Rcd tripping constantly - damp?

[333rocky333]

I liked the almost bit, I thought that was a nice touch. But your very likely right Rocky, just trying to help him eliminate things before the leccy turns up.

This could be the one case

Always the possibility that when the Mcb blew in the past that one of the spurs contacts has welded closed circuit

 

[Doggit]

Sorry Guys, you're both pretty much guaranteed to be right; I was just taught never to assume anything in diagnostic methodology, as there would always be some idiot that did what you assumed should be okay.

 

[JohnW2]

Im assuming more switched spurs are used for the Double pole isolation function just as much as the fuse down function...

I'm pretty sure that you're right.

As for why, when (in the mind of the designer!) fusing down is not required, they use an FCU rather than a DP switch, I don't know - but, as suggestions ...
(a) they might be cheaper, or
(b) one might be talking about designers who believe that '20A' switches should not be connected to 32A circuits, or
(c) one might be talking about designers who believe that (regardless of 'rating'), things that it is not permissible to connect things which are not 'accessories to BS1363' to a ring final circuit

Kind Regards, John

 

[JohnW2]

Sorry Guys, you're both pretty much guaranteed to be right; I was just taught never to assume anything in diagnostic methodology, as there would always be some idiot that did what you assumed should be okay.

No-one can knock that upbringing.

Rocky and myself will, I think, 'virtually always' be right in relation to the matter under discussion (particularly in relation to anything approaching a 'modern' SFCU) - but that's not necessarily the same as 'always'!

Kind Regards, John

 

[London publisher]

Thanks all. In answer to earlier question, yes kitchen appliances are on another rcd which appears to be fine. The tripping rcd has two mcbs. One is the socket ring that is tripping the rcd. The other is a lighting ring that appears to be fine and doesn't trip the rcd.

 

[333rocky333]

It would not be the first time that the low load lighting circuit run fine until the non faulty socket circuit that is heavily loaded trips the Rcd, though that problem is usually more time delayed rather than instant, so as a fact it can happen, im sure its not relevant to your case.
As Doggit says its worth checking outside as obviously thats where its wetter, but may not be wise pulling things apart if an electricians coming, as it could hinder his faultfinding.

 

[London publisher]

Thanks. I'm going to leave outside as beyond my capabilities. I did find a spiderweb and a few scorch marks on the earth cable inside one of the boxes inside that I opened. The web was all over the place so I wondered if this could have created a short. I've cleaned the spider's web however and the circuit still trips the rcd. I'm now pretty much at the stage of giving up till the electrician comes!

Anyway thanks all and I'll let you know the issue if he detects it.

 

[John D v2.0]

I think the electrician will be able to disconnect and ir test the circuit. Then he will know if everything is disconnected and which conductors have the fault.
Then it's just a case of subdividing the circuit until the faulty section is found.
In my case I had a couple of junction boxes on the damp oversite that had low readings. I couldn't get to them at that stage, but they weren't enough to trip an RCD.
When I eventually found them they looked in reasonable nick but had clearly been quite wet in the past.
Wiring the circuit without them solved the issue, the cables themselves were still fine

 

[333rocky333]

It is hard without proper test gear, I have in the past found shorts within sockets that appear dry, but the original fault has dried out but left a sort of carbon track in the socket, meggaring the socket with it taken off showed it faulty and externally it looked perfect, till you broke it open.

 

[London publisher]

It did turn out to be damp. The electrician found faults in two bits of wiring. So he's suggested trunking for a short stretch to connect to another viable socket and bypass problem wiring. (This bit is three feet and can't be chased because of tanking to basement.

And then chasing new wiring into insulated plasterboard for the other stretch which is in front of tanking. Hasn't provided quote yet. Could be worse but recommend not owning a property with lived in basement! Nothing to do with outside spur.

 

[London publisher]

There are some scorch marks in that socket and historically water got in that wall, so perhaps a short caused a problem with wiring close to socket.

 

[JohnW2]

It did turn out to be damp. The electrician found faults in two bits of wiring. So he's suggested trunking for a short stretch to connect to another viable socket and bypass problem wiring.

If it is the belief that damp has affected some cable, then I would imagine that the cable must be seriously (mechanically) damaged somewhere. As I said before (although it's not 'recommended'! ), PVC cable is fine when submerged in water. For problems to occur in a PVC cable due to damp (or even frank water), there would presumably have to be penetrating mechanical damage to both the outer sheath of the cable and the insulation of at least one of its cores.

Kind Regards, John

 

[Keithmac]

Mechanical fixing for plasterboard nicked the wiring sheath and water ingress over time maybe?.

 

[JohnW2]

Mechanical fixing for plasterboard nicked the wiring sheath and water ingress over time maybe?.

It would have to be the inner insulation as well as the sheath but, as I said ....

If it is the belief that damp has affected some cable, then I would imagine that the cable must be seriously (mechanically) damaged somewhere. ...

... and I suppose that a plasterboard screw or nail is as good a way of producing "serious mechanical damage" as any other!

Kind Regards, John