I do not have a copy of UL 891 so I was hoping someone could help me out trying to understand it. Switchboards designed to meet UL 891 are required to withstand the braced fault current for 3 cycles min, UL 489 molded case / insulated case circuit breakers the instantaneous range does not exceed 0.05 (3 cycles), but what does the UL 891 say about the bracing that is has to withstand the current for 3 cycle for the breaker to clear? What if you had a project where the instantaneous dial was up at 12 and the max available fault current was 28000 it would be adjusted where the instantaneous is basically turned off because it would be way above the available fault current therefore if a fault was located on the bus (3 phase bolted) the breaker would not trip until the short time (around 0.1 sec approx. 6 cycles) thus destroying the board, how does UL rate the gear for a line to ground, or line to line fault that may happen for longer than 3 cycles before the breaker / fuse clears? If you gear is braced for say 35Kaic how long could it withstand a 22Ka fault before damage is sustained? If it were below the breaker trip setting? Thanks in advance for you help.
Switchboards and UL891
- Thread starter Mike01
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I do not have a copy of UL 891 so I was hoping someone could help me out trying to understand it. Switchboards designed to meet UL 891 are required to withstand the braced fault current for 3 cycles min, UL 489 molded case / insulated case circuit breakers the instantaneous range does not exceed 0.05 (3 cycles), but what does the UL 891 say about the bracing that is has to withstand the current for 3 cycle for the breaker to clear? What if you had a project where the instantaneous dial was up at 12 and the max available fault current was 28000 it would be adjusted where the instantaneous is basically turned off because it would be way above the available fault current therefore if a fault was located on the bus (3 phase bolted) the breaker would not trip until the short time (around 0.1 sec approx. 6 cycles) thus destroying the board, how does UL rate the gear for a line to ground, or line to line fault that may happen for longer than 3 cycles before the breaker / fuse clears? If you gear is braced for say 35Kaic how long could it withstand a 22Ka fault before damage is sustained? If it were below the breaker trip setting? Thanks in advance for you help.- Status