Electrics virgin!

[noseall]

I don't think i have ever visited this section but have been troubled by a few questions regarding electrics over on the building section.

I have looked at a few posts regarding insulation (insulation as in fibreglass, rockwool, celotex kingspan etc) and electric cables.
Some posters are concerned that the two should not mix.

I personally find this difficult to believe as a heck of a lot of homes up and down the country would be spontaneously bursting into flames on a daily basis.

My own experience as a builder has seen insulation and cables in close proximity trillions of times without ill effect.

What is the TROUTH?
//www.diynot.com/forums/viewtopic.php?t=222257

 

[ricicle]

Pvc cables in contact with polystyrene can lead to the plasticiser in the cables leeching out into it. This can make the cables brittle.
Cables in thermal insulation need to be derated accordingly due to heat dissapation. It is not normally a problem on cables whose circuits are not used at full rated current eg lighting circuits but will be a problem with cables supplying such things as showers, immersion heaters and the like.
To prevent any such problems the de-rating is applied to all circuits, regardless of likely load.

 

[noseall]

Pvc cables in contact with polystyrene can lead to the plasticiser in the cables leeching out into it. This can make the cables brittle.
Cables in thermal insulation need to be derated accordingly due to heat dissapation. It is not normally a problem on cables whose circuits are not used at full rated current eg lighting circuits but will be a problem with cables supplying such things as showers, immersion heaters and the like.
To prevent any such problems the de-rating is applied to all circuits, regardless of likely load.

So, do you have a definitive answer then?

Or do you advise that customers insulate only around cables with low loads and not around those with high loads?

It seems a bit of a tentative answer to me.

What does de-rating mean?

 

[ricicle]

To prevent any such problems the de-rating is applied to all circuits, regardless of likely load.

For cables wholly surrounded by insulation for more than so many mm (can't remember exactly off the top of my head how long) the de-rating is up to 50%. Which means it can only carry half the current it is designed to clipped direct.

Also there is a regulation reagrding mutual detremental influence ie cables cannot be placed near or in contact with anything that can affect it's ability to function safely either now or in the future. The leeching effect can take many years.

 

[ColJack]

how far into "idiot guide" territory do you want us to go?
we don't want to be condescending..

ok.. cables get warm..
if they're wrapped up in a nice bit of thermal insulation then they get even hotter since they can't loose the heat as fast..
at some point they melt, but before that they melt the cable insulation and then short out and all sorts of bad things happen then.

a cable has a rating of so many amps in free air before it starts to get too hot too fast and melts the insulation.. lets say 20A for arguments sake.
if you wrap that cable with rockwool ( as you might if running in a stud wall etc ) then that cable rating drops to as little as half.. so down to 10A..
if you were using that cable to supply something that needs 17A say, then it's now going to melt the cable..

if you lay the cable on top of the insulation ( such as in a loft ), then it can still loose heat from 3 sides..

now.. pvc ( cable insulation ) and polystyrene don't like each other..
the cable insulation tends to melt into the polystyrene.

 

[ban-all-sheds]

These are the truths:

1) PVC cables and polystyrene insulation do not mix because the polystyrene leaches the plasticiser out of PVC, resulting in a sticky mess of missing thermal insulation and damaged electrical insulation.

2) Cables wholly or partially surrounded by insulation are reduced in capacity. It's simple physics, not just opinions or best practices, and there's nothing to be done about it - current flowing in cables heats them up, and they need to be able to get rid of the heat. If thermal insulation limits how much heat can be transferred away then the cables can't be allowed to generate as much heat, i.e. they can't be allowed to carry as much current.

3) Fires do happen. Maybe not on a daily basis - there's not a lot of retrofitting of thermal elements in walls, apart from CWI and cavities are not widely used as cable routes, and in the place where insulation is frequently added (lofts), it's nearly all lighting cables which are not run at capacity. But not entirely all, and the neighbour of a friend had his roof catch fire after insulation was piled on top of a shower cable.

4) If I'd hit submit when I first wrote the above it would have been the first reply.

 

[holmslaw]

Bald tyres are perfectly safe as long as you don't have to pull up a bit quick and as long as its not raining. But if you want to pull up quickly in the rain you will possibly be killed or probably horrendously injured.

Safety advise is for the obeyance of idiots and the advice of wise men.

Noseall which one are you?

