Fault currents providing instantaneous disconnection/ thermal withstand of cables.

[DBR electrical]

HI.
I have recently installed some circuits at work and i am having problems proving that the fuses will disconnect before the cables used get to their max permitted temperature.
The two circuits are as follows:
1. 6mm L,N,E singles connected to a 32 amp HRC fuse, having a Zs of 0.05 causing a fault current of 4.6Ka

2. 2.5mm 3 core swa connected to a 16 amp type B MCB, having a Zs of 0.12 causing a fault current of 1916 amps.

Both of these circuits have very similar short circuit currents, and my problem arises when i use the log graphs in the back of BS 7671
and use the equations S = sq rt of I2 x t/ k and t = k2 xS2/ I2. Now because my fault currents are so high i find that the time is always 0.1 seconds on the respective graphs for both fuses, but when i use the 0.1sec to do my calculations, the answers i get mean that my 6mm cables need to be 16mm cables and my 2.5mm cables need to be 6mm cables. Because the graphs in the regs book don't seem to go lower than 0.1 sec
my calculations don't seem to work but i have heard that if the fault current is higher than that of the instantaneous operation of the fuse then you can calculate it using 0.01sec which does make my circuits correct. Is this true because i can't seem to find an answer in the regs for what time you can use when the overload/fault currents are so high.
Many thanks. Dan.

 

[D Skelton]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

I would find out what the manufacturer of the fuse says as they will have accurate time/current graphs applicable to their fuses only but I'd be willing to steak a fair amount on the fact that using 0.01 would be more than ok. I've seen time current graphs for HRCs that go down to 0.005 and MCB's that go down as far as 0.001.

The time/current graphs in the BGB are rough at best!

 

[DBR electrical]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

I would find out what the manufacturer of the fuse says as they will have accurate time/current graphs applicable to their fuses only but I'd be willing to steak a fair amount on the fact that using 0.01 would be more than ok. I've seen time current graphs for HRCs that go down to 0.005 and MCB's that go down as far as 0.001.

The time/current graphs in the BGB are rough at best!

WOW. wasn't expecting any reply's this quickly! Really appreciated advice that i will be using asap. Many thanks. Dan.

 

[D Skelton]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

WOW. wasn't expecting any reply's this quickly! Really appreciated advice that i will be using asap. Many thanks. Dan.

Make sure you check with the manufacturer before using 0.01 though!

 

[Guest55]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

isnt it usual to do the design before the install ?
;-)

 

[telectrix]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

but what temperature will the cables withstand for the short duration of the fault current without suffering thermal damage. it's a damn sight more than the 70deg. operating temperature.

 

[PEG]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

Usually but not all the time.Look at the Scottish Holyrood carry-on...

 

[Chr!s]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

Ill give you a simple way to meet the Regs, In not less than Ib, In not to be less than Iz, protective device I2 less than 1.45 and the CPD has a rated short circuit capacvity greater than the point of utilisation.

 

[Engineer54]

Re: Fault currents providing instantaneous disconnection/ thermal withstand of cables

Your over thinking this, the protective devices for the cable sizes you have chosen will be totally adequate. The clearance/disconnection times will be more than sufficient, in not allowing the conductors rise to their max permitted levels!!