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Fuses vs. Circuit Breakers

The National Electrical Code® (NEC®) specifies Motor Branch-Circuit Protection in Article 430, Part IV. BFS designs control panels using fuses or circuit breakers, based on the NEC® requirements. BFS typically uses Class CC, time-delay fuses for branch circuits up to 10 amps and Class J, time-delay fuses for branch circuits rated higher. (Class CC fuses are considered nontime-delay when calculating the fuse rating per the NEC®.) For motor protection, the smallest available inverse-time circuit breaker is 15-amp. BFS does not typically use instantaneous trip circuit breakers.

The basic BFS System Control Center is designed with fuse blocks. Fuses are available from the factory, or may be added by the installer or end user. Factory installation ensures that the correct fuse is used, and avoids any procurement issues in the field. Some users prefer to purchase fuses locally, allowing them to choose a particular manufacturer. In any case, the recommended fuses (rating and type) are indicated on the BFS schematic.

Fuses provide excellent motor protection at a relatively low cost. During a fault condition, fuses protect the branch circuit faster than circuit breakers. A fuse element can melt faster than a circuit breaker can open its contacts. Also, the fuse rating is typically less than the circuit breaker rating for a given motor, based on the NEC®.

Another advantage of fuses is their high interrupting rating. During a short circuit, extremely high currents can pass through the branch circuit while the protection device is opening. This can weld the contacts on a circuit breaker, preventing it from opening properly. Class CC fuses have an interrupting rating of 200,000 amps symmetrical. Class J fuses are available with an interrupting rating of 300,000 amps symmetrical. By using fuse protection, equipment can be connected directly to the main power feed in a building.

Despite their higher cost and slower response time, circuit breakers have advantages as well. Circuit breakers are much easier to troubleshoot than fuses. When external handles are installed, it is obvious when a circuit breaker has tripped. The circuit may then be locked out for motor maintenance. Upon completion, there is no need to procure the proper fuses or install them. During this down time, other circuits may remain on without any risk of shock to personnel.

National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association.