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 Post subject: Flash Protection Boundary & Limited Approach Boundary
PostPosted: Tue Jan 17, 2012 5:25 pm 
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I understand the Flash Protection Boundary can either be closer than or farther than the Limited Approach Boundary. I don't really understand how that is possible. If the Flash Protection Boundary is telling you where the arc flash hazard exists, how could that be closer than the hazard of electric shock (the Limited Approach Boundary).

Granted, I'm admittedly extremely new to this material, so I'm sure I'm just not getting something very basic about these definitions. Thank you guys for your help.


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PostPosted: Wed Jan 18, 2012 5:28 am 
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Anyone please correct me if I am wrong. The shock hazard is governed by specific variables in all systems; voltage and distance. The arc flash hazard is governed by variables that can change with each system and type of fault; current at the fault and time to open the protective device.


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PostPosted: Wed Jan 18, 2012 7:43 am 
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MIEngineer wrote:
Anyone please correct me if I am wrong. The shock hazard is governed by specific variables in all systems; voltage and distance. The arc flash hazard is governed by variables that can change with each system and type of fault; current at the fault and time to open the protective device.


The shock hazard boundaries such as limited, restricted and prohibited approach are all based on voltage.
i.e. 480 volts the LAB=3.5 ft., RAB =12 inches, PAB =1 inch. Arc flash doesn't enter into the shock boundaries.

The Arc Flash Boundary is where the incident energy falls to 1.2 cal/cm2. This means it is a function of how large the arc flash is which is a matter of short circuit current, duration, equipment and a few other things.

If the arc flash is small, the LAB (and maybe RAB if it is really small) will be greater than the AFB. If the arc flash is large, the AFB can be greater than the LAB. The 2012 70E now includes language about the greater of LAB or AFB since either can be the greater of the hazard distance.


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PostPosted: Wed Jan 18, 2012 9:59 am 
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CT-To try and make it simple:
You may have a circuit that is 480 Volts AC. Looking at the Shock Tables in 70E 2012, this is going to put your LAB at 3 feet. If you either conduct a detailed analysis or use the tables (HRC categories)(With all of the assumptions and notes met by using the tables) you see that it could be possible the AFB could be 30 inches depending on the task. If you conduct a detailed analysis of this same exact circuit you could find that the AFB boundary may decrease even more!(but could potentially increase in some instances)
This is a simple example but not to be used as a standard. Involving someone that can educate or explain to you more of the standard will help you tremendously.


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PostPosted: Wed Jan 18, 2012 1:56 pm 
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I guess it's hard for me, a beginner, to understand that the potential incident energy could be so low that the AFB would be smaller than the LAB. But your explanations help a lot.

I should point out that this is very introductory training, and that we are getting some review/contribution from some experienced electrical safety guys. This was just for my own curiosity, to get an answer quicker rather than later.


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PostPosted: Wed Jan 18, 2012 5:26 pm 
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ConvergenceTraining wrote:
I guess it's hard for me, a beginner, to understand that the potential incident energy could be so low that the AFB would be smaller than the LAB. But your explanations help a lot.

I should point out that this is very introductory training, and that we are getting some review/contribution from some experienced electrical safety guys. This was just for my own curiosity, to get an answer quicker rather than later.


I have plenty of equipment, even at 4160 V, where the arc flash rating even at 18" or 24" is significantly less than 1.2 cal/cm^2. So my AFB is less than 18", sometimes only 3-6 inches.

This happens frequently near the "end points" of the system. On a relatively small fuse protected motor (say around 2-3 HP) the wire size may be only 12 to 14 gauge. The voltage is still 480 V, so the same LAB applies. But the available fault current with all the impendance of the wiring involved and especially if it is fed from a small (say 10 kVA) transformer is so small, and with only a 1/4 cycle trip time if there is a dead short which trips the fuses, my arc flash boundary and incident energy at working distance is so small that the AFB almost disappears.

Although this is not a hard and fast rule, remember that the AFB (and incident energy) are based on ENERGY. As we are all taught early on, energy is TIME times POWER. Power is of course volts times amps times power factor and perhaps a constant. Even if voltage is large, if time and current are small, the energy is also small. Where this breaks down is that the impact on incident energy is not quite linear...it is not time X volts X amps. There are curves that make the extremes not quite so smooth. But as a rough estimate, doubling or halving any of the 3 values will roughly double or halve your incident energy.

Now all the energy gets released at one assumed point or at realistic distances, it might as well be considered at a point. As it travels outwards, it does so in a sphere. So the area on the sphere is what matters, and the area of a sphere is pi*4/3*(diameter/2)*(diameter/2). So doubling the distance cuts your incident energy roughly to 1/4. Again there is a little fudge factor in here such as the difference between arcing in an MCC (which is really a hemisphere) and arcing in open air.

I use these grossly simplified examples with my own electricians to help them get used to the idea of how and why the values are what they are. They might get dizzy looking at logarithms and exponentials, but simple examples of doubling and halving, and from a very layman's point of view, help the math make a little more sense.


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PostPosted: Thu Jan 19, 2012 11:00 am 
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Paul, thanks a lot. That makes it a lot clearer!


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