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...

 

[pc1966]

I would imagine that putting in plastic trunking / conduit for the cable and plastic back-boxes would provide good protection for anything buried in a damp wall's plaster or cement, so long as some means for air circulation / vent to the pipework?

RCD protection would be sensible, as clearly that circuit is a bit dodgy. No idea if it might fail in a fire-starting manner (probably not if the wall is not inflammable) but for T&E I suspect the RCD would trip on any serious insulation failure as it would be to earth first, not live-neutral.

 

[pc1966]

There was a thread recently where some NI sparks were saying that they always put in plastic conduit for wiring in domestic installs, even if T&E is used. This would be a perfect use-case!

 

[Dartlec]

I would imagine that putting in plastic trunking for the cable and plastic back-boxes would provide good protection for anything buried in a damp wall's plaster or cement, so long as some means for air circulation / vent to the pipework?

RCD protection would be sensible, as clearly that circuit is a bit dodgy. No idea if it might fail in a fire-starting manner (probably not if the wall is not inflammable) but for T&E I suspect the RCD would trip on any serious insulation failure as it would be to earth first, not live-neutral.

The damp might put out any fire at least!

I wondered about burying plastic back boxes in the wall in this case, though I generally consider it a bodge when I see it. Conduit boxes with plastic conduit could work I guess though.

It is at least only 2 main switch drops involved, and then a sideways switch extension that was done at some point due to door moving I think.

It may be possible to end the plastic conduit up the ceiling/floor void for ventilation - the damp doesn't seem to have reached that height. But I'm never a big fan of chasing walls, even less so when the room is already decorated. At the moment I think the wallpaper may be holding the plaster on the wall!

 

[buzzlightyear]

Once you get damp in a wall with out a cure the damp will always come through.
Like rising damp. Or a bridge from wall to wall. It does not matter you could put a surface back boxes in.

 

[PEG]

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.

 

[Marvo]

There was a thread recently where some NI sparks were saying that they always put in plastic conduit for wiring in domestic installs, even if T&E is used. This would be a perfect use-case!

Conduit is the norm here in South Africa as well, it's pretty rare to see cables buried direct and I've never seen capping used.

 

[Vortigern]

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.

 

[DefyG]

More or less implying T&E is or becomes porous over a period of time but Ive known T&E installed (rightly or wrongly) outside in all weathers where its insulation has been and remained sound.
One such example, did an EICR the other day where a T&E fed a pond and was run (wont class as installed) in an open gulley (so IN water at various points and times). The IR was good >100M. Red & Black so probably in excess of 15 years old!

 

[DPG]

More or less implying T&E is or becomes porous over a period of time but Ive known T&E installed (rightly or wrongly) outside in all weathers where its insulation has been and remained sound.
One such example, did an EICR the other day where a T&E fed a pond and was run (wont class as installed) in an open gulley (so IN water at various points and times). The IR was good >100M. Red & Black so probably in excess of 15 years old!

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

 

[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...