Short circuit tripping RCD

Dancook

Distinguished Member
Since last night (high winds and rain), one of the RCDs has been tripping with a loud bang every so often, perhaps 8 times last night. I switched it back on this morning, it tripped a couple more times in the morning and 4+ times throughout the afternoon. Sometimes it'll go 10-30minutes sometimes a few hours.

There are 6/7 MCB on the RCD, the one that goes off at the same time is the downstairs ring circuit.

The sockets have been tested, seemingly OK. There are a few sockets upstairs on the downstairs ring.
I have unplugged everything from sockets except oven/dishwasher/freezer where they are hardwired but I turned them off at their switches - it still tripped.

An electrician came tonight, could not find the source of the problem and suggested I keep a track of what's happening when it goes off.. I was hopeful about the condensation they found in the box below the outdoor floodlight, they taped up the wires i think.

Since the electrician left at 6pm I've had the heating on for over 3 hours, it's 21:30 now. About 7pm I used the oven and hob to make dinner, the freezer and fridge and running, as well as running a quick wash on the dishwasher.

At 21:30 it's tripped again, Louise is on our bed using the laptop and I'm on the PC in the lounge - both on a different RCD (new parts of house), unaffected by the trip. It's not windy out and it's not raining.

I already feel like I've discount any appliance i'm aware of including the boiler, we have underfloor heating in the utility and downstairs bathroom but they have not worked properly for a few years and have been left switched off.

Sorry to ask, but I feel stuck - any ideas?
 

logonuser1

Active Member
It seems to me you have answered your own question..........you state /"
here are 6/7 MCB on the RCD, the one that goes off at the same time is the downstairs ring circuit."

so on the downstairs ring unplug everything and use one socket at a time and see if the rcd trips after a period of time , if it does unplug the item you have in the socket and plug said item into the upstairs socket if this trips the upstairs ring main, the fault is your item that is plugged in. however if it does not the fault lays inside one of the sockets in the downstairs ring main, you would have to remove every socket and look for a loose wire or a earth that may be touching the live or nuetral every now and then.

but these intermitent faults are hard to find .... have you maybe spured of the downstairs ring main to a fused spur to protect somthing outside? i ask because my suspition is water ingress somwhere outside .
 

Dancook

Distinguished Member
It seems to me you have answered your own question..........you state /"
here are 6/7 MCB on the RCD, the one that goes off at the same time is the downstairs ring circuit."

so on the downstairs ring unplug everything and use one socket at a time and see if the rcd trips after a period of time , if it does unplug the item you have in the socket and plug said item into the upstairs socket if this trips the upstairs ring main, the fault is your item that is plugged in. however if it does not the fault lays inside one of the sockets in the downstairs ring main, you would have to remove every socket and look for a loose wire or a earth that may be touching the live or nuetral every now and then.

but these intermitent faults are hard to find .... have you maybe spured of the downstairs ring main to a fused spur to protect somthing outside? i ask because my suspition is water ingress somwhere outside .

I have had everything unplugged from the downstairs ring and the circuit still tripped.

The exceptions are the oven, dishwasher and freezer where I switched off their hardwired wall switches, so unused and unpowered.

Although outside of this test Actively using the dishwasher and oven does not in itself trip the RCD, the freezer has been on for hours without doing so either.

there are no outdoor sockets only a few lights, I don’t understand the ‘protect something outside’
 

logonuser1

Active Member
‘protect something outside’ .... by this i mean , somtimes one can come of the last ring socket and spur off to say a light outside..... the light has a fused spur befor the the light itself , this fused spur is to protect the light only! However ....a mcb or rcd is alot more sensative than the fuse hence if the light has water in it the fuse will not blow but the rcd or mcb will trip , im not a house or domestic electrician but im a manufacturing electrical tech ....there may be some domestic electricians on here that may have a better idea than me . Just to clarify is it the rcd tripping on the distrabution board or the mcb marked as downstairs ring ?
 

logonuser1

Active Member
Also you state
The exceptions are the oven, dishwasher and freezer where I switched off their hardwired wall switches, so unused and unpowered.
your oven should never be on the ring main and should have its own mcb protective circuit.
 

