Lime plaster and PVC sheathed cables

[Spike]

Does anyone know if lime plaster is damaging to PVC sheathing of electrical cables - i.e. can I lime plaster across cables?

 

[BMcD]

I don't know for certain but I wouldn't have thought so since it's stored in plastic buckets anyway, though I'm no scientist. If you're worried then why not cap the cables with galvanised capping.

 

[Penners]

In the absence of a definitive answer to this (although you probably will get one), and for the sake of good practice, why not tack some cable channel over them before you plaster?

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331823&ts=03832&id=31046

 

[skier-hughes]

A mixture of our electricians and the plasterers knowledge came forth with, plastic is cheapest, stainless steel is best and galvanised would rot>

 

[MdB]

There was another thread on this. I seem to remember the consensus was the lime plaster is fine on PVC cable but plastic conduit is a better idea as it gives an added layer of protection to the cable. Wont stop a drill or nail though. Metal conduit needs to be earthed and earthed properly.

 

[Gervase]

I use plastic conduit, roughened up with a coarse wife pan scourer before I screw it to the wall. Get it as snug and tight as possible, as flexing can cause hollows and weak spots in your plaster.
If you don't use anything at all you run the risk of damaging your plaster if ever you need to work on the cable, as the slightest tug could cause the plaster covering the cable to scab off.

 

[Moo]

a coarse wife pan scourer

Cue Penners.......

 

[Nemesis]

Coarse WIFE?

In this house young man my tongue may scour at times the inhabitants, four and two legged, but my pans are scoured with wire.

 

[Gervase]

Oops! In mitigation, I have a stinker of a cold and am pooped as a pooped thing; we're slap-bang in the middle of lambing (difficult triplets at three this morning - only one still alive, Grrr!) and I've got a rush job an hour's srive away which has to be finished by next Wednesday...
It should, of course, have read 'wise' pan scourer. Or was that 'wine'? Or 'wide'?

 

[NT]

I'm not sure there's any significant advantage in capping. hence its not normally used, as long as cable runs in the permitted zones.


NT

 

[Spike]

Thanks for the replies. The reason I was asking is that my house has just been rewired and the electrician has put the cables in plastic conduit in chases. The conduits are full width to the chases and so the only area for the new plaster to bond to would be the edges of the plaster each side (assuming it will not bond to the plastic). I had considered cutting the conduits out to expose the masonry - but thus also exposing the cables. Scouring the conduit sounds like an interesting approach and I'll trial this

 

[Spike]

And another thing....what is the point of plastic cable covers/conduits anyway? They wont stop a nail going through, you can't thread new cables down them when rewiring and they flex when plastering.

 

[MdB]

The conduits are full width to the chases and so the only area for the new plaster to bond to would be the edges of the plaster each side (assuming it will not bond to the plastic).

On advise from my plastering mate, I used the following method on mine though this was for a wall with modern plaster rather than lime:

1) course gritpaper to rough the surface of the plastic

2) 4:1 water PVA mixture - let dry

3) 1:4 water PVA mixture - let go tacky

4) Bonding plaster to fill gap

5) Scrim laid across new plaster and onto original wall

6) Joint filler over top of joints to level it all up.

5 & 6 make the joint proud of the old wall but only by a small amount and you can feather it in if you are little bit more skilled then me. The idea is that it stops the joint from cracking which I am told it is very likely to do without the scrim.

Now, I've no idea if PVA and lime work together but it may help it stick. Others I am sure will say if this is a really bad idea or not.

 

[BMcD]

Why not expose an inch of masonry either side of the conduit for suction and tack 2" EML over the conduit (nailing into mortar beds not conduit) for key onto plastic

 

[NT]

Why not ask yourself if a problem exists in the first place? A whole lotta steel and plastic has been plastered over in this country.


NT

 

[Gervase]

The capping gives your (or your sparks) slightly more leeway to work on the tails where they emerge from the plaster without risking pulling off a scab of plaster. Having done both capped and naked (and had plaster crack and scab with the latter), I always now use the plastic covering. It also stops your trowel snagging the head of a clip nail or a flopping bit of cable, which is extremely annoying.
For fixing I prefer to screw rather than nail as I know it will hold, and I think I'm allergic to masonry nails! Just use a small masonry drill, a yellow plug and a little three-quarter-inch screw on alternate sides of the capping every three feet or so, with a screw each side at top and bottom. That way you can also tension the cappings to stop them flexing so much.
It all makes for a neater, less stressful job.

 

[Penners]

a coarse wife pan scourer

Cue Penners.......

Penners knows not of coarse wives - they are beyond his ken.

(The management of the Penners establishment has approved the above remark as being suitable for publication.)

 

[Penners]

The capping gives your (or your sparks) slightly more leeway to work on the tails where they emerge from the plaster without risking pulling off a scab of plaster. Having done both capped and naked (and had plaster crack and scab with the latter), I always now use the plastic covering. It also stops your trowel snagging the head of a clip nail or a flopping bit of cable, which is extremely annoying.

Absolutely agree. The capping also stops any slightly springy lengths of naked cable bouncing back after trowelling, and cracking your plaster.

 

[Flyfisher]

The consensus seems to be in favour of covering the cable to make the plastering easier. Fair enough. But is there a need to de-rate the cable because it is no longer in free air?

My guess is probably not, because it seems to be a widely practiced, er, practice. But the thermal insulation of an enclosed air jacket must surely be higher than plaster in direct contact?

Perhaps the difference is negligible, but it would be interesting to know the industry-accepted best practice, as underwritten by the relevant wiring regs.

 

[Penners]

is there a need to de-rate the cable because it is no longer in free air?

I can't help feeling that this would have been considered when the wiring regs were written. There can be few houses that have a consistent environment for cabling. Cables run from free air into plaster, out again, into conduit and so on. It's the norm.