Arc Flash Protection Boundary question

NFPA70E (2009) Article 130.3(A)(1) Voltage Levels between 50V & 600V.....The Arc Flash Protection Boundary shall be 4 ft, based on the product of clearing time of 2 cycles and the available bolted fault current of 50kA......

Is this saying if we use 4ft as the boundary, we don't have to run calculations. Incoming voltage is 480V & AmerunUE says the available Fault current is 3800A and the Bolted Fault Current is 3700 to our building. I'm not sure I understand what they are infurring to in this particular paragraph. Can anyone explain. Should calculations be done at every panel if you want the AFPB to be less than 4 ft?

Well what are your clearing times?

The clearing time for the main breaker is 1cycle according to the time current curves at the fault current level stated by UE.

No such thing as a 1 cycle breaker. Is this fused? What is the clearing time of the calulated arcing current?

Zog – In the Time-Current curve that I have in front of me the Instantaneous Trip zone for the particular breaker is from .001 seconds to .015 sec (approx 1 cycle) the current range is 27,00A-65,000A for the instantaneous trip…… then the curve move to less current to a more infinite time. Doesn’t that mean the bolted fault current of 3700A is within the 1 cycle trip for this breaker?

BrainFiller – if we can meet the calculation and establish the minimum 4 ft zone for people clearance from the panel we would like to. Am I reading the information correctly…. For example this particular Time Curve……. If I look at the min and max at 1 second the values are approx 120Amin and 300Amax so for this particular breaker (if it was the main) the breaker would trip somewhere between 120-300A in 1 second and because it is less than 1667A no other calculations would need to be done and we could use the 4 ft minimum distance for unprotected personnel. Or do I look at the values at 2 cycles (.033) which would be between 500A min or 1200A max?

I am unsure which I should use. We have many machine panels and building panels that I need to collect data on for the main breaker and wanted to start my calculations correctly to see if it qualifies for the 4 ft min distance. Any help would be appreciated as to whether I am in the right ball park!!!

If the available bolted fault current is 3800A (is this a typo?) it is nowhere near the breaker's instantaneous trip of 27,000A. If the utility actually said it is 38,000A for a bolted fault, it is probably a 'made up' number (because it looks to be rounded) used for equipment selection purposes and should not be used for an arc flash analysis.

Also, when evaluating the clearing time of the device, you need to use the arcing fault current that flows through the device, not the bolted fault current available at its line side terminals.

What type of breaker is this?

The NFPA70E specifically says Bolted Fault Current (see above for section)....... the fault current numbers and Bolted fault number was given to me by Plant Manager from the Utility Company...... does the 3700A BFC seems unrealistic........ our servive to the building MAX current is 1800A provided by transformer is what I was told. We are just a moderate size manufacturing plant. The main breakers in our distribution panels are all Siemens of assorted sizes.

Just a guess, but sounds that if your service is 480/277 Y then you might be served from a 1500 kVA transformer based on the 1800 amps you mentioned. Is this the case? 3,700 sounds way too low and 37,000 sounds like it might be infinite source for a 1500 kVA transformer with %z of about 5%.

I would definitely verify the current numbers.

That does nto help, what type of Siemens breakers from the nameplate, also what is the trip unit catalog #?

I also agree with the other guys about your fault current levels being off and the fact you need to look at arcing current too.

NFPA 70E says "When the product of clearing times and bolted
fault current exceeds 100 kA cycles, the Arc Flash Protection Boundary shall be calculated." It does not specifically say what fault current is used to determine the opening time. I agree, you can make the case for using the clearing time of the bolted fault.

Yes, the value of 3,700A SCA is unrealistic for any service much larger than about 100A.

Back to the beginning......

Thank you for your responses........ I will start again with getting some reasonable data from the utility company regarding available fault current, bolted fault current, and more information on the transformer supplying our building. I am collecting data on the main power incoming swith board and all the subpanels and circuit protection. Maybe in a few days I'll have enough information I can ask more specific questions. Is there anything else any of you are aware of that I should seek out before trying to determine whether we can apply the 4Ft rule boundary to all of our subpanels?