IEC 60364 Table 48A

AJJewsbury:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm.

I think this was discussed before, and I still think that's a dangerous assumption. Take a typical 63A BS 88-3/BS1361 fuse (commonly used for distribution circuits in domestics) - Max Zs for 5s is 0.68Ω so fault currents could be as low as 338A - is that sort of additional load really going to collapse the output of a public supply transformer?

   - Andy.

230 x 338 amps is about 78kw. For a 25kva pole transformer I think it is safe to assume a good dip, however you do have a point with larger units such as 500kva. 


This is why I think that 0.4 seconds should be at least extended to circuits up to 63 amps. Work needs to be done to determine country specific practices and the values adjusted for each local regulation.


For example, in the US and Canada 5 seconds would work for a 100 amps MCB (100 x 7) as homes and commercial properties are typically fed via 25 or 50kva pole pigs (either single phase or in a 3 phase bank), where as in the UK (100 x 5) may require the establishment of a 1.5 second clearing based on the exact drop during a fault.   

 

AJJewsbury:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm.

I think this was discussed before, and I still think that's a dangerous assumption. Take a typical 63A BS 88-3/BS1361 fuse (commonly used for distribution circuits in domestics) - Max Zs for 5s is 0.68Ω so fault currents could be as low as 338A - is that sort of additional load really going to collapse the output of a public supply transformer?

   - Andy.

230 x 338 amps is about 78kw. For a 25kva pole transformer I think it is safe to assume a good dip, however you do have a point with larger units such as 500kva. 


This is why I think that 0.4 seconds should be at least extended to circuits up to 63 amps. Work needs to be done to determine country specific practices and the values adjusted for each local regulation.


For example, in the US and Canada 5 seconds would work for a 100 amps MCB (100 x 7) as homes and commercial properties are typically fed via 25 or 50kva pole pigs (either single phase or in a 3 phase bank), where as in the UK (100 x 5) may require the establishment of a 1.5 second clearing based on the exact drop during a fault.