Cable calc min CPC

[ianphilip007]

hi there

can anyone just check my use of adiabatic please

4mm/1.5mm cpc circuit, type 2 30A MCB

zs = 0.35

pfc = 230/0.35 = 657.14

t = 0.01

k= 115


s = sqroot (657.14x657.14x0.01)/115

= 0.5714 mm

so the 1.5mm cpc will be fine. Yes ??

 

[ianphilip007]

just not sure if using right figure for t??

 

[hospital]

i think t should be 0.2(table 41.1 bs 7671) that would make it 1.14 which would also comply:bigear:

 

[tony mc]

Carefull hospital table 41.1 is for maximum disconnection times in TN &TT systems.

If you were to use the 0.2 secs have another look at the maths!

Ian you calcs look OK its just I don't have access to the time current characteristics for a 30 A mcb type 2.

Looking at the OSG I see the ZS is 0.88 ohms 230/0.88 = 261 A so your 657 A should disconnect in 0.01

No doubt another member will come on who has the time curves.

Regards

Tony

 

[MarkieSparkie]

Your calculation and conclusion is correct Ian. For the BS 3871 Type 2 MCB time/current characteristics you will need to refer to a copy of 16th Edition of the Wiring Regulations as they are not in the 17th BRB or BGB as BS3871 is a withdrawn standard replaced by BS EN 60898.