For protection of non-motor circuits.
For protection of motor circuits.
We will discuss protection of non-motor circuits first.
Protecting Non-Motor Circuits
Non-motor circuit applications usually center around cable protection and require molded case circuit breakers with both overload and short circuit capabilities. They should be able to distinguish between harmless and destructive conditions, and function appropriately for its application. It is very important that the MCCB selected be adequately rated and equipped for all the electrical and physical conditions that are likely to exist when the system is energized.
The standard selection factors for molded case circuit breakers include:
- Voltage Rating
- Frequency
- Continuous Current Rating
- Interrupting Rating
- Number of Poles
- Fixed or Interchangeable Trip Unit
- Trip Unit Functions
- Accessories
Protecting Motor Circuits
The selection of an HMCP is based on the full load current of the motor it is to protect.
Data shown in the National Electric Code (tables 430-148 and 430-150) list the full load currents of induction motors running at speeds normal for belted motors and with normal torque characteristics. However, actual motor nameplate ratings should be used for selecting the motor running overload protection.
Other considerations in the selection of HMCPs include:
- The ambient temperature outside the enclosure should not exceed 40°C (104°F).
- Infrequent starting, stopping and reversing of the motor.
- Motor accelerating time of 10 seconds or less.
- Locked rotor rating is a maximum of six times the motor FLA rating.
