(Solved): a) Determine the minimum cross-sectional area of the Circuit Protective Conductor (CPC) ...

a) Determine the minimum cross-sectional area of the Circuit Protective Conductor (CPC) for Miniature Circuit Breaker (MCB) with a value of thermal capacity constant, $$\mathrm{k}$$ is 115. i. Type B MCB with operating time and fault current of 8 seconds and 200 A. [10 marks] ii. Type C MCB with operating time and fault current of $$0.1$$ seconds and $$500\mathrm{A}$$. [10 marks] b) The electrical supply to a factory is fed by a $$1\mathrm{MVA},22/0.4?\mathrm{kV}$$ transformer that has a reactance of $$6\%$$ as shown in Figure 2. The resistance of the transformer is negligible. All the MCCBs have the same time-current characteristic as given in Table 1. The current ratings and the values of resistance and reactance of each circuit are given in Table 2. Determine; i. The current rating and the minimum breaking capacity of the MCCB for the circuit connecting to the motor, which is rated at $$25\mathrm{kW},90\%$$ efficiency and $$0.9$$ power factor. The motor utilises a DOL starter. The starting current is seven times the full-load current and such a current would exist for $$10\mathrm{s}$$. [10 marks] ii. Whether the 200-A MCCB for the $$4\times 95{\mathrm{mm}}^2$$ circuit can adequately protect against overload current. [10 marks] iii. The magnitude of the earth-fault current for an earth fault at the motor terminals and confirm whether the motor circuit has adequate protection against electric shock? Give reasons with calculation. [10 marks] Figure 2 Table 1: Time-current Characteristic of MCCBs Table 2: Cable characteristic (For the whole circuit length and no temperature correction is required)