insulation over wires or not?

ricicle · 18 Dec 2006

joe-90 said:
You can do better than that. Get a pan of water. Bring it up to the boil. Cut off a length of cable and immerse it for an hour. Take a photo and post it here. If it doesn't look the same as before it went into the pan I'll admit that I am wrong. Deal? You'll also win the Nobel Prize for proving physics wrong.

Standard pvc cables are designed to run at a maximum of 70°c any higher and the insulation will start to break down over a period of time inversely proportional to the temperature is is exposed to.Putting it in a pan of boiling water for an hour would probably not show any immediate effects but cables lay in floor voids for a lot longer than an hour.Now crawl back under your stone and stay there until you have something worthwhile to say

securespark · 18 Dec 2006

joe-90 said:
You can do better than that. Get a pan of water. Bring it up to the boil. Cut off a length of cable and immerse it for an hour. Take a photo and post it here. If it doesn't look the same as before it went into the pan I'll admit that I am wrong. Deal? You'll also win the Nobel Prize for proving physics wrong.

Immerse a pvc cable in a pan of water at 100deg for 60mins?

You think that's the acid test?

+++ unhelpful part deleted +++ ....If nothing else, it proves what a devastating effect excessive temperature can have on pvc.

+++ unhelpful part deleted +++

Big_Spark · 18 Dec 2006

joe-90 said:
Wrong. You can put cables in notches if physical protection is provided (and yes you can still buy the covering plates).

Whenever I use cable I admit that I drill the joists, for the simple reason notching them weakens them whereas drilling a hole in the centre doesn't (at least nowhere near as much). But that doesn't mean that they CAN'T be notched. Fancy that! Joe-90 knows the regs and Big Spark doesn't.

Actually Joe I did not say it was a breach of the Regs or that such plates were not available, what I said was that it was not ACCEPTABLE.

There are instances where cables are run like this, such as SWA's and larger csa Pyro's, but it should not be done for T&E, further I have seen notched beams for Conduit (Steel and Plastic) runs and yes the plaates are used then too..

But we are talking about a domestic situation where I cannot think of any valid reason for notching the joists when your able to drill them.

Regarding the damage to PVC cables by heat, of course it happens, usually over extended periods of time as the cable breaks down, it is not an overnight event, it can take several years for the plasticisers to leach out of the PVC and cause the insulation to degrade. Whilst I would conceed that I personally have not seen many examples of this, I have seen one were a cable was actually tied along a central heating pipe (6mm²) that fed a 9.5kW shower. The damage to the cable was caused by the heat produced by the cable being overloaded and by the heat from the pipe as it was in tight contact.

Here is some information for you from ERA Technology.

ERA said:
The heat radiated by pipework comprising domestic heating systems is usually not a hazard, but research carried out, and experience from Engineers over an extended period of time has shown conclusively that PVC cables, and the newer LSF-PVA type insulated cables are damaged by close proximity to this pipework.

The damage is not obvious in a short period of time in most cases, but may manifest in reduced insulation resistance and current carrying capacity of the cables over a period of several years. Once the cable sheath is heated to temperatures in excess of 48°C for PVC cables, and 53°C for LSF-PVA cables a catalytic chemical reaction is induced at the molecular level whereby the bonds of the polymers begin to degrade, releasing the plasticiser agent and thus altering the chemical and electrical properties of the material. This process is similar to that which occurs when cables are severally overloaded, except it happens from the outside inward.

Steps to mitigate such detrimental heating are easy to take, and installers of such cables must take precautions to keep such cables away from sources of heat. Tests have shown that domestic heating pipes at a surface temperature of 52.5°C (The average for such systems) should be no closer than 73mm where no insulation exists, but 155mm where glass fibre or similar insulation is installed, unless a non heat radiating barrier is installed between the cables and the pipework.

