UK RCD issue baffling me

[ChiliTom]

I have a dual board consumer unit. Both sides have a RCD. The left hand side of the board has all the MCBs for all the 6 ring mains for the property. The right side of the board has the lighting and hob and cooker for the house.

Whenever a current is drawn from a socket the right side RCD trips, but not the left side. So my lights trip but sockets stay on. It happens on any of the 6 ring mains. And seems to be any appliance that draws a large current such as a hoover, kettle, immersion heater etc.

I do have a alarm system but I have totally isolated it by unwiring it front the spur it was connected to. Still the same issue.

I do also have under cabinet lights that are ran from a ring main... I have yet to investigate these.

The MCBs switched off were because they were just straight up tripping the right side RCD. Probably because they all had stuff like a fridge, boiler etc drawing current. The right hand side lighting circuit that is turned off also automatically trips the right RCD.

Electrician has tested each circuit and can't find a fault. The board has been wired correctly with no crossed over neutrals.

Anyone got any clue where to start some more fault finding? I am a ex Army Electrician, that was 17th ed trained... But that was about a decade ago but I know enough to be safe.

Cheers

 

[brianmoooore]

Is it the same electrician who installed the CU that's doing the testing now? Not exactly impressed with the distribution of circuits between the two RCDs in that CU.
It'll be an earth to neutral fault in one of the RH bank circuits, and things with ceramic, metal enclosed elements don't always show poor IR when cold.

 

[mainline]

The MCBs switched off were because they were just straight up tripping the right side RCD. Probably because they all had stuff like a fridge, boiler etc drawing current. The right hand side lighting circuit that is turned off also automatically trips the right RCD.

Turning off single pole mcbs won't disconnect the N to Earth fault, best to disconnect individual circuits entirely.
If it trips instantly with all circuits energised, then it shouldn't take long to isolate the faulty circuit.

 

[ChiliTom]

Is it the same electrician who installed the CU that's doing the testing now? Not exactly impressed with the distribution of circuits between the two RCDs in that CU.
It'll be an earth to neutral fault in one of the RH bank circuits, and things with ceramic, metal enclosed elements don't always show poor IR when cold.

Why would the RH bank of lighting circuits RCD trips though from using plus sockets that are on the left bank.

 

[ChiliTom]

Is it the same electrician who installed the CU that's doing the testing now? Not exactly impressed with the distribution of circuits between the two RCDs in that CU.
It'll be an earth to neutral fault in one of the RH bank circuits, and things with ceramic, metal enclosed elements don't always show poor IR when cold.

No it's a different Electrician. We both actually commented on the split of the circuits as being sub par. But it shouldn't affect the RCDs as it is.

 

[brianmoooore]

Because most of the neutral current from the LH circuits will flow back the neutral tail to the meter and then the service head, but a small proportion will pass through the RH RCD neutral side, down the neutral wire of the faulty circuit, through the neutral - earth fault, back the cpc of the faulty circuit to the CU, then down the cpc to the service head, where, in the case of TNC-S, it is joined to the neutral. If this current momentarily reaches about 20mA (typical tripping current of a 30mA RCD), then the RH RCD will trip.

 

[ChiliTom]

Because most of the neutral current from the LH circuits will flow back the neutral tail to the meter and then the service head, but a small proportion will pass through the RH RCD neutral side, down the neutral wire of the faulty circuit, through the neutral - earth fault, back the cpc of the faulty circuit to the CU, then down the cpc to the service head, where, in the case of TNC-S, it is joined to the neutral. If this current momentarily reaches about 20mA (typical tripping current of a 30mA RCD), then the RH RCD will trip.

So the left hand bank that has all the sockets on it, isn't to fault, even though it trips the other sides RCD. The fault lays somewhere in one of the left hand lighting circuits and will be a E/N fault?

 

[brianmoooore]

.......... the left hand lighting circuits ..........

Typo?

 

[ChiliTom]

Typo?

Yes sorry right hand lighting circuits

 

[brianmoooore]

N - E fault will be in the RH circuits, and could be a lighting circuit, especially if any of the lights are outdoors, but I'd concentrate on the two circuits that are labelled as cookers/ovens/hobs first of all, even if it's just turning off their DP isolators when not in use.

 

[ChiliTom]

I still don't understand how a LH bank of fully normal and fully operational set of 6 ring mains trip the opposite sides RCD when they draw current. Surely the neutral current goes back to the neutral bar then back to the mains. It never passes through the other RCD.

