Co-ordination between fuses and other devices

a) In this case the current limiting fuse is used as a back-up for the other device. Thus in the event of heavy faults, only the current limiting fuse is required to operate while only the associated device is required to operate in the event of overloads or small faults. This is achieved by choosing characteristics for the fuselink and the other device, so that they produce a composite characteristic of the form shown and this clearly must give sufficient rapid clearance at all current levels to protect the associated circuit adequately.

a = other device
b = fuse

The other criteria which must be satisfied are as follows:

b) The take-over point (A) at which the curves intersect must be at a current level below the breaking capacity of the other device.

c) To deal with cases where the current limiting fuse clears the circuit, the other device must be able to carry the maximum fault current safely and where it may have to close on to a fault, it must have a making capacity adequate for the cut-off and I2t let-through values of the fuselink.