Trip class is not a label to copy from the last project. It describes relay operating behaviour under defined test conditions; the project still has to show that the selected device protects this motor on this load.
Put the curves on one time basis
| Input | Required case | Project evidence |
|---|---|---|
| Motor acceleration | Normal and worst credible start | Calculated or measured current-time trace |
| Supply condition | Minimum starting voltage and relevant source configuration | Power-system study |
| Driven load | Starting torque, inertia and process state | Load data and commissioning procedure |
| Motor thermal limit | Cold and hot withstand information where supplied | Manufacturer-approved motor data |
| Relay response | Selected class, pickup, phase-loss behaviour and thermal memory | Device curve and setting sheet |
| Repeated starts | Permitted sequence and restart lockout | Motor data and control narrative |
Compare curves using consistent current bases and CT ratios. A start time measured at one process condition should not be used to approve a heavier start without review.
Investigate margin, not just non-tripping
The start curve should clear the relay curve with a documented margin that accounts for voltage variation, process condition and model uncertainty. At the same time, the protection curve should remain inside the motor’s permitted thermal exposure for credible overload and stall conditions. If these objectives conflict, changing trip class alone is not a complete solution; the starting method, load condition or motor selection may need attention.
Approval record
- Record cold and hot starting cases and the source of each curve.
- Confirm pickup, trip class, CT ratio and thermal-memory settings.
- Test the logic for failed acceleration, stall and restart inhibition.
- Preserve the commissioning current-time trace for comparison.
For a separate stall review, see Locked-Rotor and Stall Protection. Protection changes should remain under the site’s authorised settings process.