Nuisance Tripping on a Submersible Pump: Overload vs. Short-Circuit Protection
A 15 kW, 400 V three-phase submersible pump has a nameplate full-load current of 28 A and a service factor of 1.15. It is protected by a 70 A inverse-time circuit breaker and a Class 10 thermal overload relay set at 28 A, mounted in a starter enclosure where the measured air temperature reaches 48 °C. The pump draws a steady 29 A per phase at its duty point, with balanced supply at 400 V. The relay trips after about 25 minutes of running, every time. The maintenance crew's proposal is to raise the relay setting until it stops tripping.
1. Is the 28 A overload relay setting correct?
Read the worked answer
No - it is set at 100 % of nameplate, which is too tight for this motor.
Because the service factor is 1.15 (i.e., not less than 1.15), the overload device may be set at up to 125 % of nameplate full-load current:
28 A × 1.25 = 35 A
The relay is set at 28 A, so the motor is allowed to run at only 100 % of its nameplate current. It is drawing 29 A, which is 104 % of nameplate - entirely normal and well within the 1.15 service factor. The motor is not overloaded; the protection is simply set below where the motor legitimately runs.
One further factor: the starter sits in 48 °C air. A thermal overload relay that is not ambient-compensated will trip earlier than its dial says, because its bimetal starts from a hotter baseline. Part of the 25-minute trip time is ambient, not current.
2. Is the 70 A breaker correctly sized? Distinguish what it protects from what the relay protects.
Read the worked answer
The breaker is correct. Motor branch-circuit short-circuit and ground-fault protection for an inverse-time breaker may be set up to 250 % of full-load current: 28 A × 2.50 = 70 A, and 70 A is a standard rating. It is there for short circuits, not for overload.
The two devices do different jobs and must not be confused:
| Device | Protects against | Setting |
|---|---|---|
| 70 A inverse-time breaker | Short circuit, ground fault | 250 % FLA |
| Thermal overload relay | Sustained overload, overheating, single-phasing | 125 % FLA (35 A) |
This is why the crew's instinct is wrong at the root: the breaker tripping would indicate a fault; the relay tripping indicates the overload setting. Raising the breaker would remove short-circuit protection without touching the actual problem.
The branch-circuit conductors also need checking: 125 % × 28 A = 35 A minimum ampacity. 8.0 mm² copper at 75 °C terminals gives ≈ 50 A, so the conductors are fine and do not constrain the relay setting.
3. The crew wants to set the relay to 45 A to stop the tripping. What is your response, and what do you actually do?
Read the worked answer
Reject 45 A. It is 161 % of nameplate. The code allowance goes to 125 % as of right, with a further step to a maximum of 140 % (≈39 A) only where the 125 % setting demonstrably will not carry the load. 45 A is beyond any permitted value and would let the motor bake before the relay noticed - turning a nuisance trip into a rewind.
What I would actually do, in order:
- Set the relay to 35 A (125 % of 28 A) and record it on the starter schedule.
- Confirm the current is genuinely 29 A on all three phases. 104 % of nameplate is normal; a single high phase would indicate a supply or winding problem, which a higher setting would hide.
- Address the 48 °C ambient: ventilate or shade the enclosure, or fit an ambient-compensated relay. A correctly set relay in a hot box can still trip early.
- Check the trip class. Class 10 is a 10-second trip at 6× FLC - fast. For a submersible pump with a long acceleration time, Class 20 may be the correct choice, but only after verifying the motor can tolerate the longer acceleration without exceeding its thermal limit.
- Verify the overload is not masking a mechanical problem - a fouled impeller or worn stage will push current up over time, and a raised setting would delay the diagnosis.