 

[mointainwalker]

In the above, the talk is of cables. Does that mean three cores over-moulded to make a single "unit" ?

If it does , what are the rules /guidance for individual wires within conduit when near/in insulation ?

 

[ban-all-sheds]

It means any cables - they are all subject to the same laws of physics.

Singles will have different capacities but will be reduced in capacity in very similar ways.

 

[PrinceofDarkness]

You're quite right to draw attention to this silly situation. 'nosey'. This particular 'Reg' often goes largely ignored where it comes to cables in domestic, insulated stud walls etc. Such cables rarely carry their design load for significant periods - enough to cause permanent heat damage.

You're relying on your personal observations based on practical experience, whereas others just trot out their misunderstanding of the Regulations or, worse, the hearsay repeated by others.

There's no worse old-wive's tale than that of the 'terrible damage' to PVC cables from contact with granular polystrene loft insulation. This so-called hazard is often spouted and respouted by those who have never actually witnessed such 'damage'. If they had done so, they might have realised over the years that the 'damage' amounts to slight staining and a certain stickiness to the sheath.

If a cable is installed in insulation with the knowledge that it'll be working flat-out, full-load, all the time, then certain measures would have to be taken. But, I'd also like to hear from anyone who's ever seen a stud wall or ceiling burst into flames due to this so-called problem...........


Lucia

 

[ban-all-sheds]

You're quite right to draw attention to this silly situation. 'nosey'. This particular 'Reg' often goes largely ignored where it comes to cables in domestic, insulated stud walls etc. Such cables rarely carry their design load for significant periods - enough to cause permanent heat damage.

And you think that is a good reason for ignoring the de-rating factors?

I rarely crash my car - should I ignore the rules on wearing a seatbelt?

In what way should a designer ignore 433.1?

You're relying on your personal observations based on practical experience, whereas others just trot out their misunderstanding of the Regulations or, worse, the hearsay repeated by others.

Please show where any of the posts above contain misunderstandings of the regulations.

There's no worse old-wive's tale than that of the 'terrible damage' to PVC cables from contact with granular polystrene loft insulation. This so-called hazard is often spouted and respouted by those who have never actually witnessed such 'damage'. If they had done so, they might have realised over the years that the 'damage' amounts to slight staining and a certain stickiness to the sheath.

Above the CU

• dalibu
• 12 May 2008
• 4

Polystyrene bead attack

Some cables are OK

• dalibu
• 12 May 2008
• 4

Polystyrene bead attack

This one needs replacing

• dalibu
• 12 May 2008
• 4

Polystyrene bead attack

Do they look as if 522.5 was complied with?

If a cable is installed in insulation with the knowledge that it'll be working flat-out, full-load, all the time, then certain measures would have to be taken.

Simple question, Lucia - do you think people should comply with BS 7671 or not?

But, I'd also like to hear from anyone who's ever seen a stud wall or ceiling burst into flames due to this so-called problem...........

If you never hear from anyone who has ever seen a stud wall or ceiling burst into flames due to any undersized cable situation would you disregard all the established sizing rules?

 

[PrinceofDarkness]

That's a lengthy post BAS dear, but since you've only asked the one 'simple question': No, I don't believe that people should comply slavishly with BS7671 unless they have so little confidence in their own ability to use commonsense in the interpretatation of it.

BS7671 has its uses, but it's hardly an Act of Parliament - or one of those tablets of stone, handed to Moses, is it?

I've lost count over the years of the times that you've placed your own interpretation on the 'Regs......

I'm not moved by your photos because they dont relate to my comment on granular insulation in attics - where the majority of cables are lighting cables rated at twice that of the protective device and carrying a fraction of their designed load for insignificant periods.

The O/P has a point, and I largely agree with him.

It's time for my bath now, so I'll read and respond, if necessary, later....

 

[ban-all-sheds]

No, I don't believe that people should comply with BS7671

With what attitude they comply with it is irrelevant, as is any reason they might have for choosing to or not, so I've removed those obfuscating qualifiers from what you wrote in order to cut through to the simple answer to my simple question.

So at least we know where you stand.


And I'm sure that you would want to protest, quite vehemently, that you didn't say that, and you didn't mean that, and that I shouldn't have edited your post to make it look as if you did.

Which is fair enough - I shouldn't, really, but I did it to make a very important point about qualified advice to follow BS 7671 but not slavishly and not if you know better.

