Thermal Overload Relay
A motor protection device that opens the circuit by imitating the heating of the conductor under sustained overload. It works with a delay inversely proportional to current: it waits for hours slightly above the rated current, while at several times that it opens within seconds. It does not provide short-circuit protection.
In a classic thermal overload relay the current passes through the junction of two metals with different expansion coefficients. The heated bimetal bends and mechanically opens the contact. In electronic types the same behaviour is produced by current measurement and a thermal model calculation.
- Setting: the setting dial is brought to the motor's nameplate current. Setting it higher leaves the motor unprotected; setting it lower leads to nuisance tripping at start.
- Characteristic: it has an inverse-time curve; at 1.2 times the rated current the trip time can stretch to the order of hours, while at 6 times it is of the order of seconds.
- Its limit: a thermal overload relay is slow. Interrupting short-circuit current is the job of the fuse or the breaker; the two are used together.
A thermal overload relay that trips frequently usually points to a problem in the system rather than in the relay: a jammed mechanism, current increased by low supply voltage, heating caused by a loose terminal, or an inadequate cable cross-section. Before moving the relay one step up, measure the voltage drop on the line under load.
Context in which this term is used: Installation, redundancy and field continuity