i have some 208v panels that have a trip rating of 65KA how would i go about figuring out the arc flash rating?
The 65kA trip rating that you mention is likely the interrupting rating of the panel or breaker. This is to say that the equipment is rated to withstand a short circuit condition of up to 65kA without totally destroying the equipment. That is to say, how a fault condition would affect the equipment. The arc flash value is primarily dealing with how a fault condition would affect a worker in the vicinity of the fault.
There are basically two methods used.
(1) Calculation Method.-- An incident energy analysis will provide you with the arc flash value.
(2) Table Method. -- NFPA 70E 2015 Table 130.7(C) (15)(A)(b) will provide you with an acceptable PPE Category base on typical system values...providing that your system fits within the parameters set, however it will not provide you with the arc flash value.
In order to calculate an arc flash value (expressed in cal/cm2) you would need to know the amount of available fault current the system can deliver, (magnitude) the time that it would take for the protective devices to clear the fault, (duration) and the distance that the worker is from the fault should it occur. Many other system details must be taken into account to calculate the arc flash value. In lieu of an incident energy analysis you could use Table 130.7(C) (15)(A)(b) to determine the PPE category but you will still need to know the magnitude, duration and distance to see if your system fits within the parameters set forth in the table.
I would be more than happy to discuss this further if I did not address you question to your satisfaction.
Stephen Benton, CESCP
Retrieving data ...