The 2002 Edition of IEEE 1584 contained language about limiting the duration of an arc flash to two seconds if a time current curve indicates an exceptionally long duration (or if there isn’t an upstream device or available information) Judgement needs to be made on a case by case basement if a 2 second cutoff is appropriate based on factors such as ability to move away from the area quickly. i.e. not constrained in a bucket truck of vault.

The 2 Second “Rule� was carried into the 2018 Edition of IEEE 1584 (with some text changes) and is also referenced in the 2018 Edition of NFPA 70E Table 130.7(C)(15)(b) regarding PPE Categories for DC Systems.

After all these years, there continues to be confusion by some about what the 2 Second cutoff actually represents.

Some say it represents the maximum exposure time assuming the person has moved out of the way.
Others say it is the maximum duration of the arc flash.

Btw, I have the answer. This question is to gauge the perception of others – Thanks for your views!

Full text from 2018 IEEE 1584 regarding two seconds:

If the total protective device clearing time is longer than two seconds (2 s); consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible, and 2 s usually is a reasonable assumption for the arc duration to determine the incident energy. However, this also depends on the specific task. A worker in a bucket truck, or inside an equipment enclosure, could need more time to move away. Use engineering judgement when applying any maximum arc duration time for incident energy exposure calculations, because there may be circumstances where a person’s egress may be blocked.

Here is the question:

Your interpretation of the 2 second cut of for arc flash studies is:
Time for person to move away
Duration of the arc flash
Not sure/Something else

Feel free to post your thoughts as well.

I know of people executing AF studies, that never want to make any kind of engineering/risk judgement so they only use the default time of their software package, such as 1000 sec.

Are there any statistics out there that show PPE selected using a 2 second exposure time (event is either cleared or worker moves out of the way) has been inadequate versus PPE selected based on some other time?

I don't know of any. One other issue that doesn't seem to be discussed (not sure how you would factor it in) is the actual response - jumping back, provides a rapidly increasing working distance. The 2 second duration assumes a person is at the same location for the entire duration.

In the very early years, few were quick to embrace the 2 second rule. No one wanted to be first. Then as you say, I knew a few that used 1000 seconds only because it was the longest "defined" time value - top of the TCC. Eventually more began to use it. Thanks!