RCCB tripping randomly

[StephenB]

I have an RCCB that trips off randomly. Sometimes it's two or three times a week and at other times it can go six weeks or more without tripping. It doesn't seem to be associated with switching any lights or appliances on or off and I'm worried it could trip off when we're away on holiday and we could lose all the freezer contents. Does anybody know of an instrument I could hire or buy to find the fault that is causing this problem, please? I Googled "RCCB tripping randomly" and found your thread at https://talk.electricianforum.co.uk/topic/32188-rccb-tripping-randomly/ . The person who started the thread, "Wah007" seems to have had some bad experiences with "cowboy" electricians so I'm a bit wary of calling anybody advertising in the local paper without some sort of recommendation. I do have a degree in Electrical Engineering and a MSc in Telecommunications Systems and I was a Fellow of the IEE before I retired in 2005, so I can understand technical explanations. However, I never trained as an electrician or took City and Guilds qualifications and I've never worked on domestic wiring, so I do need your advice. Can I find this problem and fix it myself, or do I need somebody with all the kit suggested to Wah007 by "Trailer Boy", and if so how do I find somebody who has the kit and knows how to use it? Thank you for any help you can offer.

 

[Andy™]

being able to connect a test lead and press test is easy. understanding what its telling you is completely different

 

[Geoff1946]

Hi StephenB. 

I'm in the same position as yourself as an electrical engineer with no electrical installation qualifications, though perhaps I've dabbled in it more than you. 

The first thing I would do is check all my class 1 appliances for insulation resistance. If you have or can borrow an insulation tester, preferably a 500v "megger" or similar, but even a low voltage multimeter with 10 megohm range then that's a straightforward move which may just reveal your problem. Pay special attention to those with heating elements, and if you have an electric cooker on the circuit that's a prime suspect.

Another thing you may want to check is whether you have moisture in any outdoor light fittings.

Beyond that, if you've never worked on installed wiring, then starting by trying to find a semi intermitant  fault is probably a bit ambitious.

 

[SPECIAL LOCATION]

I have an RCCB that trips off randomly. Sometimes it's two or three times a week and at other times it can go six weeks or more without tripping. It doesn't seem to be associated with switching any lights or appliances on or off and I'm worried it could trip off when we're away on holiday and we could lose all the freezer contents. Does anybody know of an instrument I could hire or buy to find the fault that is causing this problem, please? I Googled "RCCB tripping randomly" and found your thread at https://talk.electricianforum.co.uk/topic/32188-rccb-tripping-randomly/ . The person who started the thread, "Wah007" seems to have had some bad experiences with "cowboy" electricians so I'm a bit wary of calling anybody advertising in the local paper without some sort of recommendation. I do have a degree in Electrical Engineering and a MSc in Telecommunications Systems and I was a Fellow of the IEE before I retired in 2005, so I can understand technical explanations. However, I never trained as an electrician or took City and Guilds qualifications and I've never worked on domestic wiring, so I do need your advice. Can I find this problem and fix it myself, or do I need somebody with all the kit suggested to Wah007 by "Trailer Boy", and if so how do I find somebody who has the kit and knows how to use it? Thank you for any help you can offer.

There is NO instrument that can find a fault for you....

There are instruments that can:

a) measure the tripping times for an RCD.

b) measure the tripping current of and RCD.

c) measure the continuity of the circuit conductors.

d) measure the insulation resistance of the circuit cables.

e) measure the earth loop impedance at the supply and the furthest points on each circuit.    

f) measure various portable and fixed appliance characteristics that are connected to your circuits.

etc..

ALL of which would probably be needed to assist diagnosing a possible cause....

BUT none of the above will be able solve anything without a person competent enough to know how to use them..

AND.. correctly interpret the results from them!

From what you say..  you have a poor design of installation with a single RCD protecting multiple circuits..

which also means there may not be a single cause..

It may be multiple problems across more than one circuit..

