IEEE C37.27-2015 pdf download

IEEE C37.27-2015 pdf download.IEEE Guide for Low-Voltage AC (635 V and below) Power Circuit Breakers Applied with Separately- Mounted Current-Limiting Fuses
current-limiting (peak let-through or cutoff) characteristic curve: A curve showing the relationship between the maximum peak current passed by a device and the correlated rms available current magnitude under specified voltage and circuit impedance conditions.
current-limiting fuse: A fuse that, within a specified overcurrent range, limits the clearing time at rated voltage to an interval equal to or less than the first major or symmetrical current loop duration; and limits the peak current to a value less than the available peak current.
NOTE—The values specified in standards for the threshold ratio, peak let-through current, and I 2 t characteristics are used as the measures of current-limiting ability. current-limiting range (of a current-limiting fuse): That specified range of currents between the threshold current and the rated interrupting current within which current limitation occurs.
fuse: An overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of overcurrent through it.
NOTE—A fuse comprises all the parts that form a unit capable of performing the prescribed functions. It may or may not be the complete device necessary to connect it into an electric circuit. low-voltage fused power
circuit breaker: An assembly of an ac low-voltage power circuit breaker with either integrally mounted or nonintegrally mounted current-limiting fuses that together function as a coordinated protective device.
low-voltage integrally fused power circuit breaker: An assembly of an ac low-voltage power circuit breaker with current-limiting fuses integrally mounted to the circuit breaker and that function together as a coordinated protective device.
low-voltage nonintegrally fused power circuit breaker: An assembly of an ac low-voltage power circuit breaker and nonintegrally mounted current-limiting fuses that together function as a coordinated protective device.
low-voltage power circuit breaker applied with separately mounted current-limiting fuses: Any combination of a low-voltage power circuit breaker and current-limiting fuses other than integrally and nonintegrally fused ac low-voltage power circuit breakers as defined by IEEE Std C37.13.
peak let-through cutoff current (of a current-limiting fuse): The highest instantaneous current passed by the fuse during the interruption of the circuit.
total clearing time of a fuse: The time elapsing from the beginning of a specified overcurrent to the final circuit interruption, at rated maximum voltage.
NOTE—The clearing time is equal to the sum of melting time and the arcing time.
4. General
Combinations of low-voltage ac power circuit breakers and current-limiting fuses provide a broader range of circuit protection than either device can provide alone. Such combinations, when properly selected, can retain the versatility of switching and overload protection that circuit breakers can provide, and be applied on systems with available short-circuit current up to 200 000 A (rms symmetrical), which may be appreciably higher than the short-circuit current rating of unfused circuit breaker.
Fuses with identical continuous current ratings per NEMA FU-1 made by different manufacturers exhibit a range of clearing I 2 t, temperature rise, time-current, maximum peak let-through current, and arc voltage characteristics. Therefore, it is not feasible to select a circuit breaker-fuse combination by continuous current ratings alone. The degree of protection provided for a circuit breaker, by a fuse at current levels above the unfused circuit breaker short-circuit current rating will may vary with different values of maximum clearing I 2 t, maximum peak let-through current, and arc voltage of the fuse.
At short-circuit current levels slightly above its the short-circuit current rating of the circuit breaker, the duty on a circuit breaker may be more severe than at very high short-circuit current levels. For a specific fuse type, the use of peak current as a criterion is only an approximation for the combined effects of maximum clearing I 2 t, maximum peak let-through current, and arc voltage of the fuse.IEEE C37.27 pdf download.IEEE C37.27-2015 pdf download