Arc Flash Forum :: View topic - Using less than 2 second cut off??

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"?

Author:	C. Marsh [ Fri Sep 11, 2015 9:14 am ]
Post subject:	Using less than 2 second cut off??
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?

Author:	wilhendrix [ Mon Sep 14, 2015 8:31 am ]
Post subject:	Re: Using less than 2 second cut off??
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.

Author:	JKlessig [ Mon Sep 14, 2015 8:33 am ]
Post subject:	Re: Using less than 2 second cut off??
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"?

Author:	Voltrael [ Mon Sep 14, 2015 8:49 am ]
Post subject:	Re: Using less than 2 second cut off??
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"?

Author:	stevenal [ Thu Sep 17, 2015 11:56 am ]
Post subject:	Re: Using less than 2 second cut off??
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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Author:	PaulEngr [ Sat Sep 19, 2015 9:41 am ]
Post subject:	Re: Using less than 2 second cut off??
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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