Repeated MCB Tripping in a Bhubaneswar Flat
A Bhubaneswar flat is fed through a single 15 A miniature circuit breaker protecting the whole power circuit. The wiring is 1.5 mm² copper, which the electrician says is rated for about 11 A continuous current when run through conduit. The family owns a 2000 W geyser, a 1000 W clothes iron, a 750 W microwave oven, and a 1.5-ton air conditioner rated at 1800 W. On winter evenings, when the geyser and AC are both on and someone uses the iron, the MCB trips repeatedly. The electrician proposes replacing the 15 A MCB with a 32 A unit, saying this will stop the nuisance tripping without any rewiring charge. Supply voltage is 230 V. The family wants the cheapest fix that will survive an electrical inspection.
1. Calculate the current drawn when the geyser, iron, microwave and AC run together. Is the tripping a fault or correct operation?
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Calculation (assume unity power factor and 230 V):
- Geyser: I = P/V = 2000/230 = 8.7 A
- Iron: 1000/230 = 4.3 A
- Microwave: 750/230 = 3.3 A
- AC: 1800/230 = 7.8 A
- Total = 24.1 A
The 15 A MCB trips at ~15 A, so it is doing its job. The 1.5 mm² cable is rated ~11 A; even 15 A already exceeds the cable rating, and 24 A is more than double it.
Important physics point: an MCB protects the cable, not the appliance. The fuse/MCB rating must sit below the cable's current-carrying capacity. So the observed tripping is correct protective operation, not a fault in the MCB.
2. Evaluate the electrician's 32 A proposal and explain concretely what would happen if it were accepted.
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The 32 A proposal is dangerous and would be rejected.
With a 32 A MCB, the circuit could carry up to 32 A continuously without tripping - nearly three times the 11 A rating of the 1.5 mm² conductor. The cable would heat up, its PVC insulation would soften and carbonise, and the MCB would never respond to an overload of 12–30 A because it only opens above 32 A.
Result: overheating at joints and behind switches, insulation failure, and a fire risk inside the wall, exactly where nobody can see it. The MCB would only trip on a genuine short circuit.
Note also the short-circuit case: a 32 A device still needs the cable to withstand fault energy; thin cable plus a large breaker is a known fire pattern. The proposal trades a nuisance trip for a hidden hazard.
3. Recommend the safest feasible remedy under a modest budget, and state the principle behind your recommendation.
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Split the load onto separate circuits and never uprate the breaker above the cable.
The governing principle: every protective device rating must be ≤ the cable's current capacity, and every circuit's total load should stay within that capacity during normal use.
Practical steps, in order:
- Run a new 2.5 mm² circuit (rated roughly 20–24 A in conduit) for the AC and geyser - the two largest fixed loads - each on its own 16 A MCB, ideally with a 30 mA RCCB for shock protection.
- Keep general sockets (iron, microwave, lights) on the existing 1.5 mm² cable with the existing 15 A MCB, or upgrade that cable to 2.5 mm² if rewiring is affordable.
- As an immediate zero-cost measure, the family should stagger use: not run geyser and AC and iron simultaneously.
- Put a 16 A switch socket for the AC and geyser; ordinary 6 A sockets are not rated for 7.8 A loads.
Cheapest safe route: one new 2.5 mm² line with a 16 A MCB for the AC, keeping everything else on the old circuit, and staggering heavy loads.