Damp damage to cables

[Dartlec]

Had a nightmare of a day trying to get a small 2 bed 1900 or so terrace house through its EICR today for the landlord.

He's one I work for, so its a case of fix as I go to get a satisfactory rather than just report.

2-3 hours was sorting out the downstairs lighting circuit failing on IR L-E with 0.44MOhms (all loads removed)

Not much left of the in wall back boxes but even with everything pulled out, the main problem turned out to be a short switch extension in 3 core and earth from one room to another. (perm live to switch, 2 switched lives back)

Nice pictures of what remains of the back boxes....



Only one of the cores was failing (perm live), so with some rejigging of switch locations I managed to get things to pass for now at 2.4MOhm on 500V (though bizarrely was getting 0.7 on 250V). Had to put surface pattresses for now though to get the switches back functioning.

It's fairly clear that the switch drop cables have been affected by the damp - the ends of all cores were that dull black, and cpc was bright green in places.

Some remedial work has been done for the damp, so this may be historic damage, but given the age of the house it might also be impossible to completely solve.

The wiring at light level seems fine, it's only the switch drops that have been affected from what I can see - unfortunately I think they are in junction boxes somewhere in the ceiling, not at the pendants so rewiring will likely be fiddly and disruptive - though it will likely have to be done at some point.

Questions:

Once a cable like this has been affected, does it usually get worse over time, or will it stabilise if no more damp occurs? I'll probably be recommending an annual test on this circuit for a while anyway - not currently RCD protected though I'll be recommending a board change.

Is there a proven way to protect cables from damp in a wall?

One thought is using Quinetic or similar to avoid switch drops on the walls in question. Any other ideas in similar situations? The sockets were (badly) rewired some years back for I suspect the same reason in surface mounted 25mm conduit, which as you can imagine looks lovely... so I'd rather avoid that as an option...

 

[Mike Johnson]

I had exactly the same problem with a ring final circuit. The IR was 0.3 Mohm. The offending cable (after tracking it down) was buried in a wall which was absolutely sodden and the back box (galv) like yours was rusted away (so much for galvanised!) My answer was to chase out the cable completely and replace the cables via a hole in the ceiling I cut, and put them surface. IR was then 459 Mohm. We had to core drill through the same wall it took about four hours going through the wet wall, end terrace outside wall. It was soaked through and through. Interestingly, the more we removed of the offending cable the better the IR readings go. We chased a bit (meter) then tested, then another meter and so on when we got to the ceiling it was as above reading. So it is graded and gets worse the more cable that is in the wet wall.

Surely the first thing to do in that situation is report the sodden wall to however and rectify the fault that made the wall that wet, only then should we consider replacing any effected items?

 

[DPG]

Surely the first thing to do in that situation is report the sodden wall to however and rectify the fault that made the wall that wet, only then should we consider replacing any effected items?

At least he's put a solution in place (surface wiring) so that they have the electric back on until the damp issue is fixed.

 

[Vortigern]

Surely the first thing to do in that situation is report the sodden wall

No, the first thing to do is report to the client a failure of IR leading to an unsatisfactory report. Finally bring to the attention of the person ordering the work the reason found for the problem and possible solutions in order to rectify the situation with the clients knowledge and contractual agreement. I was subcontracting to a building firm who were apprised of the situation, who in turn took it up with the client who in turn agreed a course of action and costs associated with that action. As to getting involved with the client (LL) it would not have been within my remit to do so it is up to the builder to action the damp wall. Which as a matter of interest had been treated by rendering and coating with rainproof exterior coating. Obviously didn't work judging by the coring and water encountered while doing so. But no building work is not my bag just the installation. Anyway by the time the owner had remedied the wet wall hell would have frozen over and he would not be able to rent the property thus losing money and then having none to fix the wall so......to avoid mission creep, best get the electrics fixed first eh?