UK Why does my RCD keep tripping?

[DoctorJools]

We've been having some windy weather here in Oxford recently. It's occasionally causing the lights to flicker and sometimes it causes the RCD in the attached photo to trip (the one I've put a red box around).

First question - is that normal? It happens when we're not "doing" anything in terms of turning things on or off. And I'm pretty sure it's not triggered by any timed things like immersion heaters - it does seem to coincide with the lights flickering. (And that has only been happening recently, with the wind.)

It might be purely coincidental, but it has only started to happen since we had an induction hob fitted (by a qualified electrician). As that obviously needs a fair few amps, that's got its own, new, RCD - the one with "HOB" written underneath it.

I've been experimenting with seeing whether it's anything to do with the hob (on the basis that it's something that's changed recently) by turning that RCD off (the "HOB" one) and seeing if that stops the main one from going when the lights flicker.

So far, we haven't had a problem when the "HOB" one is off, but as the wind and flickering only happens once in a while, it's very far from conclusive evidence. But I was wondering if any of the knowledgeable people out here might have any thoughts.

Ta.

 

[buzzlightyear]

In order to find out what is coursing the rcd to trip it would be advisable to have some testing done.

 

[Ziggie]

There is some things that might add to an understanding.

The main RCD should have a different spec to more local ones.This means that the local one catches the problem.
The lights flickers are caused by voltage fluctuations in the supply coming into the house and if the voltage goes high then this might cause a problem.

One problem I had was caused by a fridge freezer on the same circuit as a local RCD.
Different RCDs with the same nominal specs have widely different actual specs.

Have you any outside equipment that could be causing a fault or has a bad connection ?

 

[HappyHippyDad]

We've been having some windy weather here in Oxford recently. It's occasionally causing the lights to flicker and sometimes it causes the RCD in the attached photo to trip (the one I've put a red box around).

First question - is that normal? It happens when we're not "doing" anything in terms of turning things on or off. And I'm pretty sure it's not triggered by any timed things like immersion heaters - it does seem to coincide with the lights flickering. (And that has only been happening recently, with the wind.)

It might be purely coincidental, but it has only started to happen since we had an induction hob fitted (by a qualified electrician). As that obviously needs a fair few amps, that's got its own, new, RCD - the one with "HOB" written underneath it.

I've been experimenting with seeing whether it's anything to do with the hob (on the basis that it's something that's changed recently) by turning that RCD off (the "HOB" one) and seeing if that stops the main one from going when the lights flicker.

So far, we haven't had a problem when the "HOB" one is off, but as the wind and flickering only happens once in a while, it's very far from conclusive evidence. But I was wondering if any of the knowledgeable people out here might have any thoughts.

Ta.

As Buzz has said, you need to get some testing done. If the RCD trips randomly just the once then I dont think an electrician is needed. If it trips more than once then you simply need to get an electrician out to test. There is a reason it is tripping and that reason is you probably have a fault somewhere. Leaving the fault in place could lead to someone getting hurt. The RCD is simply doing it's job correctly.

 

[Wilko]

Sorry to hear about your RCD tripping problem. It’s pretty rare for RCDs to be at fault, ie they normally trip for a good reason. So as per Buzz and HHD - get an experienced Electrician to assist with testing and fault finding. Someone on here may be nearby to assist if you need.

 

[sheppertonspark]

Is your property served by an overhead line (T-T Supply) ? If yes, sounds like a possible loose connection on the supply side. I suggest that you call the DNO (District Network Operator), they own the cables . The higher current demand will cause the (possible) loose connection to drop voltage across it. This is likely to imbalance the RCD. If Im talking Bo~~~~ks please someone tell me !!
Report on 105 and follow the voice prompts. There is even a number to press for lights flickering.

 

[DoctorJools]

Is your property served by an overhead line (T-T Supply) ? If yes, sounds like a possible loose connection on the supply side. I suggest that you call the DNO (District Network Operator), they own the cables . The higher current demand will cause the (possible) loose connection to drop voltage across it. This is likely to imbalance the RCD. If Im talking Bo~~~~ks please someone tell me !!
Report on 105 and follow the voice prompts. There is even a number to press for lights flickering.

Nope - there's an underground cable that comes up in to the meter box at the front of the house. Quite where that comes from, I've no idea!

Thanks anyway.