 

[brianmoooore]

As I explained earlier, there is a parallel path for the neutral current to return to the service head. One that results in a small proportion of the neutral current from the socket circuits, but not the live current, flowing through the RH RCD, thus unbalancing it.
The proportion of diverted current is the ratio of the resistance of the neutral tail compared to the resistance of the RH circuit neutral, fault resistance, RH circuit cpc, and main cpc, which can be a very small proportion, BUT 20mA is a very small current, when compared to the many tens of amps that can flow as an inrush current on the socket circuits.

 

[ChiliTom]

As I explained earlier, there is a parallel path for the neutral current to return to the service head. One that results in a small proportion of the neutral current from the socket circuits, but not the live current, flowing through the RH RCD, thus unbalancing it.
The proportion of diverted current is the ratio of the resistance of the neutral tail compared to the resistance of the RH circuit neutral, fault resistance, RH circuit cpc, and main cpc, which can be a very small proportion, BUT 20mA is a very small current, when compared to the many tens of amps that can flow as an inrush current on the socket circuits.

Ok so I should disconnect the neutrals from the RH side one neutral bar by one and see which one once removed doesnt trip the RCD? This should indicate which circuit the fault lies in?

Then I guess just investigate the circuit and see if it's wired correctly?

 

[ChiliTom]

So am I right in saying when I use a socket for a kettle the current is going from the mains through the LH RCD to the MCB to the kettle and the return current is then going down the neutral back through the LH RCD (all ok at this point) up to the neutral bar where the RH RCD neutral is also connected, but because there is a N to E fault in a circuit that side the current is instead flowing down the RH RCD neutral to the earth somewhere which then trips the RCD as there was a imbalance (due to no live current being drawn that side)

 

[brianmoooore]

If you just disconnect the neutrals, leaving the lives connected, you stand a good chance of raising the voltage on those neutrals, so that the N-E fault now becomes a L-E fault, and now thrips the RCD because of the imbalance resulting from that.
IR testing with a MFT is the usual way to track down a fault like this.

 

[brianmoooore]

So am I right in saying when I use a socket for a kettle the current is going from the mains through the LH RCD to the MCB to the kettle and the return current is then going down the neutral back through the LH RCD (all ok at this point) up to the neutral bar where the RH RCD neutral is also connected, but because there is a N to E fault in a circuit that side the current is instead flowing down the RH RCD neutral to the earth somewhere which then trips the RCD as there was a imbalance (due to no live current being drawn that side)

Yes, that's the general idea, but there might also be a balanced current flowing through the RH RCD as well, if the RH circuits are in use. May be 30A flowing through the live of the RH RCD, but if there's 30.02A flowing through the neutral of the same RCD, even for just a few milliseconds, then off it goes.

 

[ChiliTom]

Yes, that's the general idea, but there might also be a balanced current flowing through the RH RCD as well, if the RH circuits are in use. May be 30A flowing through the live of the RH RCD, but if there's 30.02A flowing through the neutral of the same RCD, even for just a few milliseconds, then off it goes.

But instead of the lighting current returning form the circuit down its neutral the sockets neutral current is going the other way trying to get to earth through the failt

 

[ChiliTom]

As I explained earlier, there is a parallel path for the neutral current to return to the service head. One that results in a small proportion of the neutral current from the socket circuits, but not the live current, flowing through the RH RCD, thus unbalancing it.
The proportion of diverted current is the ratio of the resistance of the neutral tail compared to the resistance of the RH circuit neutral, fault resistance, RH circuit cpc, and main cpc, which can be a very small proportion, BUT 20mA is a very small current, when compared to the many tens of amps that can flow as an inrush current on the socket circuits.

Would this be the case for the other lighting circuits tho because they all work fine and don't trip the RCD. Surely any current would try to use the parallel path to earth from the faulty neutral. How come the other lighting circuits are ok and work okay

 

[brianmoooore]

All depends on the resistance of the actual fault. If it's low enough, then the lighting circuits may well trip their RCD.

 

[ChiliTom]

But instead of the lighting current returning form the circuit down its neutral the sockets neutral current is going the other way trying to get to earth through the failt

All depends on the resistance of the actual fault. If it's low enough, then the lighting circuits may well trip their RCD.

Did a continuing test at the CU. Got a positive reading from neutral to earth on the right hand side. Disconnected neutrals and investigated more, found a lighting circuit with the fault. Just need to try and actually find where iit is. So far I've looked into 3 switches, and one ceiling rose and one bathroom fan. All seem ok . More hunting needed.

 

[James]

Outside lights are often the culprit
As said before.

 

[ChiliTom]

Outside lights are often the culprit
As said before.

Unlikely on this circuit. There is a outside light but I've split the circuit before it and still getting the fault. It's either in the toilet or dining room. Suspected the extractor fan but seems ok.

