Since breaker operates in transiant state & it is design to 
pass the sub transient current through it.But it is 
supposed that if breaker is just about to close & fault 
occurs at the same time then breaker will have to close in 
that sub transient period & thus amount of arc produce will 
be very high.Thus making cap. is very high.

pratik ur absolutely right but i m not able to know why this
site has given u the zero mark but ur ans was correct so
everyone can go through dis. 

well done........be ready for ur next que

when we make the circuit ON  at that time a suddern inrush 
current the steep traveling current wave) flow to the 
network or to the device due to the presence of circuit 
parameters like as inductance, capacitance.  that time the 
current value is more than the breaking current.Thats why 
the making capacity is more than the breaking capacity.

if any fault is create in systeam at the time breaker must 
be open in within seconds ( it is 2 to 3 sec. )otherwise 
very denger fault is create and we loss a very costly 
equipment.

The making capacity is more than the breaking capacity to 
maintain the RRV.