Opinion On Cable Ampacity in Thermal Insulation

[Cookie]

Ok, so a while back I came across a rather interesting article referencing an incident where MC cable had been buried in thermal insulation resulting in a small fire. The magazine claims that if the cable was de-rated to BS7671 standards over heating would not have occurred.

However, I am skeptical if that truly is the root cause. Rather, is it possible that one of the phase conductors shorted to the interlocked metal armor and due to its high loop impedance (the jacket) became a restive heater? From the photos it appears like the CPC was electrically isolated from the armor via paper.

Do such cables exist in the UK? What do others think?

 

[telectrix]

over here we derate the cable by 50% if buried for more than 18" under thick insulation, so that would derate your cable to around 110A. hence it's way over fused. the article doesn't seem to mention the actual load of the smoker..

 

[Cookie]

over here we derate the cable by 50% if buried for more than 18" under thick insulation, so that would derate your cable to around 110A. hence at 225A it's way overloaded.

But in reality will it really catch fire? Remember this is 90*C insulation- at least. I ask because its semi common for such cable in North America to be buried in insulation without burning up. Though I will acknowledge lack of overt signs do not indicate safe wiring.

 

[DPG]

But in reality will it really catch fire? Remember this is 90*C insulation- at least. I ask because its semi common for such cable in North America to be buried in insulation without burning up. Though I will acknowledge lack of overt signs do not indicate safe wiring.

Presumably, like us, you would derate the carrying capacity if it was buried in insulation though.

 

[radiohead]

Many years back I attended a call to a shower that had ceased working the day after a new owner had moved in. Investigation showed the 8.5kw shower had been wired to the upstairs lighting circuit and the 5a fuse in the board uprated to 30a. The 1.0mm wiring for the lighting circuit in the roof space was draped across the joists and 100mm thermal insulation had afterwards been installed between the joists. Where the wiring was exposed on top of the joists it appeared completely undamaged, but between each joist where the wiring was under the insulation the PVC had almost completely melted leaving just bare charred copper conductors. Also where the circuit was buried under plaster down to the DB it had also melted, damaging the other circuits alongside. But it was apparent that where uncovered, the PVC appeared unaffected.
Just to clarify, further investigation showed the shower had originally been wired in 6.0mm cable, but this had been damaged or drilled in the past, so presumably the previous owner had connected the shower to the lighting hoping the new owner wouldn't notice!
The episode brought home to me the dramatic effect sustained overcurrent will have on wiring enclosed in thermal insulation against wiring in free air.

 

[Cookie]

Presumably, like us, you would derate the carrying capacity if it was buried in insulation though.

Technically its not required. I mean in a few cases we are pushed back to the 60*C column if there is insulation in contact with the cable but nothing major.

Many years back I attended a call to a shower that had ceased working the day after a new owner had moved in. Investigation showed the 8.5kw shower had been wired to the upstairs lighting circuit and the 5a fuse in the board uprated to 30a. The 1.0mm wiring for the lighting circuit in the roof space was draped across the joists and 100mm thermal insulation had afterwards been installed between the joists. Where the wiring was exposed on top of the joists it appeared completely undamaged, but between each joist where the wiring was under the insulation the PVC had almost completely melted leaving just bare charred copper conductors. Also where the circuit was buried under plaster down to the DB it had also melted, damaging the other circuits alongside. But it was apparent that where uncovered, the PVC appeared unaffected.
Just to clarify, further investigation showed the shower had originally been wired in 6.0mm cable, but this had been damaged or drilled in the past, so presumably the previous owner had connected the shower to the lighting hoping the new owner wouldn't notice!
The episode brought home to me the dramatic effect sustained overcurrent will have on wiring enclosed in thermal insulation against wiring in free air.

Not laughing at what happened, just that someone actually thought it was a good idea.

 

[DPG]

Technically its not required. I mean in a few cases we are pushed back to the 60*C column if there is insulation in contact with the cable but nothing major.






Not laughing at what happened, just that someone actually thought it was a good idea.

I don't understand how you don't reduce the rating if the cable is run straight through thick insulation. Surely people realise the effect it has?

 

[pc1966]

But in reality will it really catch fire? Remember this is 90*C insulation- at least. I ask because its semi common for such cable in North America to be buried in insulation without burning up. Though I will acknowledge lack of overt signs do not indicate safe wiring.

I don't know for sure, but if the normal cable current capacity rating is for a 60C rise (30C ambient to 90C max insulation rating) then doubling the current is 4 times the heat dissipation so you would see 270C conductor temperature (on the above crude assumptions).

 

[telectrix]

moral of the story.... keep the insulation monkeys away from what is far beyond their combined IQ of 10 to comprehend.

 

[Lucien Nunes]

Multicore MC-style cables have never been widely used in the UK. Historically there were some semi-flexible cables for industrial locations with an interlocked, corrugated spiral metal outer, but only in small sizes. For the size of circuit mentioned in the article, we have always favoured steel wire or (previously) tape armour, or MICC.

Much more widespread is the use of interlocked spiral galvanised steel flexible conduit with separate single-core conduit cables threaded through, e.g.: Electrak tap-off with single core cables in flexible conduit
In these applications the CCC of the conduit spiral is normally adequate to clear a fault (although not permitted to be used as CPC).

I don't think we have enough information to make a useful assessment of whether the heating effect of current in the MC outer was a factor here. Where casual contact is made with a containment or shield of small CSA or thickness relative to the circuit, usually the point of contact blows itself clear, but I have no direct experience of what happens with MC if the insulation fails throughout due to overheating.

The article draws attention to the derating tables being somehow a new addition to BS7671. I am struggling to recall which edition first tabulated the factors - I think it was well before I was born.

 

[Cookie]

I don't understand how you don't reduce the rating if the cable is run straight through thick insulation. Surely people realise the effect it has?

We just don't... to my knowledge it doesn't appear to cause any problems. Maybe our insulation just isn't that great. No idea to be honest, but the fact BS7671 requires derating is just as much of a surprise to me.

 

[Cookie]

I don't know for sure, but if the normal cable current capacity rating is for a 60C rise (30C ambient to 90C max insulation rating) then doubling the current is 4 times the heat dissipation so you would see 270C conductor temperature (on the above crude assumptions).

For us 60*C, 75*C and 90*C is the temperature rating of the wire insulation itself. Nearly all modern wire is rated 90*C, but typically restricted to the 75 or 60*C column based on the temperature rating of the terminals and certain restrictions in rules going beyond 320. The 90*C column is basically used for de-rating purposes.

Here is Table 310.15 B 16:

https://imgur.com/Rd6tcPl

View: https://Upload the image directly to the thread.com/Rd6tcPl


In reality the cable doesn't actually reach 60*C, let alone 90*C. I'd imagine the cable doesn't go above these values in deep insulation.

 

[Cookie]

Don't get me wrong- we do have some rules that kick in around thermal insulation- just not as restrictive. Here is an example for romex:

https://imgur.com/Zyj1G6g

View: https://Upload the image directly to the thread.com/Zyj1G6g



And as stated in the rule- Table 310.15 (B) (3) (a):

https://imgur.com/N23PvwL

View: https://Upload the image directly to the thread.com/N23PvwL


Rule 310.15 (A) (2):

https://imgur.com/Qrgthhr

View: https://Upload the image directly to the thread.com/Qrgthhr