A 230 V single-phase 7.5 kW motor with 0.8 power factor lagging is fed from Sub...

 

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A 230 V single-phase 7.5 kW motor with 0.8 power factor lagging is fed from Sub Distribution Board (Sub DB) using single-core, non-armoured PVC copper conductors for phase conductors and circuit protective conductor (cpc) as shown in Fig. 3 on page 5. The cable length is 20 m, and the circuit is installed in trunking on its own (Reference Method 3). The ambient temperature is 40 °C. The voltage drop from Sub-DB to the motor should not exceed 1.5%. The motor circuit is protected by a Type C Double Pole (DP) MCB in Sub DB. The cable from main Distribution Board (Main DB) to Sub DB is 30 m using 70 mm² single core, non-armoured PVC copper conductor as phase conductor and 35 mm² single core, non-armoured PVC copper conductor as cpc in trunking. The protection of this main cable is by a 125 A Type B DP MCB. The external loop impedance (Ze) has a measured value of 0.4 22 at Main DB. (a) Determine the cable size of phase conductors for the motor circuit satisfying the current carrying capacity and the voltage drop. Overload of the motor is protected by the Type C DP MCB. (10 Marks) (b) Using Table 54.7 of SS638, determine the cable size of cpc. Check whether the cpc is adequate to withstand the thermal constraint under earth fault conditions. (18 Marks) (c) If there is an electrical earth fault in the motor, explain with calculations whether electric shock protection can be achieved with this design. (5 Marks) Recommend one measure to achieve shock protection. (d) ZENE L1 L2 L3 70 mm² single core PVC cable +35 mm² single core PVC cable as cpc, 30 m long 125 A Type B N Main DB E N Sub DB E single core PVC cable 20 m long M Single phase motor at 7.5 kW, pf 0.8 lagging Fig. 3: Electrical supply to a motor Available circuit breaker current ratings are: 16 A, 20 A, 32 A, 63 A, 100 A, 150 A, 200 A, 250 A, 300 A, 400 A, 600 A, 800 A, 1,000 A, 1,250 A, 1,500 A, 2,000 A A 230 V single-phase 7.5 kW motor with 0.8 power factor lagging is fed from Sub Distribution Board (Sub DB) using single-core, non-armoured PVC copper conductors for phase conductors and circuit protective conductor (cpc) as shown in Fig. 3 on page 5. The cable length is 20 m, and the circuit is installed in trunking on its own (Reference Method 3). The ambient temperature is 40 °C. The voltage drop from Sub-DB to the motor should not exceed 1.5%. The motor circuit is protected by a Type C Double Pole (DP) MCB in Sub DB. The cable from main Distribution Board (Main DB) to Sub DB is 30 m using 70 mm² single core, non-armoured PVC copper conductor as phase conductor and 35 mm² single core, non-armoured PVC copper conductor as cpc in trunking. The protection of this main cable is by a 125 A Type B DP MCB. The external loop impedance (Ze) has a measured value of 0.4 22 at Main DB. (a) Determine the cable size of phase conductors for the motor circuit satisfying the current carrying capacity and the voltage drop. Overload of the motor is protected by the Type C DP MCB. (10 Marks) (b) Using Table 54.7 of SS638, determine the cable size of cpc. Check whether the cpc is adequate to withstand the thermal constraint under earth fault conditions. (18 Marks) (c) If there is an electrical earth fault in the motor, explain with calculations whether electric shock protection can be achieved with this design. (5 Marks) Recommend one measure to achieve shock protection. (d) ZENE L1 L2 L3 70 mm² single core PVC cable +35 mm² single core PVC cable as cpc, 30 m long 125 A Type B N Main DB E N Sub DB E single core PVC cable 20 m long M Single phase motor at 7.5 kW, pf 0.8 lagging Fig. 3: Electrical supply to a motor Available circuit breaker current ratings are: 16 A, 20 A, 32 A, 63 A, 100 A, 150 A, 200 A, 250 A, 300 A, 400 A, 600 A, 800 A, 1,000 A, 1,250 A, 1,500 A, 2,000 A