UK RCD issue baffling me

[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.