Pump motor tripping the submain MCB on start: is a bigger breaker the answer?
A 7.5 kW, 415 V, three-phase squirrel-cage pump motor at a tubewell is fed by a 4 mm² copper cable about 35 m long from a submain board. The submain is protected by a 32 A Type B MCB and is itself fed from a 63 A Type B MCB at the main board. The motor is started direct-on-line and runs at a measured 13 A per phase at 412 V. Since installation it has tripped the 32 A MCB on roughly one start in ten, always within the first second; the 63 A main MCB has never tripped. The pump is not mechanically jammed. The farmer says the electrician should simply fit a 63 A MCB in the submain 'to match the main board'.
1. Estimate the motor's full-load and starting currents and explain why the 32 A Type B MCB trips occasionally but only during the first second.
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Full-load current: I = P / (√3 × V × pf × η). Taking a typical pf of 0.85 and efficiency of 0.88, I = 7500 / (1.732 × 415 × 0.85 × 0.88) ≈ 13.9 A, which agrees with the measured 13 A - the motor is not overloaded while running.
Direct-on-line starting draws roughly 6–8 times full-load current, i.e. about 84–112 A for a fraction of a second. A 32 A Type B MCB trips magnetically at 3–5 times its rating, i.e. 96–160 A. The lower end of that band overlaps the motor's inrush, so on a start when the supply voltage is slightly low or the motor is warm, the instantaneous element operates. The trip is always within the first second and never during running, which matches a starting-current problem rather than an overload or a short circuit. A 13 A running current on a 32 A device is normal - the MCB is providing short-circuit protection, while running overload should be handled by the starter's overload relay.
2. Evaluate the farmer's proposal to fit a 63 A MCB in the submain, using cable protection and discrimination.
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It would stop the tripping, but by removing protection - a bad trade.
- Cable protection: 4 mm² copper cable in this installation is good for roughly 25–32 A. A 63 A MCB allows more than twice that current to flow for a long time without operating, so a damaged cable or a jammed pump could burn without the device clearing the fault.
- Discrimination: the upstream device is also 63 A Type B. With equal ratings, a fault may trip either or both, so the whole main board can be lost instead of only the pump submain. Protection must be graded, with the downstream device smaller and faster.
- Wrong device for the job: the MCB protects against short circuit; the motor's thermal overload relay protects against overload. Making the MCB bigger does not address starting current; it only masks it.
- Safety: a 63 A device on a 4 mm² circuit feeding a motor with no verified earth is a fire and shock risk. The proposal should be refused and the actual cause of the inrush trip addressed.
3. Recommend the correct protective arrangement and one way to reduce the starting current.
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- Device: replace the 32 A Type B with a 25 A Type C MCB. Type C trips magnetically at 5–10 times rating, i.e. 125–250 A, comfortably above the 84–112 A inrush, while 25 A still protects the 4 mm² cable. A 20–25 A Type D device is an alternative if the supply is weak, but check the cable rating first.
- Running protection: confirm the starter's overload relay is set to about 13–14 A (the nameplate full-load current), not to the MCB rating, and that all three phases are sensed.
- Connections and earthing: check terminal tightness at the motor and the submain, and verify the earth continuity to the motor frame.
- Reduce inrush: a star-delta starter, soft starter or reduced-voltage starting cuts the starting current to roughly one-third or less, which also reduces the voltage dip on the feeder - useful evidence is to measure the voltage at the motor terminals during a start and confirm the dip is not excessive.
- Verify: after changes, log several starts and confirm no trip during start and no abnormal heating during running.