Why does RCD trip when fused spur switched on

[Msitekkie]

I was called out last night to an RCD trip issue. I found that switching on the fused spur to the washing machine would cause the trip & I could only reset the RCD after switching it off.
Straightforward up to that point, but after unplugging the washing machine, then disconnecting the spur itself, the trip would still occur..! An hour or so later, without finding a specific fault the problem cleared.
This circuit also tripped a few weeks ago & the fault cleared by itself during troubleshooting attempts.

A similar sounding problem posted elsewhere led to a problem with a motorised valve and a circulating pump fault was also suggested, but I am baffled as to why switching the FCU on would trigger the RCD with nothing connected?

 

[littlespark]

Fault in the cable is there’s nothing connected to it. Intermittent fault clearing itself.

I had one traced to a cable, part of the central heating. Run over the copper heating pipes under the floor and had rubbed through. So fault only showed when

A. Heating or water was calling for heat and
B. Someone stood on a certain floorboard

 

[Msitekkie]

Fault in the cable is there’s nothing connected to it. Intermittent fault clearing itself.

I had one traced to a cable, part of the central heating. Run over the copper heating pipes under the floor and had rubbed through. So fault only showed when

A. Heating or water was calling for heat and
B. Someone stood on a certain floorboard

Clearly there is an intermittent element to the fault, but the fault was consistent for maybe a couple of hours. "fault in the cable". What cable? The fault still occurred with the spur disconnected & I did insulation resistance tests to check the spur anyway.
If cable elsewhere in the circuit, why would it be affected just by operating the switch on the FCU?

 

[ipf]

Fault in the cable is there’s nothing connected to it. Intermittent fault clearing itself.
I had one traced to a cable, part of the central heating. Run over the copper heating pipes under the floor and had rubbed through. So fault only showed when
A. Heating or water was calling for heat and
B. Someone stood on a certain floorboard

Two ruddy good clues there, then....? ?

 

[littlespark]

Sorry, I missed the bit where you said spur was disconnected…. Only read washing machine unplugged

 

[Petej999]

I was called out last night to an RCD trip issue. I found that switching on the fused spur to the washing machine would cause the trip & I could only reset the RCD after switching it off.
Straightforward up to that point, but after unplugging the washing machine, then disconnecting the spur itself, the trip would still occur..! An hour or so later, without finding a specific fault the problem cleared.
This circuit also tripped a few weeks ago & the fault cleared by itself during troubleshooting attempts.

A similar sounding problem posted elsewhere led to a problem with a motorised valve and a circulating pump fault was also suggested, but I am baffled as to why switching the FCU on would trigger the RCD with nothing connected?

Except the wiring within the FCU, get the wiring checked out by a competent Electrician

 

[Msitekkie]

Except the wiring within the FCU, get the wiring checked out by a competent Electrician

So you think replacing the FCU will fix the problem? By "wiring within the FCU" I take you to mean the fuse and internal metalwork that makes the connections. The wiring as in the two legs of the ring, remains the same whether the FCU is off or on, doesn't it?

 

[timhoward]

It sounds as though there is an intermittent fault possibly involving water ingress elsewhere on the circuit that coincided with your actions with the spur / washing machine.
Could you reliably prove it trips, doesn't trips, trips, doesn't trips with the spur on / off within the same 2 minute period?
Is it RCBO's or are we talking an RCD for half a board here?

 

[Petej999]

So you think replacing the FCU will fix the problem? By "wiring within the FCU" I take you to mean the fuse and internal metalwork that makes the connections. The wiring as in the two legs of the ring, remains the same whether the FCU is off or on, doesn't it?

It's the Chicken and Egg mate, got to be something in the circuitry that's causing the RCD o trip get it checked oud out by an electrician, trying to diagnose what is wrong on a Forum is fraught with problems, to many know-alls giving advice, Im no t a know all but I KNOW WHAT I WOULD DO IF I WAS IN YOUR SHOES.

 

[Msitekkie]

It sounds as though there is an intermittent fault possibly involving water ingress elsewhere on the circuit that coincided with your actions with the spur / washing machine.
Could you reliably prove it trips, doesn't trips, trips, doesn't trips with the spur on / off within the same 2 minute period?
Is it RCBO's or are we talking an RCD for half a board here?

Yes, it was consistent for a long period, but once it went it was gone.
I was also suspecting water ingress as showers had been in use earlier & there was a previous occurrence some months ago where I believe the same circuit tripped as a result of a leak from the shower waste - on that occasion there was water through the ceiling & I'm told it was repaired.

Just very confusing in that scenario as to why the fault would go away with the FCU switched off (no neon BTW). I appreciate RCD faults can be a bit weird, but I always like to understand a fault rather than just fix it.
It's a split load consumer unit. No RCBOs. One of those MK ones, that has a mid position on the RCDs

 

[Petej999]

So you think replacing the FCU will fix the problem? By "wiring within the FCU" I take you to mean the fuse and internal metalwork that makes the connections. The wiring as in the two legs of the ring, remains the same whether the FCU is off or on, doesn't it?

A fault with whatever is connected to the LOAD SIDE of the FCU

 

[johntheo]

Yes, it was consistent for a long period, but once it went it was gone.
I was also suspecting water ingress as showers had been in use earlier & there was a previous occurrence some months ago where I believe the same circuit tripped as a result of a leak from the shower waste - on that occasion there was water through the ceiling & I'm told it was repaired.

Just very confusing in that scenario as to why the fault would go away with the FCU switched off (no neon BTW). I appreciate RCD faults can be a bit weird, but I always like to understand a fault rather than just fix it.
It's a split load consumer unit. No RCBOs. One of those MK ones, that has a mid position on the RCDs

Yes, "RCD faults can certainly be a bit weird" like mine, maybe some of the very very experienced people on here might like to (further) comment or whatever on/in the thread, my post,14,
Earth to neutral fault???? - https://www.electriciansforums.net/threads/earth-to-neutral-fault.195016/ with concentration on any conceivable outside cause.