RCD will not stay on

[rotor]

Hello there, this is my first post.

I came home from 3 weeks away, to find some of the lights in the house weren't working. I also found that the fridge was off, and had completely defrosted (I had to throw away absolutely everything). Long story short, after trying lots of different things, this is what I know:

- If I try to switch the RCD on (pushing it all the way up), it instantly trips (I can feel the tripping action as I reach the top).

- If I turn off the big red switch (Main Switch), the RCD will stay on (the indicator turns red and stays red). As soon as I turn on the Main Switch, the RCD trips. This happens regardless of whether the RCD Protected MCBs (the green ones, Nos. 9-12) are on or off. In fact, with every single MCB off (Nos. 1-5 and 9-12), as soon as I turn the Main Switch on, the RCD trips.

- If I switch the Cooker MCB (No. 9) on, and then switch the RCD on, the Cooker MCB trips with a loud pop and a visible spark (it's dark under the stairs where the CU is), and the RCD trips instantly.

- The cooker doesn't have its own switch in the kitchen. I pulled the cooker partially out, and there is a thick cable coming out the back of the cooker, and snaking away under the cabinets, which I assume goes directly to the CU.

- The cooker is a Kenwood Type C9T, Model CK 404 (gas hob with electric ignition, and dual electric ovens).

- The only things that work in the house are: lights everywhere, 1st floor sockets (for some reason they are not RCD protected). Things that don't work: anything in the kitchen (lights do work), sockets on ground and 2nd floors. In summary: all lights work, and sockets on 1st floor work. Nothing else works.

- I'm currently running an extension from a 1st floor socket to power the fridge + jug or microwave or dishwasher (only one at a time).

I'd really appreciate any feedback. I've read a few posts that imply that damp gets into the oven elements, possibly due to the house being empty for an extended period.

The main question I have: assuming the cooker is at fault, is it logical that the RCD won't stay on even when the cooker MCB is off?

Thanks in advance!

Untitled

• rotor
• 15 Jan 2015
• 1

 

[EFLImpudence]

- If I try to switch the RCD on (pushing it all the way up), it instantly trips (I can feel the tripping action as I reach the top).

You have an earth fault.

If I turn off the big red switch (Main Switch), the RCD will stay on (the indicator turns red and stays red). As soon as I turn on the Main Switch, the RCD trips.

That is because the main switch also disconnects the Neutral.

This happens regardless of whether the RCD Protected MCBs (the green ones, Nos. 9-12) are on or off. In fact, with every single MCB off (Nos. 1-5 and 9-12), as soon as I turn the Main Switch on, the RCD trips.

That is because you are reconnecting the Neutral.

If I switch the Cooker MCB (No. 9) on, and then switch the RCD on, the Cooker MCB trips with a loud pop and a visible spark (it's dark under the stairs where the CU is), and the RCD trips instantly.

This could indicate an element fault but why this would happen while not being used is strange.

It is not advisable to keep 'switching on to see if it trips'.
The fault should be found and rectified.

- The only things that work in the house are: lights everywhere, 1st floor sockets (for some reason they are not RCD protected). Things that don't work: anything in the kitchen (lights do work), sockets on ground and 2nd floors. In summary: all lights work, and sockets on 1st floor work.

They are not protected by the RCD.

The main question I have: assuming the cooker is at fault, is it logical that the RCD won't stay on even when the cooker MCB is off?

Yes. Neutral and Earth are not disconnected.


You could completely disconnect the cooker, either at the cooker or preferably in the consumer unit.
If you are not comfortable doing this then you will need an electrician with the appropriate testing equipment.

 

[JohnW2]

This happens regardless of whether the RCD Protected MCBs (the green ones, Nos. 9-12) are on or off. In fact, with every single MCB off (Nos. 1-5 and 9-12), as soon as I turn the Main Switch on, the RCD trips.

That is because you are reconnecting the Neutral.

If I switch the Cooker MCB (No. 9) on, and then switch the RCD on, the Cooker MCB trips with a loud pop and a visible spark (it's dark under the stairs where the CU is), and the RCD trips instantly.

This could indicate an element fault but why this would happen while not being used is strange.

