show the tripping current how ever testing may not show up the described fault

I think you may have hit it!
When I originally bought and installed this oven in France it had a cable with a continental plug and there was socket in the kitchen for it on a separate circuit from the hob. I put it down to differing electrical systems (e.g. they use fused circuits, individual neutrals back to the board, etc) so without further thought I just put it on to the cooker circuit as I described. Seems that could be the root of the problem. Also your theory of "cumulative small leakages" reaching the trip threshold makes a lot of sense and I will try to check it out by disconnecting everything in sight though ithat may not be conclusive as it doesn't trip "on request".
Either way it seems that it would be best to have a separate circuit installed. I don't entirely understand your wiring proposal - If it is cabled back to a 32A MCB on the board it somehow has leakage protection separately from the existing RCD? If I go that route I'll get an electrician in to do it but would like it to be a case of him implementing a viable solution not confronting a problem.

 

[charlhu]

You won't be able to see to burn the food then

Bon chance, a de suite!

Just temporarily as a diagnostic. Anyway, I am a much better cook than I am electrician. Check my thread with "nickblake" - I think he probably has made a good point about the separate circuits.

 

[charlhu]

An excellent suggestion but it has its flaws

The Salad solution may only be a partial solution

Who is to say the meat eaters in the family will accept this
A deep roast in a suitable oven exceeds the taste of cold chicken portions I'm sure

I do eat salads but "takeaways" NEVER - and anyway I need to warm my bedtime socks of a cold night,

 

[telectrix]

SALAD... Sandwiches After Localised Automatic Disconnection. SSSimples

 

[charlhu]

So you have a hob and oven connected in a junction box , mmm that always rings alarm bells to me especially if its a modern oven that can be plugged into a socket why i hear you ask ,well if and i say if the oven is the type i have mentioned then it is supplied with its own cable and normally is 1.5mm , connected to a cooker circuit protected by a 32 amp MCB ,doing this would provide no overload protection to the oven cable , this wiring should be protected by a FCU easily done behind the cooker circuit or should be on its own circuit mean while back to the problem , it could be the start of the RCD becomming fault and you may find that it will start to trip occasionally when other items are switched on or off, this should be checked , claire is right in some ways , its could well be a switch problem where the switch is sticking closed when its switched off before opening thus not making a clean break of the connection causing an imbalance /slight arch always a possability , if its an RCD fault then the only real way to find out would be to replace the RCD , a Ramp test will show the tripping current how ever testing may not show up the described fault

I have spent a merry Sat morning playing endless rounds of "Trip the RCD" mostly successfully (in getting trips that is). Nevertheless, I have made some progress. I now realize that (correctly) the cooker circuit is not tripped by the RCD - only the two power circuits go out (again correctly).That is to say, in a "RCD trip situation" the oven is working. Repeatedly switching the oven on and off, it seldom tripped (perhaps once in 20 operations. However operating the Cooker Switch almost invariably causes a trip. It can do this both switching "On" or "Off". It does it even if the oven is switched off by the programme selector switch. If I disconnect the ceramic hob from the junction box the problem is there. If I disconnect the oven and have only the hob connected there is no problem operating the cooker switch. That suggests there is an insulation problem with the original oven cable? Since when I switch the cooker switch "On" while the oven switch is "Off" all that comes into play is the cooker circuit trying to feed the oven via its cable? I am at a loss to understand why that would cause an RCD trip on the power circuits when the cooker circuit Live/Neutral amperage differential is not being measured - perhaps some electrician Hercule Poirot will explain it to me!
I would have tried replacing the cable already but it is a two-man job to get the oven out of its housing to access the terminal box and I live alone. Will do it if I am confident that is the cause of the problem.
Anyway, it is in a temporary location under-counter below the hob where the old cooker was postioned. I am remodelling the kitchen and the oven wil go into a tower cabinet on another wall and need its own switched supply installed in a few weeks time. In the light of your comments I won't combine with the hob but have a separate circuit run from the board to an FCU and power socket for the oven. Fortunately, there is one spare space in the board for another MCB.

 

[charlhu]

Probably takes the oven heating elements out of the equation, but not the internal wiring, the light itself, or even possibly the control circuit.

Could you be so kind as to read my latest reply post to "nickblake" and comment thereon?

 

[pushrod]

A cooker switch is really a double pole isolator. Isolators as such are not really intended for switching in the way that a light switch is (under any load) - speed of separation of contacts, distances and hardening of contact points. It is possible that you are getting arcing at this point that causes the rcd to trip.

 

[topquark]

As pushrod has said (we're assuming you are talking about the cooker isolator when you way the main switch) the isolator is not the way to turn the cooker on/off.

 

[sparkinlee]

As pushrod has said (we're assuming you are talking about the cooker isolator when you way the main switch) the isolator is not the way to turn the cooker on/off.

That's true, but even if the isolator is used to disconnect L & N going to the cooker, why would arcing there trip an RCD that those L & N are not going through? IE - when the RCD trips it appears from the posts above that the cooker is still working, so I assume that the cooker circuit is not (or is not only) connected to the RCD in question.

Is is possible to have some photos of the Consumer Unit both with the front on (to see the labelling) and off (having first switched off, of course)?

 

[telectrix]

could it be the surrender monkeys getting revenge for waterloo?

 

[charlhu]

A cooker switch is really a double pole isolator. Isolators as such are not really intended for switching in the way that a light switch is (under any load) - speed of separation of contacts, distances and hardening of contact points. It is possible that you are getting arcing at this point that causes the rcd to trip.

Yes, of course - I don't normally use it for anything other than its intended purpose of isolating the cooker/oven/whatever if I am going to work on it. The point is I have been trying all conceivable operations as a diagnostic. With the oven switched off it is operating under a no-load condition so should not arc. Furthermore, as stated, it does not cause a trip when only the ceramic hob is wired in - arcing would ocur irrespective of what was connected downstream.
The question remains unanswered so far - why does it trip the RCD when the cooker circuit is not on the RCD? In the trip condition the oven circuit is fine - it is the two power circuits that the RCD controls which go down.
I have checked the relevant IEE Regulation 5.9.3 and my installation (installed by a registered electician 4 years ago when I had all the wiring updated) conforms exactly. It is a bungalow so the "ground floor" power circuits are RCD protected but nothing else. There is no requirement for cooker circuits to be on RCD and mine is not. So any line/neutral imbalance is not measured. Mystery?