I came across a time current curve the other day that threw me for a loop and was hoping to get some feedback. Knowing that panel boards and switchboards [UL-891] are constructed to withstand rated fault current for 3-cycles, the OCPD for this installation was an ICCB with electronic trip characteristics located upstream of the MLO switchboard. When looking at the TCC it appears the ICCB has an instantaneous function and since it cannot be disabled but selective coordination was deemed important the setting was dialed up beyond the available fault current to basically disable the instantaneous function. With the short time delay band set at its minimum the time to clear exceeds 3 cycles. The downstream switchboard is rated at 35kAIC with an available fault current of around 18K. Now the questions is on a TCC plot there seems to be the ability to plot multiple types of components, for example cable damage curves, bus damage curves, obviously breaker trip curves, transformer damage curves, generator damage curves, etc. Is there or has there been anything developed to determine a ?bus bracing? curve? So one can compare the available fault current at different levels and determine if the equipment downstream is adequately protected? Similar to a generator decrement curve and the damage curve of the genset windings, we typically see breakers set to be able to trip based on the decrement curve provided by the generator manufacturers without damaging the windings of the genset?
Panelboards & Short Time Protection
- Thread starter Mike01
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- Wisconsin
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- PE (Retired) - Power Systems
Bus bracing has to include about magnetic forces as well as heat build up, show it would not be compatible with a TCC.
The consenus I have heard, is that the UL standards for breakers and panels/switchboards so compatible that a UL489 breaker will always protect UL891 bussing regardless of the breaker settings. This is true even if the INST function has been turned 'off', however it does not apply to UL1066 breakers or UL1558 switchgear.
The consenus I have heard, is that the UL standards for breakers and panels/switchboards so compatible that a UL489 breaker will always protect UL891 bussing regardless of the breaker settings. This is true even if the INST function has been turned 'off', however it does not apply to UL1066 breakers or UL1558 switchgear.
ron
Senior Member
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- New York, 40.7514,-73.9925
Consider an ICCB with a fixed instantaneous setting on the trip unit, or a MCCB with no solid state trip unit, which would be likely Inst 10-12x. I don't think there would be anyone requiring you to provide an adjustable Inst setting, so I think it would be ok to put the adjustment and set it high.
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