Safety first: Do not repeatedly reset a breaker that trips instantly. Isolate power and investigate.

Check motor rotation by hand: With power off, attempt to spin the motor fan or shaft. If it does not move or is very stiff, the rotor is likely seized and the motor must be repaired or replaced.
Inspect for water ingress: Look for signs of water inside the motor terminal box or rust on the motor body. If present, the motor insulation may be compromised.
Measure winding resistance: Disconnect the motor leads and measure resistance between phases (or between run/start windings on single-phase). Very low resistance or a reading to earth indicates a short.
Verify breaker size: Confirm that the breaker rating matches Calpeda's recommendations for the pump's full-load current. An undersized breaker may nuisance-trip but will not usually trip instantly.
Replace faulty motor: If a short or seizure is confirmed, replace the Calpeda motor with an identical specification unit.

Pump trips the circuit breaker immediately when switched on

Possible Causes