Damp damage to cables

[DefyG]

More likely over the years the water has wicked up into the cable by capillary action.

Agreed that can happen but unless the insulation becomes porous or is damaged I would still expect a good IR.

 

[DPG]

Agreed that can happen but unless the insulation becomes porous or is damaged I would still expect a good IR.

But the inner insulation, not the outer. Which would explain why T&E outside in water is OK.

 

[Dartlec]

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.

Sounds similar - the cable worst affected in this case had been run horizontally through the wall, though even then only one core to earth was giving me the failed readings so was able to reuse the other two for now.

I'm fairly sure I'll have to replace from ceiling down in conduit - It will be the landlords call whether I put it surface mounted or in the wall I guess - though based on drilling chasing won't be difficult, just messy.

 

[Dartlec]

Agreed that can happen but unless the insulation becomes porous or is damaged I would still expect a good IR.

I've certainly seen plenty of outside T&E to lights that's been exposed to weather for years and not damaged, but I guess you cable was probably terminated properly at both ends? I imagine this damage may have crept directly up the copper from the terminals, which were green and nasty...

 

[Dartlec]

Hi,are you sure it is e degradation of the insulation,and not a localised or single,localised bit of damage,providing a path for your IR readings. The difference in readings on 250/500v may possibly be due to that path being cleared by a higher voltage,at a small,localised part,whereas a general failure of insulation,over a run,,would give a more consistent reading,over consecutive tests.

If it is one core in particular,which is giving a low reading,it points to a more definitive reason,rather than a overall clue,of the complete cables failure,due to damp.

I know it's probably more poking around,than it deserves,but by separating and disconnecting the cores,checking continuity/resistance,to confirm equal standing,you can IR individual cores,with reference to both earth and each other,with various alternative combinations,and build up an idea of what may be occuring.

It's quite possible, though nothing obvious and by the time I'd got things to an acceptable level I'd had enough of poking around...

Unfortunately it looks like I can't get at both ends of the cable easily as they aren't junctioned at the pendant (might be above the ceiling there if I'm lucky) so not easy to separate legs.

I'm hopeful that if I track down the switch drop at ceiling height and test there things will be fine, then I can rejoint and run in conduit/trunking...

 

[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?