Grain Auger Electrical Fault and Isolation
A 3-phase 15 kW grain auger on a farm is fed from a submain in a shed. The operator reports the auger tripping the circuit breaker intermittently, more often when the grain is damp. The motor is warm but not hot. The starter has a thermal overload set to the motor nameplate full-load current. The cable run is 60 m of 4-core cable and the submain breaker is rated 40 A. The motor nameplate reads 15 kW, 400 V, 28 A, 0.86 power factor. The operator wants to 'just reset the breaker and keep going' because harvest is underway. The shed has no lockout tagout kit and the auger has no local isolator.
1. Assess whether the breaker tripping is likely an electrical protection issue or an overload caused by the load, and explain how you would decide.
Read the worked answer
The intermittent nature and correlation with damp grain strongly suggest a load-side overload rather than a faulty breaker. Damp grain increases the conveying resistance, raising motor current above normal. The thermal overload should protect the motor, but if the overload is set above the motor's actual full-load current or has the wrong trip class, the upstream breaker may trip first.
How to decide:
- Measure current on all three phases with a clamp meter under normal and damp-grain conditions. Compare with the nameplate 28 A. Sustained current near or above 28 A indicates overload.
- Check phase balance. A significant imbalance suggests a supply or winding problem, not just load.
- Verify the overload setting matches the nameplate full-load current and the trip class suits the motor and starting method.
- Check the breaker rating against the cable and motor: a 40 A breaker on a 28 A motor is plausible, but the cable's current-carrying capacity and volt drop over 60 m must be confirmed.
- Inspect for mechanical causes: blocked auger, worn flighting, tight bearings, or a partially seized gearbox.
If current is within nameplate at normal load and only rises with damp grain, the fix is operational (feed rate, grain condition, auger condition), not a bigger breaker. Increasing the breaker rating would remove protection and risk cable and motor damage.
2. The operator wants to keep resetting the breaker. Explain the risks and outline the isolation and safety steps required before any inspection.
Read the worked answer
Repeatedly resetting a tripping breaker without finding the cause is dangerous. A breaker is a protective device; repeated trips mean it is doing its job. Resetting can lead to cable overheating and fire, motor winding damage, and arc flash or electric shock if a fault is present. It also defeats the protection designed to prevent exactly that outcome.
Risks specific to this site:
- No local isolator means the auger cannot be safely isolated near the point of work, so someone could restart it remotely while another person is working on it.
- No lockout tagout kit means there is no positive assurance the supply stays off.
- Damp conditions increase the risk of earth faults and shock.
Required steps before inspection:
- Stop the auger and allow it to come to rest.
- Isolate at the submain, lock and tag the isolation point, and prove the circuit dead with a tested voltage indicator.
- Where possible, fit a local isolator so future work can be done safely.
- Only then inspect the auger, motor, and cabling. Have a licensed electrician attend for any work on the fixed wiring or motor connections.
Document the fault and the isolation, and do not return to service until the cause is found and corrected.