If you have a three phase bolted fault current of approx 30Ka, How do you calculate how long (sec. or cycles) the bus will withstand before damage occurs? Or if the bus would be able to withstand so many cycles prior to the over current device opening. I know that when dealing with switchgear / switchboard bus there are mechanical stresses and thermal stresses, ?sandwiched bus?, ?insulated bus? all that have different affects on either stress I was just hoping someone could point me in the right direction on where to start. Thanks.
Bus Bracing
- Thread starter Mike01
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- Lockport, IL
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- Semi-Retired Electrical Engineer
I once knew (and could possibly be able to retreive, if it would help) a formula for the amount of force that would be imparted onto one bus bar by the fault current flowing in another bus bar. But I don't think there is a formula to give you what you are asking for. The length of time a bus bar can withstand the forces imposed upon it is mostly dependent on the manner in which it is attached, the strength of the bolts holding it in place, the exact combination of its materials of construction, the degree to which it was "work-hardened" during its manufacturing process, its dimensions, and a host of other mechanical and structural considerations.
Suppose....
Suppose....
A topic of discussion in our office today: A facility with critical / life safety loads that need to be selectively coordinated. Incoming service is switchboard construction UL-891 (3-cycle rating) available fault current on the board 30Ka in order to coordinate the main (3000AF/2500AS/2500AT) with the feeders the instantaneous was pushed out past the available short circuit. Now the TCC would indicated that the main will not begin to operate and the minimum unlatching time is 0.1 sec approx 6-cycles, with a maximum unlatching time of 0.18sec approx. 11-cycles. My question or inquiry is if the switchboard is 35kaic rated than it is designed to withstand 35Ka of fault current for the 3 cycles for the breaker to trip, now what happens when I have a 35Ka fault for up to 10 cycles with the bus withstand the 10 cycles without major damage? The switchboard manufacturer that was contacted recommended putting in a 65Ka board and it would be able to withstand the 30Ka for more time than the 35Ka allowing for the main to trip and the bus still be protected (basically de-rating the switchboard AIC rating). Does this make any sense? This is why I wanted to find the correlation between the amount of time for a given fault current and bus bar damage curves.
Suppose....
A topic of discussion in our office today: A facility with critical / life safety loads that need to be selectively coordinated. Incoming service is switchboard construction UL-891 (3-cycle rating) available fault current on the board 30Ka in order to coordinate the main (3000AF/2500AS/2500AT) with the feeders the instantaneous was pushed out past the available short circuit. Now the TCC would indicated that the main will not begin to operate and the minimum unlatching time is 0.1 sec approx 6-cycles, with a maximum unlatching time of 0.18sec approx. 11-cycles. My question or inquiry is if the switchboard is 35kaic rated than it is designed to withstand 35Ka of fault current for the 3 cycles for the breaker to trip, now what happens when I have a 35Ka fault for up to 10 cycles with the bus withstand the 10 cycles without major damage? The switchboard manufacturer that was contacted recommended putting in a 65Ka board and it would be able to withstand the 30Ka for more time than the 35Ka allowing for the main to trip and the bus still be protected (basically de-rating the switchboard AIC rating). Does this make any sense? This is why I wanted to find the correlation between the amount of time for a given fault current and bus bar damage curves.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
As Charlie pointed out, it all depends on how the switchboard was constructed. You need to check with the manufacturer. Many times they have a design level for bus bar bracing that is well in excess of the SCCR applied to the equipment. The switchboard design may be for a minimum of 100K, but the label shows the lowest AIC rating of the branch devices installed at the time it was shipped.
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