OR multiple sources of natural leakage.. that aren't actual faults at all!!

When was the installation wiring last tested?

The bottom line is...

The BS7671 specification for how your RCD should operate states...

It MUST operate when it detects a 30ma imbalance (leakage to earth)..   (e.g. at 230v you are trying to find a path to earth of 7.6k or lower.. )

It MUST NOT operate if it detects a 15ma imbalance (leakage to earth)..  (e.g. if RCD bit more sensitive at 230v you are trying to find a path to earth lower than 14.3k..)

You need to..

prove the operating characteristics of the RCD

prove the integrity of the circuits

prove the condition of connected appliances/accessories etc..

where about are you?

:coffee

 

[StephenB]

Hi StephenB. 

I'm in the same position as yourself as an electrical engineer with no electrical installation qualifications, though perhaps I've dabbled in it more than you. 

The first thing I would do is check all my class 1 appliances for insulation resistance. If you have or can borrow an insulation tester, preferably a 500v "megger" or similar, but even a low voltage multimeter with 10 megohm range then that's a straightforward move which may just reveal your problem. Pay special attention to those with heating elements, and if you have an electric cooker on the circuit that's a prime suspect.

Another thing you may want to check is whether you have moisture in any outdoor light fittings.

Beyond that, if you've never worked on installed wiring, then starting by trying to find a semi intermitant  fault is probably a bit ambitious.

Hi Geoff1946,

Thanks for the tip. I have had to find intermittent faults in electronic equipment in my time and I know it can take a very long time. I don't relish the thought of trying to do this with domestic wiring that I have no experience with. I'd rather get a trusted electrician in to sort it for me, but after reading Wah007's account of what happened to him I'm wary of finding the "wrong" sort of electrician.  On the other hand if your suggestion that it might be moisture in outdoor fittings is right then the fault might not appear again until the winter after all the dry weather we've had recently. Even someone with all the kit and expertise might spend several hours looking and still not find it. I'm trying to gather as much information as possible at the moment to try to decide what to do.

 

[Murdoch]

TBH random tripping every 2 to 3 weeks could be extremely difficult to locate, and take a lot of time to do so

the best advice I can give you is keep a diary of when it occurs, what time of day and what was being used at the time,

how many circuits do you have on this rcd?

 

[StephenB]

There is NO instrument that can find a fault for you....

There are instruments that can:

a) measure the tripping times for an RCD.

b) measure the tripping current of and RCD.

c) measure the continuity of the circuit conductors.

d) measure the insulation resistance of the circuit cables.

e) measure the earth loop impedance at the supply and the furthest points on each circuit.    

f) measure various portable and fixed appliance characteristics that are connected to your circuits.

etc..

ALL of which would probably be needed to assist diagnosing a possible cause....

BUT none of the above will be able solve anything without a person competent enough to know how to use them..

AND.. correctly interpret the results from them!

From what you say..  you have a poor design of installation with a single RCD protecting multiple circuits..

which also means there may not be a single cause..

It may be multiple problems across more than one circuit..

OR multiple sources of natural leakage.. that aren't actual faults at all!!

When was the installation wiring last tested?

The bottom line is...

The BS7671 specification for how your RCD should operate states...

It MUST operate when it detects a 30ma imbalance (leakage to earth)..   (e.g. at 230v you are trying to find a path to earth of 7.6k or lower.. )

It MUST NOT operate if it detects a 15ma imbalance (leakage to earth)..  (e.g. if RCD bit more sensitive at 230v you are trying to find a path to earth lower than 14.3k..)

You need to..

prove the operating characteristics of the RCD

prove the integrity of the circuits

prove the condition of connected appliances/accessories etc..

where about are you?