Whilst it seems fairly clear that he cooker (or, at least, cooker circuit) is the problem, this all seems rather odd/unusual. For a start, as you imply, for the RCD to trip when main switch is switched on (even with cooker MCB is off) would tend to suggest a N-E fault - but for that also to happen when the MCBs of all final circuits ae switched off implies that there must be a significant potential difference between the incoming neutral and the installation's earth/CPCs - so presumably not a TN-C-S installation?

Secondly, a N-E fault obviously could not cause the MCB to operate, suggesting that there is also a L-N or L-E fault. Having said that, if there were an L-E fault, say on the L size of an (intact) clock motor, I suppose that would appear as a N-E fault (via the clock winding) when the MCB was switched off. If any (intact) cooker element was switched on at the time, the same would apply.

Kind Regards, John

 

[EFLImpudence]

I was thinking, perhaps wrongly, that a part of the oven was switched on.

Otherwise, without the RCD, presumably the oven MCB would have tripped while on holiday.
Therefore can we assume that the RCD just worked quicker?
Why did the oven MCB not trip when the RCD was switched on the first time.

I note that rotor said "after trying lots of different things, this is what I know".

Perhaps more details???

 

[rotor]

I was thinking, perhaps wrongly, that a part of the oven was switched on.

Otherwise, without the RCD, presumably the oven MCB would have tripped while on holiday.
Therefore can we assume that the RCD just worked quicker?
Why did the oven MCB not trip when the RCD was switched on the first time.

I note that rotor said "after trying lots of different things, this is what I know".

Perhaps more details???

I think the cooker MCB probably was tripped when I got home. It's just that I didn't think to take a photo before I started resetting everything to get the lights working. My reference to "after trying lots of things" is mainly that I don't remember what positions the various things were on the CU, so I'm just explaining what I've been able to figure out so far.

The cooker MCB will trip every time if I turn it on (with the RCD turned off), and then turn on the RCD (the cooker MCP pops and trips, and the RCD trips instantly).

To disconnect the cooker circuit:

- Turn every MCB, RCD and Main Switch off
- Remove cover (very careful to not touch anything coming from the Main Switch)
- Unscrew the live from the cooker MCB and connect to connector block to keep it out of the way
- Anything else?

Edit: I just want to isolate that the problem is indeed on the cooker circuit, and so that I can get power back to the rest of the house while I get the problem sorted.

I'm not an electrician, but I understand the logic, and I'm very careful.

Thanks again!

 

[JohnW2]

To disconnect the cooker circuit:
- Turn every MCB, RCD and Main Switch off
- Remove cover (very careful to not touch anything coming from the Main Switch)
- Unscrew the live from the cooker MCB and connect to connector block to keep it out of the way
- Anything else?

Yes, one other thing - since it's quite possible that there is a 'neutral problem' you also need to disconnect the neutral coming from the cooker cable from the 'neutral bar' in the CU (probably at the top) - and safely put that into a bit of connector block to keep it out of the way. There will be two neutral bars, and make sure to note which one it was connected to - since it will ultimately have to be reconnected to the right one! Strictly speaking, even after you have switched off the main switch, RCDs and MCBs, you should confirm (with a meter or appropriate voltage indicator) that the MCB really is 'dead' before you touch it to remove the cooker connection.

Edit: I just want to isolate that the problem is indeed on the cooker circuit, and so that I can get power back to the rest of the house while I get the problem sorted. I'm not an electrician, but I understand the logic, and I'm very careful.

Fair enough.

Kind Regards, John

 

[JohnW2]

I was thinking, perhaps wrongly, that a part of the oven was switched on.

We don't know for sure. However, as I said, even if all the elements were switched off, there could well be a clock or something that is always 'on' (and which could develop a fault).

Otherwise, without the RCD, presumably the oven MCB would have tripped while on holiday.

We are now told that it probably did.

Therefore can we assume that the RCD just worked quicker? Why did the oven MCB not trip when the RCD was switched on the first time.

I'm not sure that we know what happened when the RCD was reset 'the first time'. In fact, the OP seems to indicate that he could not reset the RCD unless he turned off the main switch (in which case the MCB obviously could not operate).

