Thermal overload relays: setting the dial
Industrial Control · Lesson 8 · Advanced
Purpose
Set a thermal overload relay from motor, connection, duty and coordination data, not by adding arbitrary starting margin.
Learning objectives
- Trace main and control paths
- Select from declared utilization data
- Coordinate protection and control
- Identify dangerous failure states
A thermal overload relay models motor heating and trips on sustained overcurrent and often phase-loss/unbalance response. It must tolerate legitimate starting while protecting against damaging thermal load.
Core theory
Establish where sensing elements sit relative to star/delta connections, the motor nameplate current for the actual connection, service factor/manufacturer instructions and ambient compensation.
Choose trip class from motor acceleration time and withstand data. Raising the setting to avoid nuisance trips can leave the motor unprotected; resolve the actual start/load/setting mismatch.
Overload protection does not provide the required short-circuit interruption. Coordinate the relay, contactor and upstream fuse/breaker using declared Type 1/Type 2 or applicable combination data.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Trip class | Declared overload operating-time characteristic under specified multiple of current | Not applicable | Not applicable |
| Phase loss | Loss of one supply phase causing damaging current/unbalance | Not applicable | Not applicable |
| Manual reset | Reset requiring deliberate local action after trip | Not applicable | Not applicable |
Assumptions: All ratings are supplied fictional design data; The machine risk assessment and manufacturer instructions remain governing inputs.
- Basis: Use the supplied direct-line sensing arrangement.
- Set: Initial setting = 1.00 × 18 A = 18 A.
- Commission: Verify load current, phase balance, starting time, trip class and manufacturer rules; do not add arbitrary margin.
Initial supplied setting is 18 A, subject to complete commissioning evidence.
Reasonableness check: The result is checked against electrical duty, starting behavior and protective boundaries rather than accepted from one nameplate number.
- Using motor kW alone to select switching equipment
- Treating an overload relay as short-circuit protection
- Assuming a stopped motor or dark display is isolated
Where this appears in practice
Industrial motor control must coordinate the motor, switching device, short-circuit protection, overload protection, control voltage, driven load and machine safety functions.
Knowledge check
Should the overload dial be increased merely because the motor trips during a long start?
No. Investigate load torque, acceleration, phase condition, connection, trip class and motor/starter suitability.
Answer: No. Investigate load torque, acceleration, phase condition, connection, trip class and motor/starter suitability.
Practical exercise
Select settings and trip classes for supplied motors with different start times and relay locations.
Summary
- Main and control circuits have different jobs
- Every rating has a declared duty and condition
- Protection, stopping and isolation are distinct functions
Sources and review
- IEC 60947-4-1:2023: Electromechanical contactors and motor-starters: IEC; 2023, corrected version 2026-03; International
- IEC 60204-1:2016+AMD1:2021: Electrical equipment of machines: IEC; Consolidated edition 6.1; International
- IEC 61800-5-1:2022: Adjustable-speed drive safety: IEC; 2022 with current corrigenda; International
Editorial review date: 2026-08-22. Professional electrical review is pending.