Intriguing problem. Shower and Cooker tripping CB not RCD

[Deuce]

How come no feedback on the oven and shower appliance testing yet? That seems like the obvious thing to rule out.

I had an isolator switch near a hob causing tripping of CB, when investigated it was due to grease that had found its way in and was arcing. Seems plausible this could be happening here, or possibly grease ingress into the cookers internal connections.

NB: it wasn't immediately apparent in my case that the cause was grease, as the switch plate had been cleaned frequently. Very close inspection showed signs of grease ingress down the side of the switch itself, probably more forced in each time it was wiped over.

 

[Wilko]

Hi - some thoughts from afar -
If MCB is tripping with no appliance load, then there is either short circuit somewhere (trapped conductors in a back box?) or as RPA has mentioned it's the MCB.

 

[NeilC]

Hi all, I am the person who owns the oven and shower appliances. Thanks Andy for sorting some stuff out and is endeavouring to sort this issue. The shower started to trip the MCB about 4 months ago. Again it only trips when switched off never when using. It doesn't do it all the time. The oven ( an 8 yrs old Rangemaster twin oven fitted by qualified spark etc) started to do the same thing about 2 months ago. At first very occasionally but now getting worse. Mostly it trips the MCB when off but has tripped on the odd occasion when on. Its doing the roast dinner as I write this and has been on over an hour. I replaced the heating element before calling Andy just in case that was at fault but no difference. Both the MCBs for shower and cooker are right next to each other RCD unit if this helps. The RCD unit is about 8yrs old.

 

[Strima]

Loose connections on the circuits or switches causing arcing? Let through current of the arc causing the MCB to trip.

 

[Deleted member 9648]

If there are definitely no fault readings on either circuit when an IR test live-neutral is carried out with appliances isolated then check the board carefully for heat in adjacent devices, which may cause other devices to operate. Otherwise with no fault readings (definite?) it can really only be faulty MCB's if there is no load current as you suggest.
I emphasize the word 'definite' because so often on here it turns out that very limited testing has actually been carried out.

 

[ipf]

For the sake of it, change both mcb's AND the rcd. Although probably ok, it's common to both circuits.
Once you've eliminated those from your problem points, you're narrowing it down plenty.
Appliances or connections.....

 

[marconi]

Ask the electrician to check the polarity of the supply at the terminals of the shower and the oven.

From #23:

Again it only trips when switched off never when using. It doesn't do it all the time.

The oven ( an 8 yrs old Rangemaster twin oven fitted by qualified spark etc) started to do the same thing about 2 months ago. At first very occasionally but now getting worse. Mostly it trips the MCB when off but has tripped on the odd occasion when on.

 

[telectrix]

i can cover the wirral if needed. the arse end is close to me. the knob end is not too far if need be.

 

[Shoei]

Is the shower switched by a two-pole cord switch? If so the switch may be faulty. I've seen them fail like this when they get a bit dirty or moist inside. Contamination on internal surfaces can impair the creepage distances, producing an arc between live and neutral when current is interrupted.

If that's suspected you can confirm by temporarily linking out the neutral wires, disconnecting them from the switch and using it as single pole. If the CB no longer trips then it points to the switch.

 

[marconi]

I hope the OP employs telectrix to hunt the problem and fix it - then the EF might might find out what it was.

 

[Intoelectrics]

If I was a betting man, from reading the information I'd hazard a guess at the problem being at the CU. It seems strange, though not impossible that completely separate circuits are suffering with the same symptoms, unless they have something in common. The only 2 common points are the point of earth and the point of supply. Since the RCD is in working order and is not tripping then you would assume that the problem is not earth leakage related. So my first process of elimination would be to start with the common source of supply.
I did once come across a CU where the mcb terminal bus bar was incorrectly fitted and some of the mcb's were not tightening on to it but just resting against it. This had been like this from the day the CU was initially fitted and then some years later some of the mcb's began to trip randomly.

 

[Andy C]

How come no feedback on the oven and shower appliance testing yet? That seems like the obvious thing to rule out.

I had an isolator switch near a hob causing tripping of CB, when investigated it was due to grease that had found its way in and was arcing. Seems plausible this could be happening here, or possibly grease ingress into the cookers internal connections.

NB: it wasn't immediately apparent in my case that the cause was grease, as the switch plate had been cleaned frequently. Very close inspection showed signs of grease ingress down the side of the switch itself, probably more forced in each time it was wiped over.

Shower has been checked and apart from not very tight connections all was okay,(no burning or arcing present). I noted the shower was a 9.5kw and being on a 40amp CB and on 6mm not the best. I have advised that as the client has a new 8.5kw mira shower waiting to go in to change the shower. The CB has also been changed for another 40 amp CB as the previous one was tripping with only a slight nudge. it tripped when I was changing the Cooker CB.

 

[DPG]

9.5KW on 6mm - how long a run and how is it run?

 

[Andy C]

How come no feedback on the oven and shower appliance testing yet? That seems like the obvious thing to rule out.

I had an isolator switch near a hob causing tripping of CB, when investigated it was due to grease that had found its way in and was arcing. Seems plausible this could be happening here, or possibly grease ingress into the cookers internal connections.

NB: it wasn't immediately apparent in my case that the cause was grease, as the switch plate had been cleaned frequently. Very close inspection showed signs of grease ingress down the side of the switch itself, probably more forced in each time it was wiped over.

Cooker has been pulled out and connections are sound. The 45 amp DP switch is in a cupboard to the side and when taken off all was sound and tests to the cooker were all within range.

 

[Andy C]

9.5KW on 6mm - how long a run and how is it run?

Not a long run, it appears to be clipped direct and buried in walls. Only the usual short length in the loft. I have advised that the client replaces the shower with an 8.5kw one he actually has in stock. This would be a much better option in relation to cable and CB ratings.

 

[Andy C]

I experienced the ‘tired’ MCB on a shower circuit.
You go there armed to the teeth with test gear and after testing and proving good, disconnected at the shower, you knock your ladder and the MCB trips! Reset, tap the CU and it trips - I made the most of the situation and sourced the correct make MCB for the CU replacement instead of the BG that was fitted.
Granted this is the only time it’s happened so far but some things do come under the acronym K.I.S.S

This actually happened when I was changing the cooker 32 amp CB. The shower next to it tripped. I then functionally checked that CB with the one next to it and there was distinct difference. So I think tired CB might be the case. Thank you.

 

[Andy C]

Ask the electrician to check the polarity of the supply at the terminals of the shower and the oven.

From #23:

Again it only trips when switched off never when using. It doesn't do it all the time.

The oven ( an 8 yrs old Rangemaster twin oven fitted by qualified spark etc) started to do the same thing about 2 months ago. At first very occasionally but now getting worse. Mostly it trips the MCB when off but has tripped on the odd occasion when on.

Done and all correct

 

[Andy C]

For the sake of it, change both mcb's AND the rcd. Although probably ok, it's common to both circuits.
Once you've eliminated those from your problem points, you're narrowing it down plenty.
Appliances or connections.....

Both CBs replaced and RCD checked again and trips correctly at both 1 and 5

 

[Andy C]

Loose connections on the circuits or switches causing arcing? Let through current of the arc causing the MCB to trip.

All connections are now tight, shower was not as tight as it could have been.

 

[Andy C]

i can cover the wirral if needed. the arse end is close to me. the knob end is not too far if need be.

Thanks for the offer Telectrix, I may have sorted it now.