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Domestic Back Box and CPC Corrosion

Former Community Member over 5 years ago

Hi all,

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

0 mapj1 over 5 years ago

damp, but why..

could be many things - do any of these look possible ?

Condensation ??

how is the dpc arranged for the house, is the wiring in conduit that perhaps communicates air to and from a colder place like a roof or floor void ?

- how is the building heated, or ventilated for that matter.

Historic flooding where plaster re-skimmed and painted over but original boxes kept - plaster does look unusually thick, may be an illusion of the pictures?

is all wiring at or below that level looking similar ?

Is it beneath a bathroom?

or above a heater or clothes horse used to air the washing ?

Dog's (or cat's) leg height can demark some odd chemical effects due to "organic deposits"

May not be too bad if you can clean up well enough to get a connection.
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0 Legh Richardson over 5 years ago

Looking at your detailed resume of the condition I might suggest that it could be some residue dampness caused by a poor DPC or/and Electroylsis caused by ground currents passing through the building fabric?

Lighting switches higher than about 1.2m will probably escape if it is a DPC problem.

Cure might be to line the hole with a bitumastic/plastic layer and 'glue' the new backing box into place using something like a mix of bonding and cement.

Legh.
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0 Former Community Member over 5 years ago

I hadn't considered "organic deposits"!

In this section of the house at least, the dpc is good. The house is H shaped and this side is all 90s new build with the other being pre-1930s and full of gremlins. The electrical installation in the 90s section is of a good standard for its time, and still. Oil-fired central heating. Poorly ventilated, but poorly insulated to compensate :o) The house could be considered watertight, dry and generally comfortable. The conduit terminates in the suspended floor void, above is bedrooms and a bathroom. Water would have to cross a joist or two to reach the conduit which would seem to lead to more extensive damage.

The plaster is incredibly thick and I don't know why. I assume the previous owners were aiming for a rustic, textured look but the end result is that there are no straight edges anywhere, at all. It's also never been re-plastered or re-skimmed. It was done once by a dedicated plasterer and nobody has wanted to go near it since. I imagine it took weeks to dry.

It seems that a historic leak into the conduit via the bathroom or related plumbing may be the most likely candidate, maybe even during the build. I will soon have an opportunity to lift the floor above which might reveal the culprit.

Thanks for your help!
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0 Former Community Member over 5 years ago

Legh Richardson:

Electroylsis caused by ground currents passing through the building fabric?

Interesting.

This particular wall is contiguous with the foundation slab. The supply is TN-C-S/PME with the main DB and earth rod about 6m from the rusty box, meter box about 18m away, overhead LV pole 26m away and HV/LV transformer maybe 70m away. Nothing else around but cows and folk found outstanding in their field.
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0 Chris Pearson over 5 years ago

I pulled out very similar, but worse this week. Clearly, there had been water ingress, be it damp, condensation or a trickle. In my case, the presence of a rainwater down pipe on the other side of the wall was a clue. The guttering, etc. was refurbished when the householders moved in and the wall was dry, so I am optimistic that there is no longer a risk.

The strange thing was the presence of leaves under the floorboards and above the mini-conduit. There was evidence of rodent damage nearby, but it seems unlikely that a squirrel (too big for a mouse) had piddled over the conduit because the cable was reasonably clean.

Last week's back box serving the same circuit and in the same wall was in good condition and even had (drum roll please) a grommet. The difference, I am sure, is explained by poor rainwater management. On the whole, in this particular property, boxes in external walls have been in poor condition, but those in internal walls have been satisfactory.
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0 Zoomup over 5 years ago

I'd not replace the metal boxes, just clean them and remove any dust or loose materials and splatter the rusty bits with car wheel bearing grease to prevent further corrosion. The black covering the copper wires can be a devil to remove mechanically, that is if you want to leave any copper behind. I suspect that the corrosion is historical where wet plaster had not dried out fully and the moisture has been trapped behind the wiring accessories for a while.

Z.
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0 AJJewsbury over 5 years ago

also a white salty deposit on the sections not rusted

My guess would be that the white stuff is from the zinc galvanizing (zinc oxide perhaps) - I'm no chemist but I suspect zinc won't "rust" like that just under the influence of just clean water (there's be no point galvanising things if it did) - so I'd expect some extra chemical at play there - perhaps salts from the plaster?

- Andy.
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0 Former Community Member over 5 years ago

Thanks for all the input folks!

Having ruminated a little more I think the most likely candidate is damp as a catalyst between the plaster and back box. There is a high chance that this plaster is a hydraulic lime mix as is used elsewhere in the house which in contact with zinc and a suitable catalyst makes zinc hydroxide and calcium.

The damp could have been an overly wet mix of plaster and/or a higher lime content which started a reaction to create the white deposits. Where enough zinc was removed it rusted creating rust underneath the white deposits. With that in play inside the back box it's not much of a reach to see how the cpc in contact with the back box via the earth nut has also corroded, although I can't explain that one fully just yet.

Might the catalyst also have been earth leakage or some other current passing through the cpc as Legh suggested? This would explain why only the cpc shows corrosion.

Edit: I forgot to add, could this also be the recently mentioned capillary action pulling the damp up inside the cpc?
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0 Zoomup over 5 years ago

Copper and Brass

If an installation requires contact between galvanized materials and copper or brass in a moist or humid environment, rapid corrosion of the zinc may occur. Even runoff water from copper or brass surfaces can contain enough dissolved copper to cause rapid corrosion.

A copper C.P.C., a brass earth terminal and a zinc coated steel box + moisture. An ideal combination for an electrical cell.

http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/electrochem.html

Z.
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0 whjohnson over 5 years ago

If it is lime plaster then plastic backboxes should have been used - I've come across a few of these where the metal backboxes have literally disintegrated over a 12 month period from new.
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Domestic Back Box and CPC Corrosion

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