Tripping MCB, an Overloaded Sub-circuit and a One-Hour Shutdown
At a workshop in a frontier sector, one 230 V single-phase sub-circuit feeds two machines: a 2.2 kW compressor (efficiency 0.85, power factor 0.80) and a 5 kVA arc welder. The sub-circuit is protected by a 16 A Type B MCB and wired with 2.5 sq mm PVC-insulated copper cable clipped directly to the wall. For the past week the MCB has tripped almost every time the compressor starts while the welder is connected but idle. A fitter suggests simply replacing the 16 A MCB with a 32 A one 'so that nuisance tripping stops'. On the same day an earth tester shows the workshop earth-pit electrode resistance as 9 ohm, against a unit standing order of not more than 5 ohm. The supervisor wants the problem settled before the next shift, with the workshop shut down for no more than one hour. You are the tradesman-in-charge.
1. Using calculations, show whether the compressor's running and starting currents explain the tripping, and explain why a Type B 16 A MCB behaves this way.
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Running current. Motor input power = output/efficiency = 2200/0.85 = 2588 W. Current = 2588/(230 × 0.80) = 14.1 A. On a 16 A breaker that is about 88 per cent of rating; an MCB is calibrated at 30 degrees C and is not meant for continuous loading that close to its rating, so the thermal element is already warm.
Starting current. A single-phase capacitor-start motor draws roughly 5 to 7 times full-load current, i.e. about 70 to 99 A for a fraction of a second. A Type B MCB trips magnetically between 3 and 5 times its rating, that is 48 to 80 A, so the inrush falls inside the instantaneous trip band. The welder's magnetising current, drawn even at no load, adds to the same circuit.
Conclusion. The breaker is doing its job: it is seeing a genuine short-duration overload on a circuit that is too small for the connected load. It is not a faulty breaker and it is not 'nuisance' tripping.
2. Evaluate the fitter's proposal to fit a 32 A MCB in place of the 16 A one. What is the correct remedy?
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Why the proposal is unsafe. An MCB protects the cable, not the appliance. The current-carrying capacity of 2.5 sq mm PVC copper cable clipped direct is roughly 24 A. Fitting a 32 A breaker means the circuit can carry a sustained fault or overload current above 24 A without the breaker operating, so the cable overheats, insulation degrades and a fire becomes possible inside a wooden-roofed workshop. The compressor's own winding is also left without overload protection; a 32 A device will never protect a 14 A motor.
Correct remedy. Split the loads into dedicated circuits: compressor on its own 20 A Type C or D MCB with 4 sq mm cable and a motor starter or overload relay set near 14 A; welder on a separate 20 A circuit with its own cable; retain 16 A protection on any remaining general-purpose socket circuit. If rewiring is not possible today, the interim control is procedural: do not run the compressor and welder together, and record the restriction.
Principle to state in the register: never uprate a protective device to stop tripping; find and correct the cause.
3. Lay down the safe sequence for the one-hour shutdown and state what you will do about the 9 ohm earth reading.
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Before work. Inform the supervisor and users, obtain a permit, identify the correct circuit and isolate at the distribution board, lock off and tag, then prove dead at the terminals with a tested voltage indicator on all conductors. Wear insulating gloves, safety shoes, helmet and goggles; use insulated tools; keep one person outside the board as standby.
Earthing. 9 ohm against the unit's 5 ohm limit is not acceptable. Within the hour: inspect the pit, clean the terminal, re-backfill with alternate layers of charcoal and salt and water the pit; re-measure. If it is still above 5 ohm, drive a second electrode at least 3 m away and bond the two in parallel, then re-measure and record the value in the earth test register. Do not energise and leave a known deficiency unrecorded.
After work. Remove the lock and tag only after the work is complete, restore supply, test insulation resistance and the protective device, hand over and log the work, the reading and the parts used. Note in writing that the motor starter and split circuits are still pending.