wroggent
Electrical
- Aug 20, 2012
- 288
When reading about series ratings I often see mention of fuse-fuse, breaker-breaker, and fuse-breaker series ratings. Two questions about this:
1.) This first question is in regards to the NEC specifically:
Since series ratings are mentioned under part VII. of Article 240 and not under sections V. or VI., does this mean that series ratings only exist when the load-side device is a circuit breaker. In other words, fuse-fuse series ratings do not exist or rather are not permitted by the NEC even if there was some tested combination that passed a UL test for series ratings?
2.) Can current limiting fuses be used to allow the use of bus that has a withstand rating lower than the available short circuit current.
For example, from a NEC requirement and technical soundness perspective, would the following scenario be adequate:
UL891 Switchboard
100kA 3 cycle bus bar withstand rating
200kA interrupting rating current limiting fuse upstream of bus bar (main disconnect)
200kA interrupting rating protective device downstream of bus bar (branch disconnect)
150kA available short circuit current on line side of main disconnect
And for argument sake assume the peak let through current of the current limiting fuse is less than 100kA.
My thoughts are from a strict code perspective this would not be allowed because the short circuit current rating (SCCR) 'shall not exceed the interrupting rating or short-circuit current rating of any component in the power circuit' (from NEMA PB 2-2011) and this, in my opinion, specifically includes (among possibly other things): individual protective device interrupting ratings, interrupting ratings for series rated protective devices (see question 1 above), and bus bar withstand ratings. Thus, the switchboard SCCR would be 100kA due to the withstand rating. From a technical perspective, and where the NEC or other code did not apply, I think some people may find the installation acceptable because the bus bars are a 'passive device' not subject to dynamic impedance thus the 'up-over-down' method could be used. Thoughts?
1.) This first question is in regards to the NEC specifically:
Since series ratings are mentioned under part VII. of Article 240 and not under sections V. or VI., does this mean that series ratings only exist when the load-side device is a circuit breaker. In other words, fuse-fuse series ratings do not exist or rather are not permitted by the NEC even if there was some tested combination that passed a UL test for series ratings?
2.) Can current limiting fuses be used to allow the use of bus that has a withstand rating lower than the available short circuit current.
For example, from a NEC requirement and technical soundness perspective, would the following scenario be adequate:
UL891 Switchboard
100kA 3 cycle bus bar withstand rating
200kA interrupting rating current limiting fuse upstream of bus bar (main disconnect)
200kA interrupting rating protective device downstream of bus bar (branch disconnect)
150kA available short circuit current on line side of main disconnect
And for argument sake assume the peak let through current of the current limiting fuse is less than 100kA.
My thoughts are from a strict code perspective this would not be allowed because the short circuit current rating (SCCR) 'shall not exceed the interrupting rating or short-circuit current rating of any component in the power circuit' (from NEMA PB 2-2011) and this, in my opinion, specifically includes (among possibly other things): individual protective device interrupting ratings, interrupting ratings for series rated protective devices (see question 1 above), and bus bar withstand ratings. Thus, the switchboard SCCR would be 100kA due to the withstand rating. From a technical perspective, and where the NEC or other code did not apply, I think some people may find the installation acceptable because the bus bars are a 'passive device' not subject to dynamic impedance thus the 'up-over-down' method could be used. Thoughts?