IEEE C37.27-1987 pdf free.IEEE Standard Application Guide for Low-Voltage AC Nonintegrally Fused Power Circuit Breakers (Using SeparatelyMounted Current-Limiting Fuses)
IEEE C37.27 low-voltage ac power circuit breakers and current-limiting fuses provide a broader range of circuitprotection than either device can provide alone. Such combinations, when properly selected, can retain the versatilityof switching and overload protection which circuit breakers can provide, and can be applied on systems with availableshort-circuit current up to 200 000 A (rms symmetrical), which is appreciably higher than the short-circuit currentrating of circuit breakers.
Fuses with identical continuous current ratings per ANSI C97.1-1972(R 1978)[3] made by different manufacturersexhibit a range of clearing 1’t, temperature rise, time-current,maximum peak let-through current, and arc voltagecharacteristics. Therefore, it is not feasible to select a circuit breaker-fuse combination by continuous current ratingsalone.The degree of protection provided for a circuit breaker by a fuse at current levels above the circuit breaker short-circuit current rating will vary with different values of maximum clearing 1 t, maximum peak let-through current, andarc voltages of the fuse.The duty on a circuit breaker may be more severe at short-circuit current levels slightly aboveits short-circuit rating than at very high short-circuit current levels. For a specific fuse type, the use of peak current asa criterion is only an approximation for the combined effects of maximum clearing l’t, maximum peak let-throughcurrent, and arc voltage of the fuse.
The criteria necessary for making the proper fuse selection are as follows:
1)Continuous current rating of the circuit breaker and fuse
2)Available short-circuit current level at the application point
3)Short-circuit current rating of the circuit breaker
4)Time-current characteristics of the circuit breaker and the fuse
5)Peak let-through current characteristic of the fuse
For each circuit breaker on a system there is a maximum fuse rating that will provide the required short-circuitprotection for the circuit breaker.Also, there is a minimum fuse rating that will coordinate with the circuit breakertime-current characteristic as well as its continuous-current rating based on temperature rise limits. The selection of afuse rating within these two limits will provide adequate protection for the circuit breaker and avoid unnecessary fuseopening for fault current within the short-circuit rating of the circuit breaker. Recommendations for establishing therange of fuse ratings to provide the required protection and coordination for a given application should be based onfuse and circuit breaker characteristics as provided in Section 4.IEEE C37.27 pdf download.