:coffee

Hi Special Location,

Thanks for the very detailed reply. You were the one who gave the most useful reply to Wah007's question back in 2016. To answer your question, we bought the house two years ago and had some electrical work done then, including extractor fans in two bathrooms and a downstairs loo, raising all the downstairs sockets a couple of inches when we had engineered wood flooring fitted on top of the old floorboards and replaced the skirtings. We also had a new gas boiler fitted and the electrician who worked with the gas fitter fitted a new controller for the boiler. He said the consumer unit was not compliant with today's standards but it was perfectly serviceable and didn't need replacing. The previous owners had a new kitchen fitted about ten years ago and I was surprised to find the kitchen sockets are on the upstairs socket circuit. They must have decided it was easier to take up a bit of carpet on the landing above the kitchen and drop the connection down that way than to bring it through the hall from the consumer unit direct.

There are actually two RCDs. The one that trips has only two MCBs, one for the the upstairs lights and the other for the upstairs sockets and kitchen sockets. The other RCD is for the downstairs circuits, but that one never trips.

We are in North Somerset, west of Bristol.

You're absolutely right this is not a job for a DIY enthusiast, even one with degrees in Electrical Engineering and an ex FIEE. On the other hand I do know that intermittent faults are notoriously difficult to find so it could take even an expert several hours. I was hoping to get some way along the road to solving the problem before calling in a specialist.

Thank you

 

[NozSpark]

I’ve had this kind of problem several times.... most of the time everything is fine when I get there and unfortunately, being so intermittent, those faults were impossible to find until they become permanent faults

other times I’ve found a couple of low (but passable) IR readings on some circuits that have been impossible to break down (very old cottage without any loft and/or access)..... in the end I changed the consumer unit to one with RCBO’s so that any fault would only affect the circuit that it was on, they never tripped again that I know of... must have been cumulative leakage taking it over the RCD’s threshold

 

[StephenB]

TBH random tripping every 2 to 3 weeks could be extremely difficult to locate, and take a lot of time to do so

the best advice I can give you is keep a diary of when it occurs, what time of day and what was being used at the time,

how many circuits do you have on this rcd?

Hi Murdoch,

Thanks for the suggestion. I'll make a note of time and date and anything being switched on or off each time it happens from now on.

Ther are two MCBs on this RCD, one for the upstairs lights and the other for the upstairs sockets plus the kitchen sockets.

 

[NozSpark]

Ther are two MCBs on this RCD, one for the upstairs lights and the other for the upstairs sockets plus the kitchen sockets.

TBH that should be pretty straight forward to fault find (if the fault is there at the time)

 

[Andy™]

thing is, sometimes the fault isnt always there to be found. been to a few intermittent where there are signs of a fault, low IR etc, which are low but high enough not to cause a problem. they have been fixed but then the actual problem reappears elsewhere later on and starts tripping again

 

[StephenB]

I’ve had this kind of problem several times.... most of the time everything is fine when I get there and unfortunately, being so intermittent, those faults were impossible to find until they become permanent faults

other times I’ve found a couple of low (but passable) IR readings on some circuits that have been impossible to break down (very old cottage without any loft and/or access)..... in the end I changed the consumer unit to one with RCBO’s so that any fault would only affect the circuit that it was on, they never tripped again that I know of... must have been cumulative leakage taking it over the RCD’s threshold

Hi NozSpark,

Thanks for the suggestion. I was already thinking of replacing the consumer unit as I think there are too many outlets on the one RCD. Maybe I should just try your solution and see if the problem goes away before trying to find an intermittent fault.

Thanks again

 

[StephenB]

TBH that should be pretty straight forward to fault find (if the fault is there at the time)

Hi NozSpark,

(if the fault is there at the time) is the critical question. Every time it trips I have simply switched it back on and it hasn't tripped again for days or weeks. Was the fault there for just a minute or two?

