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 Post subject: 2 second rule
PostPosted: Thu May 08, 2014 5:24 am 
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Hi, according to IEEE1584, B.1.2, for any location where it is lack of protective equipment or protective equipment has higher than 2 seconds fault clearing time, one should consider how long a person is likely to remain in the location. I would like to know if the "2 seconds" consider the time the person goes away to the boundary (1.2Cal/cm^2 location)? Thanks


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PostPosted: Thu May 08, 2014 6:16 am 
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The 2 second rule is a matter of judgment. As I read it, it means does the arc go on for longer than 2 seconds? If so can the person get out/away in 2 seconds? If you set the limit to 2 seconds.

The PPE level selected will reflect 2 seconds off exposure to an arc flash at a distance as low as the original working distance.

So if a person is working at 462mm from a possible arc flash, the energy will be based on 2 seconds at that distance and NOT a distance that gets higher towards the flash protection boundary.


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PostPosted: Thu May 08, 2014 6:28 am 
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If the electrical equipment is in a location where a worker cannot get away within two seconds, the time used for calculation for that point should be increased to the reasonable time for the worker to get clear. Examples I have seen are electrical equipment accessed by a man-lift and electrical equipment in a confined space.


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PostPosted: Thu May 08, 2014 6:45 am 
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The "2 second rule" as it has become known, was based simply on human reaction time. 2 seconds is a widely published value for reaction.

The concept is if the arcing current falls below a device's instantaneous tripping function, the calculated incident energy could become extremely large. The 2 second rule is a method to permit a reasonable reduction in the time by recognizing after the initial "boom" a person is jumping or running away - if there is room.

It is not really associated with the arc flash boundary, working distance or any other distance or the actual arc duration, it is a function of how fast do you react and move away. Also keep in mind as the distance is rapidly increasing (as a function of panic :) ) so the incident energy is falling quickly i.e. body movement just made this a dynamic event.

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PostPosted: Thu May 08, 2014 8:00 am 
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I've also heard it explained that it could also represent a person being physically removed from the vicinity (ie, arc blast) but this would in reality happen a lot faster than 2 seconds, more like 0.2 seconds. However failing any actual good data for arc blast, we can't do anything with that one.

Human reaction rates for things like pushing a button are usually around 0.35 seconds. I'll buy that you can take several steps in about 2 seconds.


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PostPosted: Mon May 12, 2014 8:54 am 
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"I'll buy that you can take several steps in about 2 seconds. -"
But Blinded, deafened, hurt, confused and disoriented, are those steps going to be in the right direction?
are they going to get you to an area with a low enough IE, that your PPE can protect you until the fault does clear?

I have always thought that bit of text was poorly written, and as far as I can tell, has no basis to support any particular number.
Why two seconds? why not three? why not 1, or .1?

In some installations, for a some commercial/industrial electrical rooms, depending on fault location, it may not be possible to move away from a fault, without first moving closer. [And that is assuming that the room is not filled with housekeeping supplies and other stored odds and ends.]


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PostPosted: Mon May 12, 2014 9:30 am 
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As stated previously, the amount of time it will take to get away from each installation has to be evaluated when doing your arc flash study. If there are reasons to believe that it will take longer than two seconds, than that number should be increased. If the design of an electrical room conforms with NEC requirements, than two seconds is probably an okay assumption.


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PostPosted: Mon May 12, 2014 1:47 pm 
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There are a lot of military studies looking at human performance. There is a lot of variance from around 1% to 40% error rates. The 1% rate assumes lots of training to the point where the action is reactionary, routine, etc. The 40% number is typically where you get to in an emergency situation where there is no time to think about what to do. These are the typical numbers that are used for instance in calculating the likelihood that humans will do the right thing in an emergency such as to hit the ubiquitous "E-Stop" button which in reality turns out to be one of the more useless things that we have convinced ourselves protects people and equipment from all kinds of scenarios without ever considering the actual realistic likelihood that someone will actually trigger the E-Stop. I've personally only witnessed a single case out of over a dozen where someone was able to successfully use an E-Stop in an actual emergency condition.

The more normal human response from something such as an arc flash seems to be either a "freeze" or "flee" reaction but I can't put a hard number on the likelihood of either. If it is flee, chances are that the individual is going to use the most basic perception of a threat such as light or noise and move as quickly as possible away from that point, likely with little or no navigational capabilities. It would be best to think of this in terms of only very basic "instinct" here, not higher level thinking. It would probably be unrealistic to consider someone maneuvering up or down ladders or stairs, to operate controls on an aerial lift, or to operate door knobs.

The problem here however is that research studies on human performance are pretty poor. Remember that range...1 to 40%. That's a huge range, from reasonably reliable to almost a coin flip in terms of error rates. Even the various human reliability standards that are out there give very wide ranges on the results. So since there isn't any way to calculate it, this is one of those situations where we honestly have to just do the best we can with what we have.

There is another consideration with the "2 second rule". At some point the damage to the equipment is so great that arcing cannot continue. While I recognize that often with utility equipment it seems a if it can go on almost forever, and I've personnally dealt with a case where a GE AK breaker continued to arc for over 2 seconds and eventually interrupted only because it was damaged so badly, it's hard to put a limit on it.


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PostPosted: Mon May 12, 2014 2:12 pm 
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Were the GE AK breakers equipped with EC trip devices?

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PostPosted: Mon May 12, 2014 5:08 pm 
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The idea of using a maximum arc time is not really a rule and is not an official part of IEEE 1584 - it's in the Annex. The value of two seconds is more like a suggestion than a rule at least the way I read it. Any time limit is going to be somewhat arbitrary and there will probably be outliers where a longer maximum time should have been used. But assigning some maximum duration to a theoretical calculation probably makes sense.


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PostPosted: Mon May 12, 2014 10:18 pm 
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I know of some installations were an (low margin) arc flash would be sustained for 10 seconds or until as Paul said the equipment is so badly damaged that it will not be able to arc anymore. 10 seconds is a long time and even arc with low energy at 2 seconds become pretty high at that point if taken the full range.

I have to say this is the situation as is, with only the energy calculated and not yet looked at mitigation possibilities.


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PostPosted: Tue May 13, 2014 1:29 pm 
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I don't recall exactly what those AK breakers had on them and at this point I know for sure they made it to the scrap heap. I believe they were installed around the late 1970's time frame but I don't remember which series it was. I do recall they were definitely using an electromechanical relay at the time. It's not like the current generation which works fine when they work but are so cheaply made that the contact tips fall off them and GE won't even do the investigation to find out why.


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PostPosted: Tue May 13, 2014 3:00 pm 
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Thanks PaulEngr. I was just curious as I have experience with those bkrs and trip units. The bkrs were also subject to grease hardening if not maintained.

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PostPosted: Wed May 14, 2014 5:52 am 
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Grease hardening is right. When I got to that, they had not been serviced in 6 years. I didn't know as much then and the EPRI report on AK breakers definitely points to that issue among others. If I had a choice now I'd modernize the trip units such as with a Utility Relay kit replacement and just keep the breakers if I stuck with an air breaker. The system voltage was 2300 V, so it's in that strange category of not quite really medium voltage and not quite really low voltage either. Retrofit replacement wiring is also a pain in the rump since NEC chose the MV shielding cutoff at 2 kV. None of the equipment was designed and installed with enough working space to handle stress cones so one spends a lot of time either rebuilding equipment or searching for (frequently non-UL) non-shielded cable.


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