RCD trips with all load breakers open

[santoli3]

My circuit panel has two banks of breakers, each bank protected by an RCD. One of these RCDs has been problematic in that it trips intermitantly. A few months ago it became so frequent that we had the RCD replaced. It would trip regardless of what load breakers were shut or open. No fault could be found at the time. It was fine affter replacement until today. Today it tripped, and will only reset for a short period of time before tripping again. It trips even if all of the downstream load breakers are open. I don't understand what fault could result in a trip when all of the load breakers are open. Since there is no current, there can be no current imbalance between earth and neutral. What am I missing?

 

[SJD]

Could there be some N current from one of the unprotected circuits finding it's way to the N of an RCD-protected circuit?

 

[marconi]

The circuit breakers for your final circuits are all single pole which means when they are open only the line (brown) is disconnected from the rcd, the neutral (blue) remains connected. I suspect a neutral to earth fault but before jumping the gun you should call in a good electricians with the appropriate skill and test equipments to do some dead tests and visual inspections of all the final circuits of your CU. Ask him/her also to check the tightness and solidity of all connections.

Note that the fault could be with a load plugged in or permanently wired in and not with the circuit supplying the power to it so your loads should be tested too.

 

[Avo Mk8]

As per Marconi above, an experienced electrician should be able to track the problem down.

In my (admittedly limited) experience, immersion heaters and appliances, particularly those heating water, can develop earth leakage of their heating elements. As an experiment, you could try switching off your immersion heater at its own switch, which should be double pole, and/or completely unplugging the washing machine and anything else in the utility room that would fit into the category above. That way you disconnect their neutrals, which usually don't get isolated if turning off at the plug.
It's a long shot, and probably won't locate the problem, but it's something else to try!
NB having second thoughts on the above, because if the RCD's still tripping with all the mcb's off, it's probably not the above

 

[santoli3]

The circuit breakers for your final circuits are all single pole which means when they are open only the line (brown) is disconnected from the rcd, the neutral (blue) remains connected. I suspect a neutral to earth fault but before jumping the gun you should call in a good electricians with the appropriate skill and test equipments to do some dead tests and visual inspections of all the final circuits of your CU. Ask him/her also to check the tightness and solidity of all connections.

Note that the fault could be with a load plugged in or permanently wired in and not with the circuit supplying the power to it so your loads should be tested too.

Thanks Marconi. I get that. But with all of the load breakers open, there is no current going to any of the loads. So there is zero current on the line side of the RCD. But with no current going to any of the loads, there can be no current coming back through neutral from the loads either, and therefore zero current on the neutral side of the RCD. Two zeros = no imbalance. I know I’m missing something. But what?

 

[santoli3]

As per Marconi above, an experienced electrician should be able to track the problem down.

In my (admittedly limited) experience, immersion heaters and appliances, particularly those heating water, can develop earth leakage of their heating elements. As an experiment, you could try switching off your immersion heater at its own switch, which should be double pole, and/or completely unplugging the washing machine and anything else in the utility room that would fit into the category above. That way you disconnect their neutrals, which usually don't get isolated if turning off at the plug.
It's a long shot, and probably won't locate the problem, but it's something else to try!
NB having second thoughts on the above, because if the RCD's still tripping with all the mcb's off, it's probably not the above

Yeah, It's the trippimg no matter what breakers are shut, and even when all are open that is really confusing me.

 

[santoli3]

Could there be some N current from one of the unprotected circuits finding it's way to the N of an RCD-protected circuit?

Well, they are all protected. But they are in two banks, each bank with an RCD.

 

[marconi]

If the neutral and earth conductors have a potential difference between them where they enter your property, and this pd could depend on network neutral current then even in absence of line current through the rcd there may well be a neutral current flowing ‘forwards’ through the rcd and then via an rcd load side n-e fault and then all the way back via e conductor or terra firma to the supply transformer star point which is where the network earthed neutral originates. Thus an unbalance of current through the rcd happens. A loop circuit with an emf in it causes a current to flow through the loop.

Shows us a picture of your supply intake so we can work out your home’s earthing system please.

 

[James]

As per Marconi above, an experienced electrician should be able to track the problem down.

In my (admittedly limited) experience, immersion heaters and appliances, particularly those heating water, can develop earth leakage of their heating elements. As an experiment, you could try switching off your immersion heater at its own switch, which should be double pole, and/or completely unplugging the washing machine and anything else in the utility room that would fit into the category above. That way you disconnect their neutrals, which usually don't get isolated if turning off at the plug.
It's a long shot, and probably won't locate the problem, but it's something else to try!
NB having second thoughts on the above, because if the RCD's still tripping with all the mcb's off, it's probably not the above

You are on the right track, add to the list of usual suspects
Electric oven, grill and of course any outdoor lights or sockets

 

[brianmoooore]

Does it still trip when the RCD for the other bank is off?

 

[santoli3]

If the neutral and earth conductors have a potential difference between them where they enter your property, and this pd could depend on network neutral current then even in absence of line current through the rcd there may well be a neutral current flowing ‘forwards’ through the rcd and then via an rcd load side n-e fault and then all the way back via e conductor or terra firma to the supply transformer star point which is where the network earthed neutral originates. Thus an unbalance of current through the rcd happens. A loop circuit with an emf in it causes a current to flow through the loop.

Shows us a picture of your supply intake so we can work out your home’s earthing system please.

OK. Based on that discussion, I took measurements of neutral bus to earth on each side, and I think I've sorted it. I opened the panel, and opened the main, both RCDs and all load breakers. The attached picture was taken just prior to opening all breakers. Then I measured neutral bus to earth bus resistance on both sides. The left side was overload. The right side was 20 Ohms. I then disconnected individual neutrals one at a time on the right neutral bus until the right side went to overload. It turns out the immersion heater is the culprit. I left disconnected the immersion heater neutral and insulated it. I also disconnected the immersion heater load side from its breaker and insulated it. I labeled both as “Immersion Heater – Faulted”. Now with both sides OL to earth, I buttoned it up and shut all the breakers. So far so good. Thanks all!

 

[Avo Mk8]

OK. Based on that discussion, I took measurements of neutral bus to earth on each side, and I think I've sorted it. I opened the panel, and opened the main, both RCDs and all load breakers. The attached picture was taken just prior to opening all breakers. Then I measured neutral bus to earth bus resistance on both sides. The left side was overload. The right side was 20 Ohms. I then disconnected individual neutrals one at a time on the right neutral bus until the right side went to overload. It turns out the immersion heater is the culprit. I left disconnected the immersion heater neutral and insulated it. I also disconnected the immersion heater load side from its breaker and insulated it. I labeled both as “Immersion Heater – Faulted”. Now with both sides OL to earth, I buttoned it up and shut all the breakers. So far so good. Thanks all!

Well done! Worth checking at the immersion heater (preferably with the local switch 'off') the resistance between the element connections and ground. Sounds like you need a new one!

 

[santoli3]

Well done! Worth checking at the immersion heater (preferably with the local switch 'off') the resistance between the element connections and ground. Sounds like you need a new one!

Thanks. Need is a strong word. The immersion heater breaker hasn't been shut since we moved in (maybe the previous knew something). So I certainly don't need it. But I did rethink my solution, and disconnected the immesion heater loacaly, insulated the leads, and labeled the cover "Immersion heater disconnected due to earth fault". I then reconnected everything in the breaker panel, checked that neutral to earth was still good, then labeled the breaker "immerion heater faulted". This way, the next person to live here, if they "need" the immersion heater, can sort it without having to enter the breaker panel.