 

[sheppertonspark]

Nope - there's an underground cable that comes up in to the meter box at the front of the house. Quite where that comes from, I've no idea!

Thanks anyway.

I would still consider informing DNO. Had a similar "call-out" last weekend. Neutral burnt out in joint outside the property. IT COULD ALSO BE A BAD CONNECTION INSIDE THE PROPERTY, RISK OF FIRE, GET IT TESTED QUICK !!

 

[Dartlec]

We've been having some windy weather here in Oxford recently. It's occasionally causing the lights to flicker and sometimes it causes the RCD in the attached photo to trip (the one I've put a red box around).

First question - is that normal? It happens when we're not "doing" anything in terms of turning things on or off. And I'm pretty sure it's not triggered by any timed things like immersion heaters - it does seem to coincide with the lights flickering. (And that has only been happening recently, with the wind.)

It might be purely coincidental, but it has only started to happen since we had an induction hob fitted (by a qualified electrician). As that obviously needs a fair few amps, that's got its own, new, RCD - the one with "HOB" written underneath it.

I've been experimenting with seeing whether it's anything to do with the hob (on the basis that it's something that's changed recently) by turning that RCD off (the "HOB" one) and seeing if that stops the main one from going when the lights flicker.

So far, we haven't had a problem when the "HOB" one is off, but as the wind and flickering only happens once in a while, it's very far from conclusive evidence. But I was wondering if any of the knowledgeable people out here might have any thoughts.

Ta.

Intermittent faults can be a bugger to track down unfortunately. Testing is not a bad idea, because it rules out the fixed wiring at least and will determine if the RCD is particularly sensitive.

The picture doesn't appear to include the circuit for the hob mentioned. If that is electrically wired separately to a different RCD (or RCBO) then it shouldn't be impacting the main RCD.

Everything will 'leak' small amounts of current, so it could be a combination of things all 'leaking' together that is enough to go over whatever the RCD is tripping at.

The wind and lights flickering may suggest an external cause, in which case it wouldn't hurt to call the DNO to get them to confirm there are no problems with their supply.

If the wiring testing doesn't show up a cause, then individually PAT testing your appliances on the circuits in question may narrow it down, but otherwise it becomes a question of elimination.

The picture is obscuring the rating of the RCD, but if it's 63A, then running what appears to be all your socket circuits from it isn't a great idea - though depending on what's connected it may not be a practical issue. But in theory you could be overstressing the RCD which could cause it to fail prematurely.

 

[pc1966]

One possible reason for the RCD tripping is if the supply fluctuates rapidly in voltage. RCD trip on "stray" current (i.e. the residual of L-N) and anything with capacitance such as cables or noise filters in electronics that have some capacitance to earth (and an induction hob is a strong candidate for that) will pass a small current, normally well below the 15-30mA trip threshold of such an RCD.

Often that current is computed from I = V / Xc = V * 2 * pi * f * C

Where f = 50Hz and C is the capacitance in Farads. But that is for a sine wave as the true computation for current is:

I = C * dV/dt

Where dV/dt is the rate of change of voltage. If something is causing your supply to jump quickly (problem with bad connection, volts dropping/returning suddenly as cables go down in the wind and breakers disconnect them, etc) you might get enough current to trip the RCD.

However, there are other more common issues such as a N-E fault that normally shows no trip behaviour (as N and E are so close together in voltage that negligible current circulates) but can cause strange trips when high current loads go on even on unrelated RCD circuits.

So I would suggest you get a professional in first to check your RCD trip characteristics and to see if you have any insulation faults (poor L-E insulation or N-E faults).

If that draws a blank then probably it is just too much off that RCD and if it bothers you enough you might need to have the circuits divided to another RCD, or better still look longer-term at replacing the board with an all-RCBO style so each circuit has its own RCD protection and far less accumulated leakage per circuit to cause spurious trips.

 

[DoctorJools]

Intermittent faults can be a bugger to track down unfortunately. Testing is not a bad idea, because it rules out the fixed wiring at least and will determine if the RCD is particularly sensitive.

The picture doesn't appear to include the circuit for the hob mentioned. If that is electrically wired separately to a different RCD (or RCBO) then it shouldn't be impacting the main RCD.

Everything will 'leak' small amounts of current, so it could be a combination of things all 'leaking' together that is enough to go over whatever the RCD is tripping at.