logonuser1

Active Member
My advice would be to switch off the mcb thats marked as downstairs ring then go outside and see if any of the outside lights are off....if they are they are on that circuit .....i suspect one of them is causing your mcb to trip.

if not

see what items are not powered when you turn off the mcb marked downstairs ring and one of these will be the cause of your problem
 

John7

Well-known Member
Unless the permanently wired appliances have double pole isolators, an earth leakage in any of them, will trip the RCCD. The only way to eliminate them is to disconnect the L & N wires form the outlet connector of each appliance.
 

noiseboy72

Distinguished Member
It may be that the MCB is simply tripping due to going faulty and is taking the rcd with it when it trips. Try manually tripping the MCB and see if the RCD trips as well.

For an MCB to legitimately trip, you need very high currents to be flowing - much more than a bit of damp or water ingress. I think I would replace MCB in the first instance and see if that cures things.
 

Dancook

Distinguished Member
Just to clarify is it the rcd tripping on the distrabution board or the mcb marked as downstairs ring ?

Both, the mcb trips the RCD when it goes. yes the oven is on it own as I put the rcd back at 23:15 when it went again but left the mcb off. The oven came on.

got an extension for the fridge from the lounge and went back to sleep. I’ll check the lights when I get up thanks
 

Dancook

Distinguished Member
It may be that the MCB is simply tripping due to going faulty and is taking the rcd with it when it trips. Try manually tripping the MCB and see if the RCD trips as well.

For an MCB to legitimately trip, you need very high currents to be flowing - much more than a bit of damp or water ingress. I think I would replace MCB in the first instance and see if that cures things.

when the mcb trips the rcd there is a loud bang, I figured was normal when tripping but the electrician told me it was due to a short circuit.

I will try as you say
 

Dancook

Distinguished Member
Tripping the MCB manually 20-30 times did nothing.

I discovered the utility room sockets aren't on that MCB, with the MCB off the washing machine and freezer (relief) are still working. So are the outside lights.

So the only things affected by the MCB off are

Sockets in Kitchen, Dining Room, Hallway, upstairs Landing and front bedroom.

Currently the only devices powered by that MCB are the dishwasher (hardwired), induction hob (plugged in) and a utility 'smart control' unit (plugged in).

So if it's an appliance, I've unplugged the other two before - so there's only the dishwasher that has not been disconnected throughout.
 

Dancook

Distinguished Member
I also note the underfloor heating for the utility and downstairs bathroom must both be on the downstairs MCB which trips.

These have not seemingly worked in years since we had water damage when the roof was off during the extension. They have been turned off at their isolation switches since.

so in summary this tripping MCB controls

Dishwasher
Underfloor heating utility/downstairs bathroom
Sockets for Kitchen/Dining Room/Hallway/Landing/Front Bedroom

I can leave it off today to see if the RCD trips for any other reason?
 
Last edited:

RBZ5416

Distinguished Member
I went through this a while back & eventually isolated it to a Bosh dishwasher. First time it went was with a bang but nothing on subsequent trips. But they were only every couple of days. Like yours, the dishwasher continued to function. Fortunately it was on a 13A plug so I just inserted a plug-in RCD so it didn't take everything else out.

The odd thing is that the part that was suspect is end of life & no longer available. While trying to hunt one down incidences decreased & then went away. Might even have been a stray mouse as I did find one across the mains block of the oven when I replaced the thermostat recently. I really ought to pull it out & check but it's a real ballache.
 

Dancook

Distinguished Member
If it take the fuse out of the dishwasher hardwired switch is it the same as disconnecting it? I could try that easily if it means much.

pulled the dishwasher out, there's a plug socket behind it - the one on the work top, just isolation switch then? I don't know if i'm using the right words :)

unplugged dishwasher, turned mcb on - will test for a day.
 
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noiseboy72

Distinguished Member
The only thing I've ever had that tripped our household rcd was a dishwasher. Water had leaked into the base and shorted out the flood switch ironically enough!!

See if that is your issue, but it doesn't explain the mcb tripping as well.
 

Dancook

Distinguished Member
The only thing I've ever had that tripped our household rcd was a dishwasher. Water had leaked into the base and shorted out the flood switch ironically enough!!

See if that is your issue, but it doesn't explain the mcb tripping as well.

