A recent thread this past week lead to this week's question:

Long device clearing times can lead to some "interesting" results for arc flash studies. One area is with the arc flash boundary. Although a 2 second cut off might be used, it is not used all the time by everyone. Here is this week's question:

What is the largest arc flash boundary that you have calculated/seen/heard of?

Less than 20 feet
21 to 100 feet
101 to 500 feet
over 500 feet

Stories Welcome!

One of my personal best:
ARC FLASH BOUNDARY (in): 156,231
INCIDENT ENERGY (cal/cm2): 4136

In the old days using 1000sec cutoff. 2.5 mile AFB. Good thing substation was on the shore of a lake, we only had to worry about unqualified people in one direction.

Well I can’t beat a 2.5mile AFB but here is my biggest AFB in the short time I have been busy with arc flash analyses.

6 parallel 690V generators feed main switchboard

AFB = 10210 mm = 33.5 feet
Incident Energy = 195 cal/cm2
Trip time = 0.632 s

If you enter the room, you are in the arc flash boundary. Well the doors are usually closed (at least I hope for the people that have to spend time in the room). Installation was build in ’04 so no arc flash requirements for the maritime sector back then.

Incident energy is based on a static current while in reality the current is very dynamic.

We have a station in Lebanon, OH that has a 69kv incoming line. Because our technicians need to operate the manual 3 gang air break switch we asked the engineering firm performing our analysis at the station to calculate the hazard on our substation. At the switch they calculated 5138cal/cm^2 with a boundary of 120ft. The fault current was 15,500ka and they calculated 2sec clearing time instead of using the utility companies relay settings. They use SKM software so we contacted SKM to find out if their software will work accurately at voltages above 15kv and they told us they use the Lee Equations at those voltage levels. They biggest factor in this energy was the 2 sec clearing time. Had they used the actual relay settings from the utiltiy company, this woudl have been considerably lowerer. Unfortunately they sold us on a $600,000 substation upgrade with automatic switches and relays to mediate a hazard we didn't really have. Our substation is updated and very nice now, it was unnecessary.