Kind Regards, John

 

[rotor]

Whilst it seems fairly clear that he cooker (or, at least, cooker circuit) is the problem, this all seems rather odd/unusual. For a start, as you imply, for the RCD to trip when main switch is switched on (even with cooker MCB is off) would tend to suggest a N-E fault - but for that also to happen when the MCBs of all final circuits ae switched off implies that there must be a significant potential difference between the incoming neutral and the installation's earth/CPCs - so presumably not a TN-C-S installation?

Secondly, a N-E fault obviously could not cause the MCB to operate, suggesting that there is also a L-N or L-E fault. Having said that, if there were an L-E fault, say on the L size of an (intact) clock motor, I suppose that would appear as a N-E fault (via the clock winding) when the MCB was switched off. If any (intact) cooker element was switched on at the time, the same would apply.

Kind Regards, John

Thanks John. Unfortunately not a lot of that makes sense, I'm not an electrician.

My current line of thought is to disconnect the cooker circuit so I can hopefully get the rest of the house working, as it's going to take several days to get an electrician in to diagnose the actual problem.

 

[rotor]

Yes, one other thing - since it's quite possible that there is a 'neutral problem' you also need to disconnect the neutral coming from the cooker cable from the 'neutral bar' in the CU (probably at the top) - and safely put that into a bit of connector block to keep it out of the way. There will be two neutral bars, and make sure to note which one it was connected to - since it will ultimately have to be reconnected to the right one! Strictly speaking, even after you have switched off the main switch, RCDs and MCBs, you should confirm (with a meter or appropriate voltage indicator) that the MCB really is 'dead' before you touch it to remove the cooker connection.

Thanks very much John. I'll take pictures before I remove anything.

I don't have a meter. Will one of those little tester screwdrivers that you put your finger on the end of with a little light inside work/be safe?

Thanks again!

 

[rotor]

We don't know for sure. However, as I said, even if all the elements were switched off, there could well be a clock or something that is always 'on' (and which could develop a fault).

There's no clock in this cooker.

 

[stillp]

Will one of those little tester screwdrivers that you put your finger on the end of with a little light inside work/be safe?

No. It won't confirm that the circuit is dead, might tell you it is when it isn't, or might tell you it isn't when it is, if that makes sense.
You can buy multimeters for as little as £3.99, which might not be accurate but will tell you what you need to know if set on a voltage range that includes 240 volts AC.

 

[JohnW2]

Thanks John. Unfortunately not a lot of that makes sense, I'm not an electrician.

I realised that would be the case - but that little exchange obviously wasn't really intended for you!

My current line of thought is to disconnect the cooker circuit so I can hopefully get the rest of the house working, as it's going to take several days to get an electrician in to diagnose the actual problem.

Yes I understand that. If the problem is with the innards of the cooker, whether or not an electrician will necessarily be the right person to mend it is a different matter.

Kind Regards, John

 

[JohnW2]

Thanks very much John. I'll take pictures before I remove anything. ... I don't have a meter. Will one of those little tester screwdrivers that you put your finger on the end of with a little light inside work/be safe?

Those things are unreliable, hence not safe, but as an extra bit of 'belts and braces' it would do no harm to use it on the terminal of the MCB before you disconnect the wire. If, by any chance,it lights up, you certainly shouldn't proceed - but the reason that it's 'unsafe' for this purpose is that the absence of lighting up does not necessarily 'mean what it says'.

I will undoubtedly attract some flak by saying this but, with three levels of 'defence' between the electricity supply and the MCB terminal (main switch, RCD and the MCB itself all switched off), the risk of any electricity getting to the MCB terminal despite all that is, as they say,'vanishingly small'!

Kind Regards, John

 

[JohnW2]

We don't know for sure. However, as I said, even if all the elements were switched off, there could well be a clock or something that is always 'on' (and which could develop a fault).

There's no clock in this cooker.

Fair enough - and nothing else which is 'always on' even when all the heating bits of the cooker (oven, grill, hotplates etc.) are turned off? Were all those 'heating bits' turned off when you got the trips, particularly the tripping of the MCB?

Kind Regards, John

 

[rotor]

Fair enough - and nothing else which is 'always on' even when all the heating bits of the cooker (oven, grill, hotplates etc.) are turned off? Were all those 'heating bits' turned off when you got the trips, particularly the tripping of the MCB?

Not that I'm aware of. Nothing was on, the two ovens have the usual selector and temperature knobs each (everything off/set to 0).