Well the dishwasher is unplugged and the MCB/RCD just both tripped with it's normal bang (easily heard from upstairs opposite corner of house), the only thing left on that circuit are empty sockets and the 2 x underfloor heating

Electrician on phone just now has said to turn off all MCBs apart from the one that trips, and see if it still trips on its own.
 

Will I Aint

Active Member
do you have a kettle plugged into those kitchen sockets?
 

Will I Aint

Active Member
I've just been through a similar issue where RCD was tripping but not the MCBs. Turns out it was earth leakage with fridges and aircon on the same RCD. Even when powered off the earth leakage still trips the RCD if the device is plugged in so i've swapped out all the MCBs and RCDs for RCBOs so any future trips will only hit the relevant fuse. that also meant i now had room for surge protection in the consumer unit.
 

ChuckMountain

Distinguished Member
Each has their own thermostat, but they've both been switched off for years at their isolation switches.

Strange, is it a dual pole isolator?

Given you have isolated most things it does seem weird but the bang does suggest a high current flow.

I had an RCD that had become oversensitive and that would trip, replacing it resolved the issue after many attempts at trying to identify something on the circuit.

Currently also got an MCB that trips 60% of the time on a spur when a surge protected extension with 12 devices (total max normal load < 500W) when it is switched off at night and then switched back on in the morning.
 

Dancook

Distinguished Member
Strange, is it a dual pole isolator?

Given you have isolated most things it does seem weird but the bang does suggest a high current flow.

I believe it's not, I don't know what a dual pole isolator is - but when I suggested to the electrician about taking the fuses for the underfloor heating, he didn't think that would help as it wasn't dual pole isolated.
 

ChuckMountain

Distinguished Member
A dual pole isolator is effectively a switch that works on both the neutral and live wires. A normal switch will just operate on the live wire. (as does the fuse)

That way you have you completely remove the neutral and live wires from the device from the rest of the circuit.

With an RCD it trips when there is a current imbalance between live and neutral, hence if you short a neutral wire to earth it will trip the RCD.

However, in your case, it is tripping the MCB so suggests there is a reasonable current flowing somewhere.
 

The Dreamer

Distinguished Member
I'm not an electrician, so any ideas I have is just me thinking 'aloud'....

While your underfloor heating may be switched off, it is only disconnecting the live wire from the circuit. The neutral is still connected.

I believe your thermostats switch both neutral and live on/off (possibly - see below for second theory with similar results).

Which means your neutral is still being connected/disconnected from your household supply as and when the thermostat engages/disengages.

A sparky can confirm this - but if there's a leak to earth from your neutral, that would cause a trip.

Personally, as you haven't used the underfloor heating in years (and you've said that was due to a fault after water ingress I believe), I would switch off your power, disconnect all wires to/from your thermostats (tape them up and tuck them away from each other), and then switch your power back on.

Even if your thermostats aren't switching both neutral and live, the neutral is still being fed into the circuit. Assuming the thermostat is switching the 'disconnected' live only, it is still potentially connecting that part of the live circuit to your always connected neutral. If that part of the live circuit has an earth fault on it, it will again trip the circuit breaker.

The cure would be as above.....

HTH
 

Dancook

Distinguished Member
I'm not an electrician, so any ideas I have is just me thinking 'aloud'....

While your underfloor heating may be switched off, it is only disconnecting the live wire from the circuit. The neutral is still connected.

I believe your thermostats switch both neutral and live on/off (possibly - see below for second theory with similar results).

Which means your neutral is still being connected/disconnected from your household supply as and when the thermostat engages/disengages.

A sparky can confirm this - but if there's a leak to earth from your neutral, that would cause a trip.

Personally, as you haven't used the underfloor heating in years (and you've said that was due to a fault after water ingress I believe), I would switch off your power, disconnect all wires to/from your thermostats (tape them up and tuck them away from each other), and then switch your power back on.

Even if your thermostats aren't switching both neutral and live, the neutral is still being fed into the circuit. Assuming the thermostat is switching the 'disconnected' live only, it is still potentially connecting that part of the live circuit to your always connected neutral. If that part of the live circuit has an earth fault on it, it will again trip the circuit breaker.

The cure would be as above.....

HTH

thanks, i'll look into that - first i'll see if i can find some tape :D
 

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