A kitchen circuit is to be protected by a single fuse at the distribution board. The appliances likely to be ON together are:
• Bread toaster: 800 W
• Mixer-grinder: 500 W (starts and runs immediately at its rated power)
• Refrigerator: 150 W running power; however, its motor draws a starting current three times the running current for the first 0.5s each time it starts
• LED strip lights: 120 W total
Assume the supply voltage to remain steady at 230 V and that each appliance behaves as a pure resistive load at its rated value.
(i) Compute the steady running current of each appliance and hence find the total steady current in the circuit after the refrigerator has settled to running condition.
(ii) Determine the instantaneous current drawn at the moment the refrigerator motor starts, taking into account all other loads being ON.
(iii) Standard re-wirable fuse links of 10 A, 15 A and 20 A are available. Select the minimum fuse rating that will allow the starting surge in part (ii) without rupturing yet will rupture promptly if a fault causes the current to rise to 30 A.
(iv) Comment on the protection offered if a fuse of the next higher rating were used instead.