The traditional molded case circuit breaker uses thermal magnetic trip units that may be fixed or interchangeable. An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism.
Depending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the following conditions:
In an overload condition, there’s a temperature buildup between the insulation and conductor. If left unchecked, the insulation’s life will drastically reduce, ultimately resulting in a short circuit. This heat is a function of the square of the rms current (F), the resistance in the conductor (R), and the amount of time the current flows (t).
If you monitor current flow and time, you can somewhat predict and detect overload conditions.
Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used.
■Ground fault condition.
A ground fault actually is a type of short circuit, only it’s phase-to-ground, which probably is the most common type of fault on low-voltage systems (600V or less).
Usually, arcing ground-fault currents are not large enough to be detected by the standard MCCB protective device. But, if left undetected, they can increase sufficiently to trip the standard protective device. When this happens, it usually is too late, and the damage is already done. An example of this is a motor having an internal insulation failure. While the current flow may be small, it must be detected and eliminated before major motor damage takes place.