IEEE C37.27-2008 pdf free

IEEE C37.27-2008 pdf free.IEEE Application Guide for Low- Voltage AC Power Circuit Breakers Applied with Separately-Mounted Current-Limiting Fuses.
For each circuit breaker on a system there is a maximum fuse rating that will provide the required short- circuit protection for the circuit breaker. Also, there is a minimum fuse rating that will coordinate with the circuit breaker time-current characteristic as well as its continuous-current rating based on temperature rise limits. The selection of a fuse rating within these two limits will provide adequate protection for the circuit breaker and avoid unnecessary fuse opening for fault current within the short-circuit rating of the circuit breaker. Recommendations for establishing the range of fuse ratings to provide the required protection and coordination for a given application should be based on fuse and circuit breaker characteristics as provided in Clause 5.
5. Coordination of circuit breaker and fuse
A current-limiting fuse should be chosen that has operating characteristics that will protect the circuit breaker and will allow the circuit breaker to interrupt overloads and faults within its ratings to avoid unnecessary fuse operation. A fuse of the type recommended by the circuit breaker manufacturer should be selected with a rating that complies with the criteria outlined in 5. 1 and 5.2.
5.1 Maximum fuse rating to be used
The maximum continuous current rating for the fuse that will adequately protect the circuit breaker should be determined by the information outlined in subclauses 5.1.1, 5.1.2, 5.1.3, and 5.1.4.
5.1.1 Current limiting region
The lower limit of the current limiting region (rms symmetrical value) of the fuse (Point A of Figure 1) shall be lower than the circuit breaker short-circuit current rating (unfused) at the specific application voltage and shall extend up to and include the maximum available short-circuit current at the point of application of the system.
5.1.2 Instantaneous peak let-through
In the absence of current limiting fuses, the peak current available is 2.3 times the system available shortcircuit current (rms symmetrical). The unfused circuit breaker is tested to demonstrate that it withstands 2.3 times the 254 V rms symmetrical short-circuit rating when equipped with instantaneous direct-acting trip elements. The preferred values of the 254 V short-circuit current rating (symmetrical) are shown in ANSI C37.16.IEEE C37.27 pdf free download.