Ramesh's flour-mill shed: sizing the load before blaming the MCB
Ramesh runs a small flour mill from a shed attached to his house in a Rajasthan village. The shed is wired as a single 15 A circuit ending in one power socket; the house lighting runs on a separate 5 A circuit carrying four 60 W bulbs. His equipment is a 1.5 kW mill motor, a 2 kW water heater and a 1 kW clothes press. The supply is 230 V. Last week, when the motor, the heater and the press were all run together from that one socket through a multi-plug, the 15 A MCB tripped after a few minutes. Ramesh blames the MCB and wants to replace it with a 25 A unit. His nephew, a Class 10 student, insists the MCB is doing exactly its job and the real fault is the wiring plan. Ramesh also grumbles that the bulbs dim for a moment every time the motor starts. He has already had one shock from the motor body when his hand was wet.
1. Calculate the current drawn by each appliance and by all three together at 230 V. Decide who is right about the MCB and justify the answer in terms of what the MCB is protecting.
Read the worked answer
Using I = P/V for each appliance:
- Mill motor: 1500/230 = 6.5 A
- Heater: 2000/230 = 8.7 A
- Press: 1000/230 = 4.3 A
- Total with all three running: 6.5 + 8.7 + 4.3 = 19.5 A
- Lighting circuit (separate): 4 × 60 = 240 W, so 240/230 = 1.04 A, comfortably within 5 A.
19.5 A exceeds the 15 A rating, so the MCB tripping is correct behaviour, not a fault. The nephew is right.
Fitting a 25 A MCB would be dangerous. The MCB is chosen to protect the wire, not the appliance. The shed wiring is sized for about 15 A; at 19.5 A or more the copper conductor heats up, insulation softens and can catch fire, and a 25 A MCB would allow that current to flow without tripping. A protective device must never be rated above the safe current capacity of the cable it protects.
Note that each appliance alone is fine (motor 6.5 A, heater 8.7 A, press 4.3 A). The problem is the combination on one circuit, and the multi-plug is what makes the combination possible.
2. Explain why the bulbs dim for a moment when the motor starts, and why the motor's metal body must be earthed.
Read the worked answer
A motor draws a starting current several times its running current (often 5–6 times) for a fraction of a second while the rotor accelerates. That surge flows through the same service cable, and the cable has resistance. The voltage drop along the cable is V = IR, so a larger current means a larger drop, and the voltage actually reaching the bulbs falls briefly. Since the power consumed by a bulb depends on the square of the voltage across it, the bulbs glow dimmer until the motor reaches normal speed and the current settles.
Earthing: the metal casing is connected by a green/yellow earth wire to a copper plate buried in the ground. If the live wire's insulation fails and touches the body, the body becomes live. Without earthing, a person touching it completes a circuit through their body to the ground and receives a severe shock. With earthing, the fault current flows through the low-resistance earth wire, which is a near short circuit - the current becomes large enough to trip the MCB almost instantly, and the body is held at earth potential. This is why the wet-hand shock Ramesh got must be treated as a wiring fault to be repaired, not ignored.
3. Suggest two changes to the wiring arrangement that would let Ramesh run his equipment safely, and state one thing he must not do.
Read the worked answer
Sensible changes:
- Give the heater and the press their own 15 A power circuits, separate from the motor circuit, each with its own MCB, and use properly rated three-pin sockets instead of a multi-plug. Then no single circuit carries the full 19.5 A.
- Use cable of adequate cross-section for each circuit and separate the lighting circuit completely from the power circuits, so motor starting surges do not affect the bulbs.
An alternative if rewiring is not immediately possible: run at most one high-wattage appliance at a time - motor plus press is 10.8 A, motor plus heater is 15.2 A (still marginal), heater plus press is 13.0 A. This is a stopgap, not a solution; the heater and press are both power appliances that should not share a lighting-grade circuit at all.
What he must not do is simply fit a higher-rated MCB or fuse on the existing thin cable. The protective device must match the cable; uprating it removes the protection and converts an inconvenience into a fire risk.