electrical wiring cables and ratings

[kurnal]

Was looking at a shop today, the electrical wiring was a nightmare and needs total overhall.

Anyway there were cables hanging everywhere unsupported and as I moved one bunch of 2.5 T&E which was hanging loose obstructing the door I noticed that one of the cables was warm to touch. I took out the infra red thermometer and found that one cable in the bunch  was running at 40 deg C whereas the others were all showing 18.  

Through google it appears that this type of cable is specified with a maximum conductor temperature of 70 deg C. So is a running temperature of 40 degrees with the cable hanging in open air (not clipped or supported as per BS7671) in an ambient of 18 degrees indicative of an overload? Have recommended a periodic inspection in any case to identify and resolve the many issues but curious as to the current  needed to heat it to 22 deg c above ambient?

[Clevelandfire 3]

do you have any idea what that cable was supplying?

[kurnal]

No nobody did - there was air conditioning and much halogen lighting, there were no labels on the dis boards and no way of tracing it easily- which was not my job anyway. Have advised them to get in an electrician today.

The cable was taped to another and then in a loose bundle of about 8 secured only by being raped together.  
My gut feeling is that the two taped together may be a ring main and perhaps one leg of the ring has become disconnected somewhere.  Just interested to know how much current it would take to heat up the cable to 40 degrees -  22 degrees above ambient in this context.

[John Webb]

Table 4D5 of the current BS 7671 (2008 edition) gives the rating of 2.5 sq mm T&E clipped to a surface as 27 Amp. Not specifically for T&E but for multicore cables in table 4E2A, the rating for the same size cable of two cores in free air or clipped to a perforated cable tray is 36Amp.

Take the latter as more representative of your situation, if it takes 36 A to get to 70^O from 20^O then very roughly to get to 40^O (and bearing in mind heating effect is proportional to the square of the current) then there may have been a current of around 23 A in that cable?

By the way, was the cable grey or white? If the latter it's possible that the IR thermometer may have under-measured the temperature as the emmissivity may have been lower?

[kurnal]

Thanks John. Yes it was a white cable but the thermometer is a cheap one and uncalibrated so its only going to be ball park anyway. Your estimate is useful thank you very much. If I found a cable running at 70 deg C I would be seriously worried but it seems  it would be ok provided it was within its ratings and suitably mounted throughout its length.

[John Webb]

My pleasure. If it was warm to the touch than that suggests around 40 to 50deg. Above 50 it would feel 'hot' to the inaccurate human hand.

By the way, don't know how conversant you are with BS7671 (The Wiring Regs) but
(a) the current rating of cables is based on an ambient of 30deg;
(b) if you have a bunch of cables, any cables carrying 30% or less of their grouped rating current can be discounted for the purposes of potentially derating the adjacent cables according to Table 4C1.

[lyledunn]

I am not sure that I would be too concerned about the temperature of the cable providing overload or fault current protection is appropriate. My concern would be directed to unsupported cables especially in proximity to halogen downlights (the lamps of which can reach temperatures close to 200 degrees C).
I have been inspecting electrical installations for most of my working life and I have often witnessed heat damage to cables that have been carelessly installed above ceilings. Electrical contractors are sometimes the culprits but more often than not it is those who install the burglar alarms, computer systems, sound systems and the like who through sheer ignorance of the requirements of BS7671 are responsible for increased fire risk due to their slap-dash wiring methods. You are right to ask for a Periodic Inspection Report to be conducted. It is, however, often difficult to get something done about the problem!

Regards,
Lyle Dunn

[Graeme]

Lyle

Being from the fire alarm trade most of the "slap dash" work i see is from sparkies with their sheer ignorance of BS5839-1; 2002. So lets get the balance right.
They are as much guilty of throwing cables in as much as other trades with the exception of Data installars who i agree are the worst.

[lyledunn]

Graeme,
Ah balance! You will note that I did'nt even mention FA engineers in my post. They are required, are they not, to secure their cables to provide appropriate integrity with respect to fire attack (26.2 (e) 5839-1 2002). Thus I cannot conceive of any self-respecting fire alarm engineer installing, signing off commissioning or verification documentation where cables are not so installed! Any of those draping FP200 often found above suspended ceilings were installed by sparks and the systems must have been signed off by them as well! Damn tongue caught in my teeth again!!

