This might be a little too much detail but I'm stumped as to why I have rusty back boxes and cpc corrosion in part of my house and want to present the problem in full. I believe this may have been covered in the old forums but am having trouble locating the topic.
In two ground floor rooms of a two-storey extension I have some switch and socket back boxes with a fair bit of corrosion accompanied by oxidation of the cpc and I'm wondering what forces were at work to cause this. There is clearly rust on the back box but also a white salty deposit on the sections not rusted and what looks like drops of water sitting on the rust but are actually crystals of some sort.
The cpc is blackened to the same degree where it was exposed and when it is snipped and stripped, for how far I don't know yet. The cpc drops from the pendant above and it's fine up that end. The earth nut on the box shows some copper carbonate but this appears to have come after the copper oxide. The switch back box in the next room is rotated with the earth nut in the top-left but has the same rust, copper oxide and copper carbonate problems. All live conductors are in good condition showing only a little dullness having been installed in mid to late 90's.
The wall pictured in an internal wall of solid plaster with singles or t&e in oval conduit leading to a suspended floor above with a similarly plastered ceiling. The switch pictured is in what used to be a kitchen but is now an office. The switch and socket back boxes with similar corrosion are in the next room, a living room with a fireplace, but only the switches and sockets on one side of the room show corrosion.
In my mind I have ruled out:
- Water ingress as it would likely have pooled causing a different corrosion pattern and left some other evidence.
- Atmospheric moisture as it would have left some mark on the live conductors also.
- Chemical vapour (bleach) as it may have been at a kitchen switch but probably not at a living room socket and it would affect the live conductors again.
- Minerals leaching from the plaster as it would leave a mark on all back boxes in the room.
- High fault current as it would leave some other evidence but there is no heat damage to the insulation or switch.
Maybe it's a combined small amount of some of the above?
In any event, replacing the cpc and back boxes is doable but not something I would relish. I am hoping to clean it up a bit, test resistance and move on happy that at least I found a cpc behind the switch.
Any ideas?!
Neil
