Here, we see that the thermal or overload element of the MCCB will initiate a trip in 1,800 sec at 135% of rating (shown here as Point 1), or in 10 sec at 500% of rating (shown here as Point 2). 1, you can see the boundary between the normal and overload conditions. By using a time-current curve, as shown in Fig. If you monitor current flow and time, you can somewhat predict and detect overload conditions. This heat is a function of the square of the rms current (I squared), the resistance in the conductor (R), and the amount of time the current flows (t). If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit. In an overload condition, there's a temperature buildup between the insulation and conductor. Both these devices act mechanically on the trip mechanism.ĭepending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the following conditions: An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. ![]() The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable.
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