Regards, Lyle Dunn

[Graeme]

Graeme,
Any of those draping FP200 often found above suspended ceilings were installed by sparks
usually

 and the systems must have been signed off by them as well! Damn tongue caught in my teeth again!!

picked up by the commissioning engineer but no one takes any notice. That's how it usually works

Regards, Lyle Dunn

[David Rooney]

Graeme,
Ah balance! You will note that I did'nt even mention FA engineers in my post. They are required, are they not, to secure their cables to provide appropriate integrity with respect to fire attack (26.2 (e) 5839-1 2002). Thus I cannot conceive of any self-respecting fire alarm engineer installing, signing off commissioning or verification documentation where cables are not so installed! Any of those draping FP200 often found above suspended ceilings were installed by sparks and the systems must have been signed off by them as well! Damn tongue caught in my teeth again!!

Regards, Lyle Dunn

On a supply and commission job the electrical contractor takes responsibility for the cabling and appropriate fixings by signing the installation certificate (tho can't say I've found an electrician that's ever heard of one), the commissioning engineer isn't there to carry out a "verification" of the system and wouldn't therefore inspect all the cabling and appropriate fixings.

He's there to test the system to the designers specification and report.

[lyledunn]

David,
Not quite right! In CTs wee book on the subject he does say that the commissioning engineer is also expected to ensure that installation workmanship is generally of an adequate standard...(see also 39.2 (22) BS5839-1 2002 where it requires this check to be made, so far as is reasonably practicable). Whilst assessing such things I dont think that it would be too much to ask the commissioning engineer to stick his head above the suspended ceiling.
Regardless of who might be responsible for the untidy (and perhaps potentially unsafe) workmanship, I think that, especially in the smaller commercial installations, there is a definite problem that is'nt being sorted as part of the certification process. Maybe we need more fire officers doing what the original poster did and having a good old butchers above that ceiling!
Regards,
Lyle Dunn

[Allen Higginson]

Well,at commissioning the commissioning engineer is expected to - " ....as far as it is reasonably practicable to ascertain, the specified cable type has been used in all parts of the system and the workmanship complies with 37.2" and that ".....It is not, in general, the responsibility of the commissioning engineer to verify compliance of the design, or of the installation work, with this part of BS 5839 (i.e. with Section 2 and Section 4 of this standard). In general, the responsibility of the commissioning engineer is to verify that the system operates correctly in the manner designed and that the installation workmanship is generally of an adequate standard."
Saying that,if we are working on a supply,test and commission basis then I make sure that the installer knows what he needs to be doing to meet the recommendations of section 4 or I will highlight it directly to the client at time of commissioning and at any time thereafter.

[David Rooney]

David,
Not quite right! In CTs wee book on the subject he does say that the commissioning engineer is also expected to ensure that installation workmanship is generally of an adequate standard...(see also 39.2 (22) BS5839-1 2002 where it requires this check to be made, so far as is reasonably practicable). Whilst assessing such things I dont think that it would be too much to ask the commissioning engineer to stick his head above the suspended ceiling.
Regardless of who might be responsible for the untidy (and perhaps potentially unsafe) workmanship, I think that, especially in the smaller commercial installations, there is a definite problem that is'nt being sorted as part of the certification process. Maybe we need more fire officers doing what the original poster did and having a good old butchers above that ceiling!
Regards,
Lyle Dunn

As it says Lyle - "as far as is reasonably practicable".

Looking above a couple of ceiling tiles - presuming they're accessible - at a couple of metres of cable in a 10 storey block may give you an insight as to the general installation standard but as said, the installer is to supply the certificate certifying compliance of the installation with current standards.

As the commentary to 39.1 explains, the commissioning engineer should check "the installation workmanship is generally of an adequate standard" but accepts this is impractical in practise. 

A commisisoning engineer is not going to inspect a complete cable installation, he'll just report on anything obvious.

[Graeme]

Amen