electrical engineering construction

The following animation shows how a conventional thermal/magnetic circuit breaker works. Current flowing through the circuit heats the bimetal current sensor, causing it to bend. When the amount of bending exceeds a limit, the armature is released and a spring forces the contacts to open. The load current also flows through a magnetic current sensor (coil) which creates a magnetic field that will trip the armature faster than the bimetal strip can respond when very large currents flow. When the contacts open, an arc will be generated that is dissipated in the arc channel. This opening to the outside prevents pressure buildup inside the breaker's case.
The conventional circuit breaker's performance is limited by:

1. Slow response time of bi-metal current sensors.
2. Mechanical tolerances that affect both the thermal and the magnetic trip limits.
3. Inability to differentiate between arcing and normal start up transients due to large motors or incandescent lamps.
4. The magnetic trip threshold must be set high to prevent nuisance tripping because:
   
   (a) normal transients require a time delay, not available with magnetic sensing, and
   (b) there is a strong interaction between thermal history (bi-metal strip position) and magnetic sensitivity.