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 Post subject: 2 second time delay
PostPosted: Thu Jun 26, 2014 3:32 pm 
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Joined: Tue Oct 02, 2007 7:23 pm
Posts: 40
Location: Oak Ridge, Tennessee
Dear folks,

Doing arc flash analysis, I have heard referenced that 2 seconds, as long as their is a clear egress path, is an acceptable trip delay time when you are not sure if the breaker will operate (not properly maintained) or if the real time delay is several seconds. The argument is that someone will move away or be blasted away from the equipment within 2 seconds. Does anyone else use this approach, and how can I reference use of the 2 seconds time delay.

I feel strongly that if this 2 second time delay maximum is just made up as an assumption and there is nothing to back it up, that it cannot be utilized for an assumption to a calculation. Does anyone know where this 2 seconds reference was established and are you making this assumption in your arc flash studies?

Good practice or not?

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 Post subject: Re: 2 second time delay
PostPosted: Thu Jun 26, 2014 11:02 pm 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
The IEEE 1584 states:
Quote:
If the time is longer than two seconds, 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 two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away.

The NFPA70E ANEX D D.6(2) states the following:
Quote:
If the total protective device clearing time is longer than 2 seconds, 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 seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away. Sound engineering judgment must be used in applying the 2-second maximum clearing time, since there could be circumstances where an employee’s egress is inhibited.

The bold part is what the NFPA mentions and the IEEE doesn’t (I made it bold myself) and it is the annex so it is not part of the official requirements of the NFPA 70E, it is purely informative.

What is possible is that the gap will become too big for an arc to sustain itself (because off the melting of the conductors). The 2 second rule is a pain because you can’t just use it, you need sound engineering judgment. As a student I feel I do not have that. For the ship I am currently doing a (dummy) arc flash study for I talked to experienced engineers and we decided that because off the space and doors that fully open, the 2 seconds is a reasonably assumption.

Nevertheless the 2 second rule is not a way to make the installation safer, it is an assumption. The important part is to always try to make the incident energy as low as possible even if your calculated energy with the 2 second rule is very low. I recommend doing an arc flash study without 2 second rule to see were you have these problems. Solve them as good as possible and do an arc flash study with the 2 second rule if you conclude that this is a reasonable assumption. I only have experience with SKM Powertools and it has the option of making scenarios so running 2 studies (1 with 2 second rule and 1 without) is not much effort.

This is how I negotiate the 2 second rule, I am looking forward to your take on this and how other engineers work with the rule.


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 Post subject: Re: 2 second time delay
PostPosted: Fri Jun 27, 2014 8:45 am 
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Joined: Tue Oct 02, 2007 7:23 pm
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Location: Oak Ridge, Tennessee
great. thanks for the reference and the information.

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 Post subject: Re: 2 second time delay
PostPosted: Mon Jun 30, 2014 8:52 am 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
I have always taken the "2 seconds" as a rather lame attempt to both quantify and ignore what is really a complex issue.
In most cases I doubt an arc will last 2 seconds, but lets assume it does.
For many situations it is still probably unreasonably long.
A small panel board in an open area, yes you are going to move away. Big panel board, in a small electrical closet, perhaps not so much.

In some cases (not some one at the working distance tho) a worker may have to get closer to the arc in order to get away.

But as another thought example, take a typical windowless small office conference room, with people, all standing.
Some at the white board (dont they all have whiteboards these days) others watching.
This is the model of our electrical room. The whiteboard is the switchgear.
Flip out the lights (now its pitch black),
throw in a couple of swat team flashbangs,
and play bad rock music at earsplitting levels.

How many of these (now deafened and blinded) people get out in two seconds?
How many people got closer to the white board?
How many froze in place for two seconds?


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 Post subject: Re: 2 second time delay
PostPosted: Wed Jul 02, 2014 12:10 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
I agree on the point you are trying to make but I do not agree with your example.

1 In your example people don’t know about the impending danger, people need to be educated about the dangers of an arc flash.
2 You talk about leaving the room, but let’s start with getting out of the flash protection boundary or even just increasing the distance is a plus.
3 There is no force pushing people away from the whiteboard, in reality there is a force pushing you away from the flash.

My first comment does not hold true if the workers have not gotten the proper training.


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 Post subject: Re: 2 second time delay
PostPosted: Mon Jul 07, 2014 11:02 am 
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Joined: Wed Jun 10, 2009 5:00 pm
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Location: Toronto
mnmurphy wrote:
Does anyone else use this approach, and how can I reference use of the 2 seconds time delay.


I personally believe the 2 seconds limit is justified in a prevailing number of cases. If you believe a person serving the equipment may have no egress and therefore may be exposed to the arc for longer then two seconds, you should do the analysis at more than two seconds. I would also reference the use of 2 seconds (or more) time delay in the report and I would put the exposure time after calculated incident energy value to reflect the arc duration cut-off value. For example, XYZ cal/cm^2 @ 2 sec.

Also, please note that large incident energy resulting from long exposure to radiant heat does not necessarily translates into a big trouble. For example, XYZ amount of calories per cm^2 received in 0.01 seconds are much more dangerous then the same amount of incident energy received within 1 or 10 seconds. Alternatively, an exposure to less than the industry accepted 1.2calc/^2 threshold incident energy level for a second degree burn could very well result in incurable burn. Please check this forum library article at http://arcflashforum.brainfiller.com/viewtopic.php?f=34&t=2221 for more information. You may also want to reflect this fact in your calculations and report.


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