Discrimination Between Upstream Circuit Breaker and Downstream Fuse

A Fuse is normally faster acting than a circuit breaker.

If the CB trip settings are correct for the application, the fuse element should open/melt before the upstream circuit breaker trips.

Based on this can we say that pre-arcing i2t of the fuse should be less than i2t of the upstream breaker

If the design is correct I would say yes in most cases.

However; You must look at the trip curves for each device and compare them accordingly based on specific applied current load to determine the trip time difference.

Using the DATA you can plot the operational curve of each device for further clarity.

Aren't we missing something important out here. There seems seems to be a transformer between the 40A MCB and the 50A fuse.

The 40A MCB (if it needs to be as rough as a type D) is sized purely for the inrush current of the transformer. You need to ensure the earth loop impedance is such that a circuit with negligible impedance (a short) will bring your 40A device out. It might be that the supply at the board can't clear this device.

Your extremely long length of cable is going to add to your problems regarding earth loop impedance. You can use an RCD but this won't react to a short circuit. The long length of cable will also reduce the voltage and hence inrush current so an opportunity to revisit the 40A type D.

The fuse is intended to protect the secondary winding of the transformer. Look to the curves to see what will discriminate with it. I don't have the curves to hand but would suggest downstream of the 50A fuse use no greater than a 32A MCB and possible a type B.

regards

The Fox