Current protection

1. It depends on the location of the fault. If the fault is "downstream" of some of these components but "upstream" of others, only the former will trip.

2. Among the breakers or fuses of the same type, those of lesser amp ratings would trip first.

3. For items of different types,the time/overcurrent trip curves need to be compared. In this case, the 160A fuses would be likely to trip first. However, if they are time-delay fuses, it is conceivable that even one of the 200A breakers would trip faster. Hence the need for comparing the curves.

You have to grade those fuses properly. In Incomer, you have 160A Fuses or MCCB's and in down streams, you have MCCB's of 200 or 250A. Grade it properly. Use the max rated MCCB at I/C and then gradually decrease the value as per requirement. This will help system to isolate the faulty part only and all other part will reaman live.

SC calculation is not possible without few data.

I agree with with Sidhu's explanation. In case you are designing the system, u need to redesign the same with highest rating of of upstream Incomer , to downstream incomers in decreasing order, based on the equipment/consumer requirement on each stage. However the sytem is laready designed then I doubt there will proper co-ordination between the breakars durng fault conditions and u need to check with person who has designed the sytem.

Thanks all for your help, I think what I really require is the curves for each, as I am pretty sure that due to a MCCB being more sensitive than a fuse there is a means of discrimination here. Also I may have found a reason/explanation for the lack of protection, if the main fuse is to be the sole source of protection and all other MCCB's are to be classed as equipment i.e. switch disconnectors, and not as they are intended then this would be correct.

Many thanks for all your responses.

Kind regards

Rossk