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Thermal overload relays


The thermal overload relays up to 100 A have been designed for current-dependent protection of loads with normal starting (see Functions) against excessive temperature rises due to overload or phase failure.
An overload or phase failure results in an increase of the motor current beyond the set motor rated current.
Via heating elements inside the device, this current rise heats up the bimetal strips which then bend and as a result trigger the auxiliary contacts by means of a tripping mechanism. The auxiliary contacts then switch off the load by means of a contactor.
The break time depends on the ratio between the tripping current and operating current Ie and is stored in the form of a long-term stable tripping characteristic (see Characteristics).
The "tripped" status is signaled by means of a switch position indicator (see Functions).
The contactor is either reset manually or automatically (see Functions) after the recovery time has elapsed (see Functions).
They comply with important worldwide standards and approvals

Thermal overload relay components

Thermal overload relay components (SOURCE: Siemens)