The wind and lights flickering may suggest an external cause, in which case it wouldn't hurt to call the DNO to get them to confirm there are no problems with their supply.

If the wiring testing doesn't show up a cause, then individually PAT testing your appliances on the circuits in question may narrow it down, but otherwise it becomes a question of elimination.

The picture is obscuring the rating of the RCD, but if it's 63A, then running what appears to be all your socket circuits from it isn't a great idea - though depending on what's connected it may not be a practical issue. But in theory you could be overstressing the RCD which could cause it to fail prematurely.

Thanks for your reply. Yes, I know about intermittent faults. I work in IT and intermittent bugs are the worst!

Apologies about picture - attached is one of the whole fuse box. Both of the RCDs are rated as 80A and 30mA 230V.

Seems like I need to call the electrician then!

 

[DoctorJools]

One possible reason for the RCD tripping is if the supply fluctuates rapidly in voltage. RCD trip on "stray" current (i.e. the residual of L-N) and anything with capacitance such as cables or noise filters in electronics that have some capacitance to earth (and an induction hob is a strong candidate for that) will pass a small current, normally well below the 15-30mA trip threshold of such an RCD.

Often that current is computed from I = V / Xc = V * 2 * pi * f * C

Where f = 50Hz and C is the capacitance in Farads. But that is for a sine wave as the true computation for current is:

I = C * dV/dt

Where dV/dt is the rate of change of voltage. If something is causing your supply to jump quickly (problem with bad connection, volts dropping/returning suddenly as cables go down in the wind and breakers disconnect them, etc) you might get enough current to trip the RCD.

However, there are other more common issues such as a N-E fault that normally shows no trip behaviour (as N and E are so close together in voltage that negligible current circulates) but can cause strange trips when high current loads go on even on unrelated RCD circuits.

So I would suggest you get a professional in first to check your RCD trip characteristics and to see if you have any insulation faults (poor L-E insulation or N-E faults).

If that draws a blank then probably it is just too much off that RCD and if it bothers you enough you might need to have the circuits divided to another RCD, or better still look longer-term at replacing the board with an all-RCBO style so each circuit has its own RCD protection and far less accumulated leakage per circuit to cause spurious trips.

Thanks for your reply. My A-level understanding of this area did make me wonder about capacitance in conjunction with spikes in the supply being the cause. But that's my limit! So I'll call a professional.

 

[Lucien Nunes]

Pc1966's explanation is good and I agree that transients on the line (caused by external line faults) are a likely cause of tripping via the RF interference suppressors in the induction hob. As he points out, and I think you already have a good grasp of, high rates of change of supply voltage cause proportionately high momentary currents through the capacitors, much higher than a steady-state 50Hz. That RCD has multiple socket-outlet circuits on, plus the normal leakage of the hob, so might be running close to its tripping threshold already. If switching off the hob MCB (not RCD) does actually stop the nuisance trips, that is useful evidence as it would not have any effect on the other likely cause; a neutral-earth fault.

The higher current demand will cause the (possible) loose connection to drop voltage across it. This is likely to imbalance the RCD. If Im talking Bo~~~~ks please someone tell me !!

Erm, you're talking.... (not quite)
Voltage drop by itself won't cause an imbalance or nuisance trip, as it doesn't create leakage. The load current is reduced by the resistance but it all stays within the circuit. However, an intermittent connection that arcs can cause the type of voltage spikes we're considering, which in turn cause the interference suppressors in electronic appliances to create leakage.

 

[Dartlec]

Thanks for your reply. Yes, I know about intermittent faults. I work in IT and intermittent bugs are the worst!

Apologies about picture - attached is one of the whole fuse box. Both of the RCDs are rated as 80A and 30mA 230V.

Seems like I need to call the electrician then!

Seeing the whole board, it's not a great idea to have all those high current circuits on the one RCD, usually they are split across the two RCDs with the lower capacity ones like lights, etc.

Is it a huge house, or is there a reason why there are 4 ring circuits?

Although it's unlikely you will be using a load anywhere near the full capacity, it is possible at times that you could be pulling a significant load on the left RCD. An 80A RCD should be able to cope with your likely usage, though if it had been a 63A I might have wondered if it had been overloaded enough to affect it.

But with that many circuits, presumably with things plugged in, it might not take much 'extra' leakage above the normal background for it to trip.