Larry Stutts wrote:
Can you not just identify on the hazard label the increased working distance? I mean if the incident energy is 51 cal/cm^2, can you not just increase the working distance to 36" to get an incident energy of 16.5 cal/cm^2?
Yes.
I'll give a rather complicated example of where the default distances fall apart and you have to figure it out yourself for illustration, including a case where you get into the issue of hazards vs. risks.
I have around 50 portable substations in a mine. Everyone has their own way of doing it and there are lots of variations but by way of illustration, the following web sites have photos on them from equipment manufacturers of the type of equipment I'm talking about:
http://www.linepower.com/http://www.azz.com/electrical-and-industrial-products/mining-switchgear My site has equipment from both manufacturers as well as others. You park the substation under a pole line and then run jumpers up to the power lines. The jumpers that we use attach with a "saddle clamp" which is a clip that is installed on the power line itself, but there are other types of clips. When you want to move it, just take the load off, remove the jumpers (with a hot stick), and then remove the saddle clamps either with rubber glove-only method or with hot sticks depending on local work rules. The alternative is to cable everything together using SHD-GC cable but pole lines are significantly less expensive, keep the power lines away from heavy equipment, handle much higher currents for the cost, are easier to add branches, and have much lower capacitance. The SHD-GC approach is used within the pit itself where it's not practical to run pole lines.
On top of the mast we have a set of lightning arresters, a gang operated unitized disconnect switch with a ground mounted manual operator, and a set of cutout fuses, in that order. All of this is wired up with bare wire. From there, we have stress cones and bring the wiring down off the mast in shielded MV-105 mounted in cable tray. The MV-105 terminates into the enclosure on the transformer primaries. The bottom of the stress cones is at least 13 feet above the top of the deck so that the wiring on the mast is not exposed unless ladders or bucket trucks are in use.
I end up with 5 different working distances that depend on the task and worker positions:
1. Doing rubber glove work and troubleshooting while working from a bucket truck. Working distance is only 24" based on what we measured.
2. Doing hot stick work from a bucket truck. Working distance is 8 feet (8 foot hot stick). For example, attaching the jumpers falls into this category.
3. Using a telescoping hot stick to pull the drop fuses from the deck. Working distance is 16.9 feet for the line side, 13 feet for the load side. Given that fuse spacing is quite wide there is practically no risk of L-L faults and it's all L-G. This system is delta-wye distributed with a 2000 A resistance ground so the maximum fault current is 2 kA rather than the normal L-L case.
4. Operating the disconnect using the deck mounted operator. Working distance is around 20 feet (fuses are below the disconnect).
Item #1 is typically what you'd get from an arc flash study other than having to turn on the flag for open air gear.
Item #2 has to be calculated using the adjusted working distance.
Item #3 has to be calculated using a different fault current and a different working distance.
Item #4 technically has to be calculated but the likelihood of failure is well under 10^-6 for this based on IEEE 493 data.
On top of this, IEEE 1584 is useless (22.9 kV working voltage) and defaults to Lee which calculates a result that is grossly incorrect (vastly too high) so some consideration as to how to realistically model >15 kV can be done.
Now this gets even more interesting if I point out that it's 23 kV so IEEE 1584 does not address it. There is really no test data that I know of for anything over 15 kV. That pretty much leaves the tables in NESC or ArcPro (Lee is totally impractical and invalid). On top of that, the mine moves at a speed of approximately 3/4 acre per day and loads are constantly moving around. So the overhead line length and potential inductive loads are constantly changing. Modeling this is challenging. For convenience, say I end up with H/RC 3 for case #1, H/RC 1 for case #2, and H/RC 0 for case #3 and #4. However due the fact that the likelihood of failure meets the corporate acceptable risk criteria, no PPE is required for item #4.
Next, we have to consider the actual tasks involved. Here is one way:
1. Working directly on equipment while energized using rubber-glove-only method. This ends up being H/RC 3 case but is almost impractical to do because the only way to do it is to wear a 25 or 40 cal suit and stand on an insulated ladder or in a bucket truck. You can't see very well, operating controls or going up/down a ladder (never mind tying off) while wearing rubber gloves is cumbersome. And there's a strong possibility of heat exhaustion in warm weather.
2. Use saddle clamps on the overhead line. These have to be attached using rubber-gloves only working from a bucket truck so this is a floating object issue for the lineman and both gloves and bucket truck have to be 23 kV rated. No passing tools to lineman in this scenario (avoids other shock consequences). Arc flash potential is nil since line spacing makes it practically impossible to create a line-to-line fault and we're double insulated against line-to-ground faults. So we treat this as no arc flash PPE required.
3. Connecting or removing jumpers from the line. Do it with an 8 foot hot stick. Arc flash is reduced to H/RC 1 due to distance. Lineman can be required to wear FR shirt and pants as standard PPE for linemen (recommended in NESC since 2009).
4. Closing in or opening fuse cutouts. This is either H/RC 1 or 0 depending on whether it's done from the bucket truck or the ground. If FR pants and shirts are standard PPE for lineman, it doesn't matter.
5. All others are not trained to use hot sticks and thus fall under item #4 (they have to use the switch or call a lineman for assistance). Since likelihood of arc flash and shock falls under the corporate definition of acceptable risk using IEEE 1584 data for fault rates, no PPE is required.
6. After removing power using methods 3, 4, or 5, testing for absence of voltage, and grounding, work dead instead of live, and taking into account working distances to adjacent equipment again, no PPE is required.
Labeling has a few options. We can put 5 labels on or one big label with all the work tasks. We can put on just one label with the worst case scenario (#1). We can put on the generic (warning! arc flash and shock hazard) label which forces someone to look elsewhere for guidance. We can implement all this under NESC since it's distribution gear and scoped out of 70E, and thus no label required. Either way, clearly the work rules and procedures have far more value here than could be condensed down to fit onto a label.
