Description
2.7 Thermal-magnetic trip unit
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
85
2.7
Thermal-magnetic trip unit
A thermal-magnetic trip unit consists of a thermal trip unit for protecting against overload,
and a magnetic trip unit for protecting against short circuits. Both trip units are series-
connected.
2.7.1
Thermal trip unit (L)
The thermal trip unit consists of a temperature-dependent bimetal that heats up as a result of
the flow of current. This means the trip unit is current-dependent. The temperature rise in the
bimetal strip depends not only on the current magnitude, but also on the ambient
temperature of the molded case circuit breaker. All current values specified for thermal-
magnetic trip units of 3VA circuit breakers refer to an ambient temperature of +50 ℃.
2.7.2
Magnetic trip unit with short-circuit protection (I)
The magnetic trip unit with short-circuit protection comprises a yoke mounting through which
a current path runs, and a flap armature that is kept at a distance from the yoke mounting by
a tension spring.
If a short-circuit current flows along the current path, the magnetic field thus generated
causes the flap armature to be moved towards the yoke mounting against the opposite force
of the tension spring. The tripping time is almost current-independent and instantaneous.
The flap armature releases the breaker mechanism and thus opens the switching contacts.
Immediately after tripping, the flap armature is moved back to its starting position by the
restoring force of the tension spring.