You can find ERA HERE

joe-90 · 19 Dec 2006

So we are all in agreement that running cables near pipes is a definite hazard, far more so than running a cable a few feet in a drafty shed.

Oh and Big Spark. If the regs say it is OK to run cables is protected notched joists - then it IS acceptable. Anything else is simply your own personal opinion

+++ Unhelpful part deleted ++++

joe-90 · 19 Dec 2006

+++++++++
As warned earlier, I am deleting unhelpful posts

There is no need for anyone to say "It wasn't my fault" "He hit me first" "I was only reacting to the other guy". Obviously, all that goes without saying.

Mod Rupert
+++++++++

Pensdown · 19 Dec 2006

JohnD said:
Who let these people into our Electrics forum?

That's the most sensible post in this topic

sm1thson · 19 Dec 2006

joe-90 said:
..far more so than running a cable a few feet in a drafty shed.

the shed is drafty now, he wants to insulate it, the point is he needs to work out how much this will derate the cable byto ensure all is still fine (else upgrade the cable or plan it in such a way he doesnt insulate the cable). as i understand a few feet of insulation is enough to have to derate the whole length of cable.

i wont comment on notching as i dont think its an issue in his garage.

Big_Spark · 19 Dec 2006

Joe, I take your point on the notching...however I do not know a contractor that would do it except in exceptional circumstances.

Regarding the derating for cables in insulation, generally your right, the pipes will have more of an effect than insulation, the point about heat damage by pipes I am glad to see you have accepted can be as bad as we were saying..

, the problem is that if the cables are highly loaded toward their theoretical limits, then the cable will produce heat as a result, and the insulation will cause this heat to be trapped in the cavity made by the cable(s)..this will then have the same effect as the pipes would if placed adjacent to the cables...

joe-90 · 19 Dec 2006

I agree with all that Big Spark said above.

securespark · 19 Dec 2006

Bring back the old argumentative joe -

Big_Spark · 19 Dec 2006

joe-90 said:
I agree with all that Big Spark said above.

Blooming hell..you hit your head fella?

+++++++++++++++
Moderator's note:

Keep calm, this is the Electrics UK forum, and we're all civilised here.

M R
+++++++++++++++

ltheb · 19 Dec 2006

Blimey, I haven't seen so many handbags out since monty python did the battle of hastings!

Nice to see reference to the regs, of not much detail. I bet the poor bloke with the shed wished he'd never asked!

Cable surrounded by insulation for more than 0.5m must be derated by 50%, ie a 20amp cable is rated at 10amps. For the ring circuit rated at 32amps current capacity is 16amps - a good 3kw of load.

If the cable is only 'in contact' with the insulation or clipped to a thermally conductive surface or in conduit then no problem.

Please also note that a garden shed is a 'special location' under the regs. and power outlets MUST be protected by a 30mA RCD.

Hope this helps.

Big_Spark · 19 Dec 2006

Mod Rupert said:
+++++++++++++++
Moderator's note:

Keep calm, this is the Electrics UK forum, and we're all civilised here.

M R
+++++++++++++++

Rupert...Why don't you keep calm, Can you not see the smily faces...IT'S A BLOODY JOKE....Now stop being a bully, develope a sense of humour and take a chill pill...

Oh by the way...In future, do not post in my posts....Why do you not simply post like other members unless you actually edit a post... What you do is rather rude to be honest.

Spark123 · 19 Dec 2006

ltheb said:
If the cable is only 'in contact' with the insulation or clipped to a thermally conductive surface or in conduit then no problem.

You'd still need to consult the correct table for the installation method though.

ltheb · 19 Dec 2006

Yes. It sounds like exisitng in Reference method 1 (clipped direct). I cross referenced this with the building regs as the on-site guide just gives you the tables and guidance. Ref methods 6 & 15 respectively are cables or cables in trunking "in contact with a thermally conducting surface on one side" rather than physically surrounded. This appears to be acceptable, with no de-rating required.

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insulation over wires or not?

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