PSSC & CPC size confusion

[santidreamer]

The PSSC of a final ring circuit is 2KA so what is the min CPC size?

For PVC singles k = 115
CB 32A (BS EN 60898) will disconnect in 0.1 second
Fault current of low impediance = 2000A

S = square root of PSSC x PSSC x disconnection time (t) / Cable constant (K)

CPC size = 5.50mm (6mm cable)

So 2 x 1.5mm commonly used in final ring circuits is too small!

Earth fault loop impediance (Zs) is 0.4 Ohm (below the 1.5 ohm min)

By my calculations if an earth fault of low impediance was to occur anything less than a 6mm CPC would be damaged by the energy let through.

Should I now do as the regs say and consult manufacturers data for more accurate tripping times?

 

[Guest123]

PSCC has nowt to do with CPC selection. If anything it would be PEFC, if it were at the origin. For a final circuit, divide the supply voltage by the circuit Zs to obtain (I).

 

[telectrix]

your pefc is 230/0.4 = 575A. try that in the adiabatic.

 

[santidreamer]

I hope that is where I have gone wrong!

Actual it is where I have gone wrong fault current is related to Zs not PSSC.

The PSSC measures between phase and neutral which are 16mm at CU, then 95mm at distribution board.

CPC is 4mm from distribution board CB 50A to consumer unit. (This is a hotel).

During the calculation stage I worked out 1.5mm was big enough but though it a might get damaged during installation.


Thank you so much.

I can sleep easy now its 1.30 Am and I am in Thailand.

 

[Engineer54]

Are you in Thailand for this job your doing, or are you living there?? Oh, ...And careful, not too many girlfriends while your there, ...though they are very, very tempting lol!!!:tounge_smile:

 

[telectrix]

and watch out for the lady boys.