Fault Currents

That sure sounds like a homework question, and I'm not sure whether answering it for you is the best way for you to learn, but, maybe...

A fault in a circuit can occur in any location, and be a fault between phases (short circuit) or a fault to ground (ground fault). Depending on the location and type of fault, the fault current that flows can be very high or fairly low and be hazardous (and even be very low (in the mA range) and be life threatening if it is flowing through someone's body).

In designing a power distribution system, you have to consider the faults that can occur and how to protect against them. Sometimes there is more than one consideration--for a short circuit fault (very high current) you want at least two things, you want a circuit breaker or fuse (and sometimes the fuse is best) to remove power from the circuit very quickly, but you also want the equipment carrying the short circuit current (the transformers, circuit breakers, fuse holders, etc.) to "withstand" that current until the short circuit is de-powered.

Fault currents vary with time, equipment can withstand fault currents for varying amounts of time (depending on how large the fault current is), and fuses and circuit breakers can break the circuit in various amounts of time, so there are quite a few things to learn about and consider.

On the lower fault current end, a circuit breaker designed to detect and open on a large short circuit fault current will not even notice the very low fault currents of a ground fault, so you need different detection systems (relaying) to notice those faults and disconnect the circuit to protect personnel (and equipment).