The thermal trip
unit protects against a continuous overload. The thermal unit is comprised of a
bimetal element located behind the circuit breaker trip bar and is part of the
breaker’s current carrying path. When there is an overload, the increased
current flow heats the bimetal causing it to bend. As the bimetal bends it
pulls the trip bar which opens the breaker’s contacts.
The Magnetic
trip unit protects against a short circuit. The magnetic trip unit is comprised
of an electromagnet and an armature. When there is a short circuit, a high
magnitude of current passes through the coils creating a magnetic field that
attracts the movable armature towards the fixed armature. The hammer trip is
pushed against the movable contact and the contacts are opened. The opening of
the breakers contacts during a short circuit is complete in 0.5 milli-seconds.
So,both of the tripping units are necessary for a circuit breaker regarding complete protection.
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