Submersible pump tripping: one dead phase, one angry secretary
A multi-skill assistant at a housing society is called because the 5 HP, 415 V, 3-phase submersible pump that fills the overhead tank hums but delivers no water, and its thermal overload relay trips about 20 seconds after every start. The nameplate reads 5 HP, 415 V, 50 Hz, 7.0 A, power factor 0.85, efficiency 88%, star-delta starter. Clamping the three outgoing phases at the starter, the assistant reads 6.8 A, 6.9 A and 0 A. Line voltage is 412 V between two phases but only about 238 V when the third phase is included in the measurement. The pump ran normally the previous evening. One fuse carrier in the starter is discoloured and the enclosure is thick with dust. The society secretary wants the overload relay set higher so that the pump "stops tripping" before the tank overflows, because about 400 residents are waiting for water.
1. From the nameplate data, calculate the motor's full-load current, then use the clamp and voltage readings to state what has actually happened. Why does the overload trip when there is no short circuit?
Read the worked answer
Step 1 - rated current from the nameplate
- Output = 5 HP x 746 = 3730 W
- Input = 3730 / 0.88 = 4239 W
- I = P / (√3 x V x cos φ) = 4239 / (1.732 x 415 x 0.85) = 4239 / 611 = 6.94 A, which agrees with the 7.0 A stamped on the plate. So the calculation is validated by the nameplate.
Step 2 - read the pattern Two lines at about full-load current and one at zero is not a motor fault; it is an open circuit in one line (blown fuse, burnt contact, loose terminal). A shorted turn would push all three currents up; an earth fault would operate the ELCB. The voltage evidence confirms it: 412 V between the healthy phases, but only about 238 V when the open phase is included, because that phase is being fed back through the motor windings.
Step 3 - why the relay trips With one line open the motor keeps turning as a single-phase load. The remaining windings carry more than their share, negative-sequence current heats the rotor, and torque pulsates at twice slip frequency - hence the hum. The overload relay is not misbehaving; it is the only thing preventing the winding from passing its insulation temperature class. If it did not trip, the next event would be a burnt winding or a fire.
2. State the correct sequence before the pump is returned to service, and answer the secretary's request to raise the overload setting.
Read the worked answer
The setting must not be raised. The overload is set at roughly 1.0-1.15 times full-load current as a protection parameter, typically around 7.5 A on the dial for this motor. Turning it up converts a cheap fault (a fuse) into an expensive one (a rewound or replaced pump) and removes the only protection the winding has. The correct answer is to remove the cause.
Sequence before restart
- Inform the users, then isolate at the correct switch, lock and tag it, and prove dead with a tester.
- Replace the discoloured fuse carrier (a discoloured carrier means a loose, hot contact) and fit an HRC fuse of the rating on the starter chart - never a rewired fuse or a higher rating to 'stop it blowing'.
- Inspect and tighten all terminals; look for water, dust and tracking in the starter.
- Insulation-resistance test the motor at 500 V DC: 1 MΩ to earth is the minimum, 5 MΩ or more is healthy. Dry the winding if it is low.
- Check winding resistance balance, starter contacts, overload heater and the pump itself (not blocked, not running dry, delivery valve open).
- Trial run and clamp all three phases; current imbalance should be within about 10%. Record the readings and the fuse change in the register.
3. The secretary is standing at the panel insisting on a higher setting because 400 residents are waiting. How do you handle this on the spot, and what interim arrangement can you offer?
Read the worked answer
On the spot
- Acknowledge the problem first - the residents' water is real - then state the refusal in one plain sentence without jargon: "If I turn this up, the pump will run today and burn out within a week; a rewind costs far more than the wait."
- Do not open the panel in front of residents or leave the door open; do not bypass the relay with wire or hold the contactor in.
- Show the evidence rather than argue: the 0 A clamp reading and the discoloured fuse carrier are visible proof that the tripping is not a setting problem.
- Offer options, because a refusal with no alternative gets overruled: switch to the standby pump or booster, restrict supply to ground-floor taps, arrange a tanker for the evening, and give a firm time by which the fuse will be replaced.
Documentation Write the complaint, the readings, the advice given and the time in the maintenance register, and send a short message to the maintenance in-charge and the secretary so the instruction and your response are on record. If the pressure continues, escalate before the panel is touched, not after.