 

[StephenB]

thing is, sometimes the fault isnt always there to be found. been to a few intermittent where there are signs of a fault, low IR etc, which are low but high enough not to cause a problem. they have been fixed but then the actual problem reappears elsewhere later on and starts tripping again

Hi Andy,

Thanks for this. There seems to be general agreement that intermittent faults are near impossible to find, even for the professionals, so it looks like I'm on a hiding to nothing. Maybe I'll just replace the consumer unit wit RCBOs as suggested by NozSpark and hope for the best.

 

[kerching]

In my experience intermittent faults are usually 

water based

kettle

water heater

oitside lights

damp,in sockets

damp in switches

washing machine

boiler

vibration based

wire catching on a screw/backbox

wires touching 

...vibration causes a better contact and RCD trips

OR it could be cumulative and something just tips it over the edge

so I've  not really helped any, sorry

 

[Andy™]

Hi Andy,

Thanks for this. There seems to be general agreement that intermittent faults are near impossible to find, even for the professionals, so it looks like I'm on a hiding to nothing. Maybe I'll just replace the consumer unit wit RCBOs as suggested by NozSpark and hope for the best.

not always - there often is signs of the fault being there 95% of the time and it can be found, its jsut sometimes the 'fault' youve found is something thats not actually causing the problem you have now, (however at least it wont then cause an issue in the future), you just dont know it until the original fault thats causing the issue reappears

 

[SPECIAL LOCATION]

There are actually two RCDs. The one that trips has only two MCBs, one for the the upstairs lights and the other for the upstairs sockets and kitchen sockets. The other RCD is for the downstairs circuits, but that one never trips.

We are in North Somerset, west of Bristol.

You're absolutely right this is not a job for a DIY enthusiast, even one with degrees in Electrical Engineering and an ex FIEE. On the other hand I do know that intermittent faults are notoriously difficult to find so it could take even an expert several hours. I was hoping to get some way along the road to solving the problem before calling in a specialist.

Thank you

If there are only two MCBs' off this suspect "RCD" it may be feasible to reconfigure them as RCBO's instead..

So if it re-trips you would know definitely if it is socket or lights related..

If you do a few more posts (get you count up to 10)..    You should be able to add images..

then add a photo of your fuse-box which would help sussing our if RCDO's is doo-able or not..

:coffee

 

[UNG]

Seen this type of intermittent fault many times where the RCD trips and resets with no problems for days or weeks before it does it again.

I would go straight to the fridge and freezer and check the defrost heating elements that keep them frost free. Once the power has tripped to a fridge or freezer it resets the timer / defrost controller and there is no apparent fault to find and until it operates and energises the heating element again and then RCD trips and the cycle starts again

Took me a few weeks the first time I had this fault to find but since then it's the first thing I check when there is a fridge or freezer on the circuit. Don't be fooled by the age of the unit had this on a 3 - 4 year old american style fridge / freezer once

 

[Geoff1946]

I would go straight to the fridge and freezer and check the defrost heating elements that keep them frost free. Once the power has tripped to a fridge or freezer it resets the timer / defrost controller and there is no apparent fault to find and until it operates and energises the heating element again and then RCD trips and the cycle starts again

Now that's a good one to bear in mind. Hadn't heard of that before.  I wonder i f switching the freezer off and on again before going on holiday is a sensible precaution?  What is the defrost time interval?

 

[Murdoch]

Hi Murdoch,

Thanks for the suggestion. I'll make a note of time and date and anything being switched on or off each time it happens from now on.

Ther are two MCBs on this RCD, one for the upstairs lights and the other for the upstairs sockets plus the kitchen sockets.

noting what’s being switched on or off could send you down a blind alley

you need to make notes and see if there is any pattern.
 

does it happen at night?

when your house is empty?

when the eg washing machine is on?
when the heating is on?

when it’s raining?
 

From what you’ve said there are many sockets on the circuit so using an earth leakage clamp meter AND ramp testing your RCD is a MUST

my advice to you is find a local spark through recommendation, check they have an earth leakage clamp meter as well the test kit and knowledge to check your rcd. That shouldn’t cost much and IR testing should be done too

hope this helps