NEMA Definition – A circuit breaker is defined in NEMA standards as a device designed to open and close a circuit by non-automatic means, and to open the circuit automatically on a predetermined overcurrent, without injury to itself when properly applied within its rating.
ANSI Definition – A circuit breaker is defined in ANSI standards as a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions, and also making and carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit.
The molded case circuit breaker is one of the two basic low voltage classes of circuit breakers. The other class is the low voltage power circuit breaker, which is covered in Module 7.
What Are Molded Case Circuit Breakers?
Molded case circuit breakers are tested and rated according to the UL 489 Standard. Their current carrying parts, mechanisms and trip devices are completely contained within a molded case of insulating material.
MCCBs are available in various frame sizes with various interrupting ratings for each frame size.
Molded case circuit breakers are designed to provide circuit protection for low voltage distribution systems. They protect connected devices against overloads and/or short circuits.
They are used primarily in panelboards and switchboards where they are Fixed Mounted. Some of the larger MCCBs are available in Drawout Mount design.
Molded case circuit breakers are available with special features making them suitable for the protection of motor circuits when used in conjunction with a separate overload protection device. In these applications, they are often referred to as motor circuit protectors (MCPs).