Split the circuit at the dining room switch for the down lights. So the issue actually must be in the bathroom seeing as the dining room down lights no longer connected to neutral.

 

[ChiliTom]

Possibly the alarm bell box. Can't find where this cable runs too. It's not for the lights or the fan.

 

[mainline]

Possibly the alarm bell box. Can't find where this cable runs too. It's not for the lights or the fan.

I've never seen an Alarm bell box that has a mains supply, it's derived from a transformer in the alarm panel.

 

[ChiliTom]

I've never seen an Alarm bell box that has a mains supply, it's derived from a transformer in the alarm panel.

Alarm panel is on a 13 amp spur from a ring main. Rest is wireless.

 

[mainline]

Alarm panel is on a 13 amp spur from a ring main. Rest is wireless.

You said the fault was on a lighting circuit and the Alarm panel is a spur off the ring final ??.

ChiliTom said: Possibly the alarm bell box

An alarm bell box is unlikely to have a mains supply to it, as already said.

 

[mainline]

Unlikely on this circuit. There is a outside light but I've split the circuit before it and still getting the fault. It's either in the toilet or dining room. Suspected the extractor fan but seems ok.

Split the circuit at the dining room switch for the down lights. So the issue actually must be in the bathroom seeing as the dining room down lights no longer connected to neutral.

Have the light switches got a Neutral in them ?

 

[ChiliTom]

You said the fault was on a lighting circuit and the Alarm panel is a spur off the ring final ??.

An alarm bell box is unlikely to have a mains supply to it, as already said.

Alarm panel is on a spur. The bell box i suspect is wired into the lighting circuit. Would explain that when I set the alarm of the fault began. Probably caused a short as the bell box is obviously outside.

 

[ChiliTom]

Have the light switches got a Neutral in them ?

Yes. House has spot lights so all the neutrals are wired up at the switches as there's no ceiling roses.

 

[mainline]

Alarm panel is on a spur. The bell box i suspect is wired into the lighting circuit. Would explain that when I set the alarm of the fault began. Probably caused a short as the bell box is obviously outside.

A bell box will be powered by 12v and will also have an internal battery.
If there is a mains supply to this bell box it, will either have an internal transformer or will have one inline.

It's highly unlikely that the bell box would be giving an N to E fault, it probably doesn't even have an earth.

 

[brianmoooore]

It could be an abandoned cable going nowhere. In a case like this, I'd fully disconnect it and wait to see if anyone complains.

 

[mainline]

It could be an abandoned cable going nowhere. In a case like this, I'd fully disconnect it and wait to see if anyone complains.

In a case like this, I would advise calling a decent electrician.

 

[ChiliTom]

Sparky is coming back tomorrow. Now I've found the problem cable he can try and figure out what it does, I've absolutely no idea. It was actually in a connection block behind a mirror, but runs off somewhere random.

 

[MikeSp]

I have a dual board consumer unit. Both sides have a RCD. The left hand side of the board has all the MCBs for all the 6 ring mains for the property. The right side of the board has the lighting and hob and cooker for the house.

Whenever a current is drawn from a socket the right side RCD trips, but not the left side. So my lights trip but sockets stay on. It happens on any of the 6 ring mains. And seems to be any appliance that draws a large current such as a hoover, kettle, immersion heater etc.

I do have a alarm system but I have totally isolated it by unwiring it front the spur it was connected to. Still the same issue.

I do also have under cabinet lights that are ran from a ring main... I have yet to investigate these.

The MCBs switched off were because they were just straight up tripping the right side RCD. Probably because they all had stuff like a fridge, boiler etc drawing current. The right hand side lighting circuit that is turned off also automatically trips the right RCD.
View attachment 118549

Electrician has tested each circuit and can't find a fault. The board has been wired correctly with no crossed over neutrals.

Anyone got any clue where to start some more fault finding? I am a ex Army Electrician, that was 17th ed trained... But that was about a decade ago but I know enough to be safe.

Cheers

I had a more confusing version of this. Switching on a kitchen socket caused a RCD on a different CU to trip.

The cause was found to be a squirrel nibbled cable, with the neutral and earth shorting. Easy to fix after the exceptionally difficult task of finding exactly where the cables were chewed.

 

[ChiliTom]

Turns out it was a wire that fed the dinning room lights from the connection behind my mirror... Strange. Stripped the old cable out no visible damage at all but was faulty. New wire installed and all good. Just a few holes in my wall now

 

[brianmoooore]

no visible damage at all but was faulty.

Seen this where T&E has been fixed with a flat bend of too tight a radius.