1) Describe, with the aid of sketches, the mechanisms built into the design of an HRC fuse link to enable it to respond efficiently to a sustained overload as well as to a severe short-circuit condition

1) Describe, with the aid of sketches, the mechanisms built into the design of an HRC fuse link to enable it to respond efficiently to a sustained overload as well as to a severe short-circuit condition

2) Explain the significance of ‘let-through energy’ when assessing discrimination between major and minor fuses in a system of feeder protection. Describe how manufacturers’ data can be used in this assessment. 

3) Attached figure shows a time-current characteristic typical of miniature circuit breakers. Explain how such a characteristic can be achieved in the construction of an MCB. In what respect would the actual characteristic of Type D MCB differ from that of Type B of the same rating? What significance does this difference have on the application of each type?

4) In general, HRC fuses provide back-up protection for the overload protection relay incorporated in an induction motor starter. Define the two types or levels of back-up protection that could be provided and explain the difference between these two forms of protection and show how the two protective devices should be co-ordinated.