شرح MCB
A Miniature Circuit Breaker (MCB) is a resettable protective device that prevents electrical circuits from catching fire and causing damage to personnel and property. It is a device designed to isolate a circuit during an overcurrent event without using a fusible element.
There are two types of overcurrent events; a thermal overload and a short circuit.
Thermal Overload: A thermal overload is a slow and small overcurrent situation that causes the ampacity and temperature of the circuit to gradually increase. This type of event is characterized by a slight increase in the load (ampacity) on the circuit and is interrupted by the thermal trip unit of the breaker.
Short Circuit: A short circuit is an intense overcurrent situation that causes the ampacity of the circuit to increase. This type of event is characterized by a dramatic increase in the load (ampacity) on the circuit and is interrupted by the magnetic trip unit of the breaker.
MCB’s tripping characteristics are represented graphically in a trip curve chart. The chart shows the response of the thermal and magnetic trip element to various overload and short circuit situations.
Common MCB Trip Curves
There are several types of MCB curves that manufacturers provide for applying circuit protection in different applications. The most common curves are B, C, and D. One MCB manufacturer also produces a K and Z curve.
B curve breakers: Trip between 3-5 times rated current in a short circuit situation. B curve MCBs should be applied where loads are resistive and do not have in-rush current. The ideal application is lighting or electronic circuits.
C curve breakers: Trip between 6-10 times rated current in a short circuit situation. C curve MCBs should be applied where the loads have a small amount of in-rush current on start-up. The ideal application is a circuit with a small transformer load.
D curve breakers: Trip between 10-15 times rated current. D curve MCBs should be applied where loads have a high level of in-rush current on start-up. The ideal application is a circuit with a motor load.
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