D
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.
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.