Why Thermal & Magnetic trip units both in Circuit Breaker ?

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.