RMU

Normally a RMU will be fitted with a automatic load break switch. IE if one fuse blows it will trip all 3 phases. Some are fitted with CT's with time-lag fuses across the trip coils. Your best bet is to look at manufacturers web sites.

What do you mean with CT? Current transformer?

Yes

Check to see if your RMU has overload protection. It would be built in the RTU. If so that will proptect the unit. You need to have protection for the conductor.

In practice a directional over current or a directional power relay (or both) would be required in an RMU.

When the systems are large not only you have to consider the excessive fault currents but also there should be an element of time grading between successive interrupters (time grading of relays).

For reliability of a large system an array of main and backup relays are considered necessary.

For reliability you need to automatically isolate a faulty section or might be you have to install an auto-reclosing.

A circuit breaker is more versatile, load break fuse switch is no match to a CB when it comes to system reliability.

If your's is a small and unimportant scheme might be your thinking is valid but as a rule generally not.

Because, load-break switches have a limited breaking capacity. If you provide a overcurrent relay and arrange to trip the load-break switch thro this relay, then it may so happen that even for currents beyond the breaking capacity of the LBS, the relay might send a trip signal & thus the LBS would damage (flash-over, etc.). That's why one doesn't want to take a chance & only provides fuses for LBS RMUs. Fuses are fast acting. As rightly said in an earlier reply here, it could so be arranged thro micro-switches, that even when one of the three fuses blows, trip supply thro the fuse-blown-microswitch would go to the LBS trip coil and all the three poles of the LBS would open. But, then, here, the LBS would not be opening the short circuit current which alreday was cleared by the fuse. In the worst case, the other two poles would only break the load current, for which, anyhow, the LBS is deisgned.

If you have a circuit breaker in the RMU, then it is prudent to go for relays.

there is a TRIP COIL in parallel with the fuse.

When the fuse blows, the CT's will drive the coil that will trip the circuit breaker.

I hope this clarifies the question.

Manufacturer ABB has lot of information on its web, may check it address is abb.com