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 Post subject: Using less than 2 second cut off??
PostPosted: Fri Sep 11, 2015 9:14 am 

Joined: Fri Sep 25, 2009 5:00 pm
Posts: 43
I was curious if anyone has taken it upon themselves to use a cut off less than 2 seconds in the event a protective device / time current curve does not provide a short arcing duration. It seems 2 seconds is quite a long time when you actually time it out vs. someones actions and as the person moves away, wouldn't the energy be decreasing anyway? I think for liability reasons it would be prudent to stick with 2 seconds but for practical reasons, this doesn't seem all that realistic.

Any thoughts?


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 Post subject: Re: Using less than 2 second cut off??
PostPosted: Mon Sep 14, 2015 8:31 am 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 138
Yes, we use that exception. The 2 second rule does seem long. However, in our arc flash documents we talk about the exception and have specific language concerning use of the 2 second rule. Generally, the places we see excessively long trip times are with the secondary OCP for 480 volt step down transformers. Personally, I don't think we should rely on the 480 volt primary OCP to determine trip times. So, even though the trip might be much longer for a secondary, we'll use it and label accordingly.


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 Post subject: Re: Using less than 2 second cut off??
PostPosted: Mon Sep 14, 2015 8:33 am 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
My thoughts tend to run the opposite direction, i.e. that two seconds isn't NEAR long enough.
[although for small events, 2 seconds is a very long time, and the way I read it is ISNT a "RULE", its more of an offhand comment.]

Take someone, put them in a small room,
kneeling,
physically restrict theirmovements to one sside (simulating a panel door),
performing a task that requires their attention,
THen
flip out the lights,
set off a couple flash bang grenades in the room,
while playing crappy music at high levels,
and see how long it takes them to do any thing.

That is the model I use when thinking about it.

You are deaf, you are blind, you are concussed, and you probably are already burnt,
and you may have started in a location that takes you more that two seconds to get out of
[it probably took you more than two seconds to get into it].
Are you still going to think two seconds is "too long"?


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 Post subject: Re: Using less than 2 second cut off??
PostPosted: Mon Sep 14, 2015 8:49 am 
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Joined: Tue Nov 12, 2013 6:31 am
Posts: 238
Location: Port Huron, Michigan
If it will take longer than 2 seconds for an individual to clear an arc flash, the actual estimated time should be used when doing the study.



JKlessig wrote:
My thoughts tend to run the opposite direction, i.e. that two seconds isn't NEAR long enough.
[although for small events, 2 seconds is a very long time, and the way I read it is ISNT a "RULE", its more of an offhand comment.]

Take someone, put them in a small room,
kneeling,
physically restrict theirmovements to one sside (simulating a panel door),
performing a task that requires their attention,
THen
flip out the lights,
set off a couple flash bang grenades in the room,
while playing crappy music at high levels,
and see how long it takes them to do any thing.

That is the model I use when thinking about it.

You are deaf, you are blind, you are concussed, and you probably are already burnt,
and you may have started in a location that takes you more that two seconds to get out of
[it probably took you more than two seconds to get into it].
Are you still going to think two seconds is "too long"?


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 Post subject: Re: Using less than 2 second cut off??
PostPosted: Thu Sep 17, 2015 11:56 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
The "2 second rule" comes from this statement in in 1584:

"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".

I see no allowance here for going shorter than 2s, only longer.


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 Post subject: Re: Using less than 2 second cut off??
PostPosted: Sat Sep 19, 2015 9:41 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Less than 2 seconds would occur only if for some reason the arc extinguishes in a shorter period of time. The obvious example is over current protection but there are other mechanisms such as 87 relays, or arc flash relays. At low voltages the arc can self extinguish but without test data we have no way to determine that threshold.


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