Loft Insulation

[jenions]

went to a job today and they customer had recently had her loft insulated, after a brief look i noticed that the lighting was wired in 1mm t&e, now i ent done no calcs or anything but just off my head i was i taught 1.5mm if loft insulation is to be installed. now just say for instance if anything wa to happen to the cables who would be responsible? would it be the insulation company for not checking the cable was suitable? just got me thinking

 

[rocker]

Theoretically, yes. Just like if Part F isn't adhered to by us by not installing the right extractor fans, or part B by not using the right fire rated downlights, other trades should be adhering to part P.

In reality, though, that's never going to happen. I can't imagine any massively disastrous things happening from using 1 mil in insulation, you may get a bit of melting and if so, the customer is going to be the one who has to foot the bill.

Now, because WE all have to have qualifications and it's all official (rather than going up a loft with a ladder and telling the old dear 'you've only got 20 mil, anything less than 300 is illegal') if we were to install 1 mil after insulation has been put down, we'd be shagged and burned.

for the record I've not ever heard a hard and fast rule that says no 1 mil if you think there may be insulation.

 

[morph]

went to a job today and they customer had recently had her loft insulated, after a brief look i noticed that the lighting was wired in 1mm t&e, now i ent done no calcs or anything but just off my head i was i taught 1.5mm if loft insulation is to be installed. now just say for instance if anything wa to happen to the cables who would be responsible? would it be the insulation company for not checking the cable was suitable? just got me thinking

loft insulation people probably struggle to find their way back out of the loft , so asking them to check and calculate cable sizes is probably a touch un realistic

 

[JD6400]

A little of track i know but does any one Else think their job could possibly be one of the worst ones in the world !?

 

[jenions]

very good point rocker, i know nuffin says it cant be used i was just saying theoretically. i just think some these companies take the mick!! thing is morph there supposed to survey the area 1st and if no insulation is already down then its easy to see.

 

[geoffa]

If you read section 523.7 of BS7671 'Cables in thermal insulation' it says:
A cable should not be installed in a location where it is liable to be covered by thermal insulation. Where a cable is to be run in a space to which thermal insulation is likely to be applied it shall, wherever practicable, be fixed in a position such that it will not be covered by the thermal insulation. Where fixing in such a position is impracticable the cross-sectional area of the cable shall be selected to meet the requirements of Chapter 43. Where necessary, the nature of the load and diversity shall be taken into account.

From that i'd say that any accessible loft space is likely to have insulation added or disturbed thereby covering circuit cables so as a matter of routine all cables should be sized to allow for them being covered by more than 100mm of insulation Reference method 101# in the cable selection table for flat cable

 

[NoSparks]

I'm sorry, am I missing the point here? or just looking at the rong table? but In table 4D5 the rating of 1mm T&E under > 100 mil of insulation (Reference method 101) the capacity is 10.5 amps, so if protected by a 6Amp breaker (or even a 10Amp come to that) what is the problem? I need to learn and understand!

 

[davetherave1979]

what about if you change a fuse board and the cables have insulation on them and say a ring has a 32rewirable on it you could change it to a 20amp mcb would that5 be ok or what you going to do on the situation

 

[Jimmyb]

nopsparls, i was just thinking the same thing

 

[geoffa]

note though that reference method 101# is for the cable laying on the plasterboard and covered with insulation.
If there's already insulation in the loft of say 100mm and new 1mm cables are installed running over the top of the insulation then you should expect that someone at sometime is going to add more insulation on top so Reference method 103# would be more appropriate to use, which rates 1.0mm flat cable to 8A.
As most CU's are protected by MCB's I refer to the tripping curves on page 249 of the BS7671. A 6A type B circuit breaker will not trip out until 8.6A are passing through it and then it will take over 2.7hrs to trip.
10A passing throught the MCB will take over 9 min's for it to trip.

I use 1.5mm where cables pass through lofts.
Why take the risk when the client pays for the materials anyway.

 

[jenions]

as i said i havent looked at regs and dne calcs etc it was just sumet floating round in my head and thought it b a good thread, and if u were to see this job theres a tad more than 100mm, must b more like 4-500mm

 

[geoffa]

If the cables are installed correctly as shown on page 265 of BS7671 Table 4A2, then in lofts the cables should only ever be clipped to the ceiling joists or laying flat on the upper surface of the plasterboard so there shouldn't be an instance where the cables are completly surrounded by 100mm of insulation.
But as i can't be sure who is going to be working in the loft after me i prefer to allow for cable being pulled about

 

[JD6400]

note though that reference method 101# is for the cable laying on the plasterboard and covered with insulation.
If there's already insulation in the loft of say 100mm and new 1mm cables are installed running over the top of the insulation then you should expect that someone at sometime is going to add more insulation on top so Reference method 103# would be more appropriate to use, which rates 1.0mm flat cable to 8A.
As most CU's are protected by MCB's I refer to the tripping curves on page 249 of the BS7671. A 6A type B circuit breaker will not trip out until 8.6A are passing through it and then it will take over 2.7hrs to trip.
10A passing throught the MCB will take over 9 min's for it to trip.

I use 1.5mm where cables pass through lofts.
Why take the risk when the client pays for the materials anyway.

I am with you on this one , to be honest i did not think that 1 mm was still available untill i went to check a refurbishment this week that one of our new lads is doing and found he had used 1 mm .
Have told him never to use again , i just can not see the benefit of using it.
Granted i have not compared the price of 1 mm opposed to 1.5 mm but it surly must be pence why scrimp ?

 

[NoSparks]

geoffa has a point but you should not be comparing design charicteristics with nominal values. The time delayed characteristics of MCBs should be compaired with the time time delay of the heatng effect of particular currents in a cable (ref BS6004, but unfortunately I don't have that document). I believe that there is a reg somewhere that states that the nominal current carring capacity of a cable should not be less than the nominal current rating of the associated cut out device, but I could be wrong.

 

[geoffa]

"The time delayed characteristics of MCBs should be compared with the time time delay of the heatng effect of particular currents in a cable"
That sounds true to me.
Surely the IEE would've accomodated that factor into the safe current carrying capacity of the cable in the table tho'
Why state the capacities if the cables are prone to further effects due to over-heating?
I'm not saying you're wrong nosparks.
it was just something i hadn't considered