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Do you use the 2 second cut off or an alternative cutoff?
 2 Seconds 1 Second ½ Second Use actual (long) arc duration Don’t perform studies
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 Post subject: Alternative to 2 Second Cutoff?Posted: Sun Feb 06, 2022 4:27 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1648
Location: Scottsdale, Arizona
The IEEE 1584 “2 second” arc duration language has been coming up lately with questions about using an alternative duration. Some have mentioned 1 second, some ½ second.

Excerpt from IEEE 1584:
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 this week’s question:

Do you use the 2 second cut off or an alternative cutoff?
2 Seconds
1 Second
½ Second
Use actual (long) arc duration
Don’t perform studies

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 Post subject: Re: Alternative to 2 Second Cutoff?Posted: Sun Feb 06, 2022 11:38 pm

Joined: Wed Dec 12, 2018 12:59 am
Posts: 4
Location: Germany
Hello Jim,

when we do studies for our german customers according to DGUV Information 203-077 we use 1 second (only if the type of work to be expected allows it). The first preference is of course to try and minimize the clearing time.

Quote from DGUV Information 203-077:
Quote:
With short-circuit durations of longer than 1 s, it can be assumed that the person will be able to withdraw from the immediate danger area, if necessary. For this reason, longer periods will not need to be considered. This does not apply, however, if the person’s departure from the work environment is precluded or restricted (e.g. working in tight cable trenches or canals, narrow work corridors, work from ladders or lifting mechanisms).

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 Post subject: Re: Alternative to 2 Second Cutoff?Posted: Fri Feb 18, 2022 3:54 pm

Joined: Wed Nov 11, 2015 9:18 am
Posts: 9
I have been meaning to ask this so this seems a good time as any considering the question.
Lets say we have a dry type transformer, 480-120/208 that serves a MCB panel. I have applied 2-seconds at this panel in these cases. Now lets say we have sub panels served by breakers in this panel. Would the 2-seconds apply for the sub panels too? Technically they are being served by this transformer, but through a couple breakers along the way. Where would you draw the line, especially if the sub panel is a long feeder run with a high incident energy because of it. If the calculations are allowed to run, it indicates about a 4-second clearing time. Curious on thoughts.

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 Post subject: Re: Alternative to 2 Second Cutoff?Posted: Tue Apr 05, 2022 5:22 am
 Sparks Level

Joined: Tue Feb 17, 2009 8:38 am
Posts: 57
Location: Westminster, MD
rmgbob wrote:
I have been meaning to ask this so this seems a good time as any considering the question.
Lets say we have a dry type transformer, 480-120/208 that serves a MCB panel. I have applied 2-seconds at this panel in these cases. Now lets say we have sub panels served by breakers in this panel. Would the 2-seconds apply for the sub panels too? Technically they are being served by this transformer, but through a couple breakers along the way. Where would you draw the line, especially if the sub panel is a long feeder run with a high incident energy because of it. If the calculations are allowed to run, it indicates about a 4-second clearing time. Curious on thoughts.

Two seconds is considered the amount of time it takes for a person to fall away from the arcing incident, so yes, it is logical to use that value for everything, except where the fall away could be prevented - like in a bucket truck.
But it's also somewhat unique for a breaker to hold during an arcing fault where there's not enough current to make it trip.
There was a fellow who taught that one should use a 1000-second trip time, and then explore those cases where 1000 seconds occurred and make adjustments to assure tripping. Then readjust the Study for the 2-second timeout. I know because someone in our group forgot about returning to the 2-second value and that created a label that had a 1/4 mile arc flash boundary.

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 Post subject: Re: Alternative to 2 Second Cutoff?Posted: Mon Apr 25, 2022 7:06 pm

Joined: Thu Oct 30, 2014 11:07 am
Posts: 6
Location: Ohio
For utility arc-flash studies we used 2-second self-extraction times for pad-mounted equipment, where the worker has options for self-extraction. For over-head line work, we settled on a 5-second self-extraction time due to the more limited options for extraction when in a bucket or strapped to a pole.

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 Post subject: Re: Alternative to 2 Second Cutoff?Posted: Mon May 02, 2022 11:27 am

Joined: Fri Sep 28, 2018 7:32 am
Posts: 19
Location: Sioux Falls, SD
A customer asked that a 2-second max clearing time be used (where practical) for equipment 250V or less. For equipment operating at a higher voltage they want to use the full clearing time of the upstream OCPD. The rationale is that they have many 480V switchboards and MCC's with compartments near the floor. It is considered normal for maintenance staff to be on their hands and knees troubleshooting in these compartments while they are energized. In my opinion, a worker should not put themselves in an overly comprising position. Even if kneeling on the floor to troubleshoot in a compartment I feel I would be knocked back or be able to egress within 2-seconds. I'm curious what everyone's thoughts are on this.

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