Determining the cas of the CPC

[lofty84]

A circuit is protected by a BS88 fuse and the prospective fault current is 171 A. How do I determine the csa of the CPC assuming 70 oC themoplastic cables with copper conductors.

Can anyone tell me where I need to be looking in the BS 7671 or onsite guide please ?

 

[lofty84]

Thanks a million mate, its really decent of you to take the time to go through that for me.

If anyone can help me out here again it would be appreciated

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

 

[pushrod]

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

Now i don't like stuff like that lol, but i'm guessing it's do with presence, right size and connection of things like earthing conductor, protective bonding conductors, cpcs and may be to do with Size of Zs and nature and type of protective device. But not certain by any means!

Edit - identification of earthing system would also be important

 

[gamblor6]

see where lofty says max vd allowable 4.4? surely thats the (mv a m) he.s quoting not the max vd.

 

[pushrod]

see where lofty says max vd allowable 4.4? surely thats the (mv a m) he.s quoting not the max vd.

He is just quoting where he got the 4.4 figure from, which is good practice in the open book exams. He goes onto work out the Vd

 

[Deleted member 26818]

A couple of problems with the calculation.
Why have you chosen 0.1s, and why have you chosen 115 for k?
By choosing 0.1s you are limiting the fuse rating to a maximum of 16A, whereas in reality as these types of fuses are generally used for distribution circuits, 5s can be used, which allows a maximum fuse rating of 40A.
The 115 value of k is for multicore cables, whereas the 143 value is for single core cables.
I'd expect your calculation to work out to about 2.67mm².

 

[lofty84]

Im still trying to learn how to do it mate so thanks for picking that up for me, Ill have another look at it after some more revision.

 

[lofty84]

can anyone describe the process of measureing Ze on a TNCS system

 

[pushrod]

Turn off installation/open main switch and disconnect earthing conductor and measure with EFLI tester - should be less than 0.35 ohms for tncs. TBH you can get all this and more from your OSG chr 10. If you haven't got one your next question should be who is selling the OSG cheap

 

[lofty84]

nice one I know no where to look

 

[lofty84]

pushrod what is a EFLI tester

 

[pushrod]

earth fault loop impedance tester

 

[tony mc]

Thanks a million mate, its really decent of you to take the time to go through that for me.

If anyone can help me out here again it would be appreciated

Ive been asked to list the particular criteria specified in BS7671 with regard to inspection and testing and disconection times.

Im struggleing tho

Have a look at the Schedule of inspected items and supply characteristics Earthing & bonding arrangements.

TN systems & TT systems have different disconection times have a look .