The conventional molded case circuit breaker utilizes thermal magnetic or electromechanical trip units which are interchangeable or may be fixed. MCCB can provide a protection by using the combination of an electric sensitive electromagnetic device and a device sensitive to temperature. These two devices can act mechanically on trip mechanism.
The use of MCCB will depend upon the required protection and application. It will use one of different combinations of the trip elements in order to protect against different certain conditions which includes:
- Thermal Overloads – In a thermal overload condition, there is a build-up temperature between the conductor and the insulator. If it is left unchecked, the life of the insulator will reduce drastically that may result to short circuit that’s why a molded circuit breaker is used.
- Short Circuit – This usually occurs when there’s an abnormality on the electric flow which is very high than usual. As a result the insulation of the system fails. In order to stop the immediately stop this kind of current flow to avoid major damages, instantaneous element or short circuit of MCCD is utilized.
- Ground Fault – This is actually categorized as a short circuit, the only difference is that it is phase to ground, which is probably the most common fault type on the low voltage systems.
Usually, the arcing ground fault current aren’t large enough for molded case circuit breaker protected device to be detected. If it is left undetected, the tripping of standard protective device can sufficiently increase. Usually, under this kind of situation, it is already too late and major damages are already done. A good example of this is a motor that have an internal failure on insulation. Even a very small current flow must be identified and eliminated before the major damages on the motor takes place.
The protection devices for ground fault were used nowadays in order to provide an extra protection level. In today’s modern circuit breakers, ground-fault protection device are now integrated as a part of molded case circuit breaker trip unit.
Trip Overload Action
Thermal trip or overload action utilizes a bimetal piece that is heated through load current. Actually, this bimetal is two strips metal that are bonded together, and each of them has different thermal heat rate expansion. They’re not field adjustable but factory calibrated.
Trip Short-circuit Action
Trip short circuit action utilizes an electromagnet that has a winding series with load current. When short circuit occurs, the flowing current through circuit conductor causes the strength of electromagnetic field to rapidly increase and attract more armatures. When this situation happens, the trip bar will rotate due to armature that will cause the molded case circuit breaker to trip.
Trip Thermal Magnetic Action
As what the name implies, this trip action combines the magnetic unit and thermal unit features. As a result the current time curve combines that characteristics performance. A trip thermal magnetic unit is perfectly suited to almost all general purpose application because of its temperature sensitive and automatic safe follow cable and loadings of equipment. These kind of loadings can differ in ambient temperatures. The thermal-magnetic units don’t usually trip if overload condition is not really dangerous, but will instantly trip with very strong short-circuit currents.