# electrics maths



## ianherts (Jan 31, 2010)

please could some one help with this:

a 12mm squared cable with 90 degrease thermosetting rubbers is to be used in conjunction with lighting,small power, data and fire alarms insullations. it is protected by a 40 A fuse to BS 88 part2 in an installation where loop impedence between lines of print where the fuse is installed is 0.14 olms. If the supply is 415V three phase, what will the protective short circuit be? Also determine the time for the current to raise the conductor temp to the permissible level.
this is what i have made it but not sure if its right:

I VL over Z A = 415v over o.12 olms = PSC 3.46 KA

T k squeared x S squeared over I squeared = (143x143) X (12x12) over 3460 = 0.245970129306

please can u let me know if right or wrong anf if wrong how is it done thanks :thumbsup:


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## piperunner (Aug 22, 2009)

ianherts said:


> please could some one help with this:
> 
> a 12mm squared cable with 90 degrease thermosetting rubbers is to be used in conjunction with lighting,small power, data and fire alarms insullations. it is protected by a 40 A fuse to BS 88 part2 in an installation where loop impedence between lines of print where the fuse is installed is 0.14 olms. If the supply is 415V three phase, what will the protective short circuit be? Also determine the time for the current to raise the conductor temp to the permissible level.
> this is what i have made it but not sure if its right:
> ...


Dont know were to start but are you a electrician and if so one thing is wrong off the top 90 deg wire is the wrong choice lets start over and pick a 75 deg wire or cable first ?

But iam new to this forum you codes are different then ours yes/ no ?

Unless your equipment is rated for 90 deg ?


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## ianherts (Jan 31, 2010)

i work with an electrician and doing a course but this sum was in my home course work i have wrote it from the course work as it says


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## piperunner (Aug 22, 2009)

ianherts said:


> i work with an electrician and doing a course but this sum was in my home course work i have wrote it from the course work as it says



Oh homework well i wont give you the answer but first look at you wire temp ?

Take your code book out and tell me what the correct wire should be ?


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## ianherts (Jan 31, 2010)

im in the uk and that sum is from the book lol


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