The debate on 0.01 selectivity

[mshields]

Does anyone know of any articles on this subject arguing against the merits of coordinating to 0.01 seconds. I.e. perhaps discussing the rational for NFPA 99 and Article 517 have done away with the requirement as compared to Article 700 and 701 and 708 where it is still required.

 

[publicgood]

517 is typically a much larger system then 700 and 701 where 0.01 is less practical. The point of 0.1 in 517 is for overloads - not as concerned about bolted faults; maybe since maintenance in these applications is better established by single management.

708 is a different animal and really should consider every possible condition down to 0.01 (IMO).

 

[ron]

In NYC, I've heard that the amendment to 0.1 was good lobbying by the circuit breaker manufacturers, since the original requirement to 0 seconds was by the fuse manufacturers.

Coordination (Selective) Add a new sentence at the end of the definition of Coordination (Selective) to read as follows:
For the purposes of this code two overcurrent protective devices shall be deemed selectively coordinated if their respective time-current characteristic curves do not intersect at a time of 0.1 seconds (6 cycles on 60 Hz systems) or longer.