Problem with New RCBO

[kevnurse]

Hi Guys,
I have a workshop attached to my house. There is a 2.5mm2 spur from a double socket on the downstairs final ring passing through the wall into the workshop. The ring is a standard 2.5mm2 cable, but without RCD protection. In the workshop the cable was, until recently, connected to a FCU from which there was a 2.5mm cable to a double socket in parallel with a 1mm2 cable to a single light circuit.

I recently re-configured the wiring, such that the spur now feeds a wall mounted RCBO (with 13A fuse) from which there is a 2.5mm cable to the double socket and a 3A FCU connected to to the lighting circuit. I want to cater for electric tools (lawn mower, hedge trimmer, etc) in the garden.

Yesterday, in the workshop, when I pressed the OFF button to stop my table saw, the RCBO cut out. I removed the plugs from the sockets and removed the fuse from the lighting circuit FCU. I checked the 13A fuse in the RCBO, which was good. The RCBO would not reset. Approx 10 mins later, with no actions from me, it reset (it has a red power on light).

I reconnected everything one at time to re-create the situation to help in finding the fault, but it did not cut out until I once again hit the OFF button on the table saw. I've decided that there is a problem in the table saw, which I will investigate at another time. Thats not my immediate problem....

What bothers me, right now, is that, with nothing connected to it, the RCBO will not reset. The fuse is good and there are no loose cables (that could have been trodden on) connected to it. The T&E cables to the socket and the light circuit are properly pinned to the wall (although not mechanically protected) and they have not been touched. The red light in the RCBO glows very dimly.

Is it possible that the pinned cables on the wall have been penetrated by damp? Or do I have a dodgy RCBO? I bought it, on the spur of the moment (pardon the pun) in B&Q

 

[333rocky333]

Is it one of them rcd spur units.

I found with the socket versions if the supply earth to them is poor continuity , they sometimes dont reset.

Before you bin it you could try wiring it to a plugtop and plug it in elsewhere , to see if it resets.

 

[kevnurse]

Is it one of them rcd spur units.

Yes. Its the same size as an FCU. It has the standard fuse carrier, a test button, a reset button and a red power on neon.

Before you bin it you could try wiring it to a plugtop and plug it in elsewhere , to see if it resets.

Thanks. I'll try that tomorrow evening.

 

[pdcelec]

It probably isnt the RCD that is faulty, it is more likely to be the existing wiring. What you need to do is complete the testing procedures i.e Insulation resistance, earth continuity, earth fault loop impedance, polarity and then conduct the RCD tripping times tests. This would then ensure that your wiring is ok before blaming the RCD unit. (The RCD is tripping for a reason and telling you that something aint quite right, that is what they are there for)

 

[333rocky333]

might be worth trying where it is first,disconnect ALL the live and neutral outputs from it first.

Dont forget to isolate each time!

see if it resets then.

If it does try connecting the lights on there own.

then repeat with just the socket.

It might be on your workshop wiring or light.

 

[kevnurse]

Thanks for your input, guys. I'll try everything that has been suggested and let you know the outcome.

 

[kevnurse]

I found the fault. The neutral wire termination on the supply side of the RCBO was improperly fitted (by me). I had pushed the wire too deeply into the teminal and I had compressed the insulation as well as a little bit of the core. I guess the increased impedance on the neutral caused a current imbalance of some sorts. I'm not an expert but I put it all back together (correctly this time) and everything is working, so the bad temination proved to be the cause.

Thanks for your input, guys.

Regards
Kev