When you design and install a circuit you have absolutely no way of knowing how it will be used over its expected lifetime of several decades.

If you put a circuit in with a cable of Xmm² and a breaker of Y amps then on what basis can you sign an EIC to say that it complies with BS 7671 if it cannot, as installed, be used to carry Y amps all day long, every day?

Where is the qualifier in the declaration on an EIC to say that you have exercised reasonable skill and care and that to the best of your knowledge and belief your work is sort of in compliance with BS 7671 but not slavishly so because you have used your common sense and decided that the de-rating factors for thermal insulation are an old wives tale?

I'm not moved by your photos because they dont relate to my comment on granular insulation in attics

It's polystyrene insulation - what does it matter where it is? It damages PVC cables whether it is in the loft or not.

where the majority of cables are lighting cables rated at twice that of the protective device and carrying a fraction of their designed load for insignificant periods.

How is their Ib:In:Iz relationship, or their cyclic load characteristics, relevant to whether polystyrene leaches the plasticiser out of PVC?

 

[PrinceofDarkness]

BAS, I'm most disappointed with you for cropping and quoting a sentence from my earlier post to suit your own ends. I've always thought of you as a fair and decent person.

A "quote" is of no use, unless it's an accurate and complete quote.

I still say that the O/P has a point about cables in insulation, because this matter is often overstated by spouting the Regs on derating etc., rather than considering the likely load on such cables. I'm sure that there are many electricians here who have first-fixed cables in a stud wall without too much regard for the builder following on with rockwool insulation.

The O/P has drawn on his great experience and observations of 'trillions' of installations - and I'm happy to support him with my own observations of a few thousand similar installations. I've already said that cables in insulation that are known to carry their full load for significant periods should have special consideration

The issue of cables surrounded by granular polystrene is largely historical now. But the old myths still abound. I'm not moved by your borrowed photo images because none of those cables appear to have caught fire - or show the slightest amount of heat damage - and they were never likely to simply due the attraction of a few granules of styrene.

There's not one instance on record of an actual 'fire' due to these particular circumstances, and there's still hundreds of thousands of cables nestling in those granules as they have done for some thirty to forty years.

Do me a favour, dear: quote accurately or don't quote at all. Thank you.


Lucia.

 

[ban-all-sheds]

BAS, I'm most disappointed with you for cropping and quoting a sentence from my earlier post to suit your own ends. I've always thought of you as a fair and decent person.

A "quote" is of no use, unless it's an accurate and complete quote.

I still say that the O/P has a point about cables in insulation, because this matter is often overstated by spouting the Regs on derating etc., rather than considering the likely load on such cables.

How can you do that when you have no control over how the circuit will be used in the decades following your installation of it?

Ib ≤ In ≤ Iz, and that's all there is to it.

If you think that in an installation not under the supervision of a skilled or instructed person you can install a cable where Iz < In but that's OK because you don't think that Ib will often be too high then you are a stupid and dangerous fool.

I'm sure that there are many electricians here who have first-fixed cables in a stud wall without too much regard for the builder following on with rockwool insulation.

Yes - I'm sure that there are many stupid and and dangerous fools out there who are falsely claiming to be competent electricians.

The O/P has drawn on his great experience and observations of 'trillions' of installations - and I'm happy to support him with my own observations of a few thousand similar installations. I've already said that cables in insulation that are known to carry their full load for significant periods should have special consideration

NO THEY SHOULD NOT

Is that clear enough for you?

Any cable, installed in anything, anywhere in an installation not under the supervision of a skilled or instructed person, and where you cannot rely on any calculations you do in the context of 533.1 being valid in the future, should be able to carry its full load, or its diversified load limited by the OPD, 24x7.

The issue of cables surrounded by granular polystrene is largely historical now. But the old myths still abound. I'm not moved by your borrowed photo images because none of those cables appear to have caught fire - or show the slightest amount of heat damage - and they were never likely to simply due the attraction of a few granules of styrene.

I never said they would catch fire because of damage from polystyrene.

Nobody else said they would catch fire because of damage from polystyrene.

I never said they would suffer heat damage because of the effects of polystyrene.

Nobody else said they suffer heat damage because of the effects of polystyrene.

Why do you keep trying to divert the issue of PVC damage to one of heat damage?