The load current in a system is 100 A. the short-circuit current for the smallest fault is     100 A. An overcurrent capable of carrying a current of 5 A continuously is to be used. (a) Suggest a suitable C.T. ratio                  (b) What will be the current for the smallest fault?            (c) Will the protective C.T be able to faithfully reproduce the fault current?

7- What is the uses of circuit breaker?

ISE 2. A bolted short circuit occurs in the series R-L circuit of Figure 1 with V=18kV, f=50Hz,X=1092, R=1.092, and with maximum dc offset. The circuit breaker opens 3 cycles after fault inception. Determine: (a) the rms ac fault current, (b) the rms "momentary" current at T=1.0 cycle, which passes through the breaker before it opens, (c) the rms asymmetrical fault current that the breaker interrupts. 1 R mm i(t) ~e(t)=√√2U sin(@t+a) t=0 X² Figure 1 series R-L circuit

(d) The load current in a system is 100 A. The short-circuit current for the smallest fault is 1000 A. An over-current relay capable of carrying a current of 5 A continuously is to be used. (i) Suggest a suitable CT ratio. What will be the relay current for the smallest fault? (ii) (iii) Will the protective CT be able to faithfully reproduce the fault current?

Discuss the concept of "fault tolerance" and its practical applications.

Difference in circuit breakerand isolator.