NESC Arc Flash Boundaries

hello,

The tables in NESC list the PPE requirements but there is no mention of what the arc flash hazard boundary should be. What is everyone else doing for the arc flash hazard boundary?

With the NESC you only have the MAD for shock hazard. No AFH boundary is defined or exists.

I guess you could say if one exists it would be the MAD.

For most of the people that we work with, PPE is already required ground to ground for overhead work. We are recommending for ground/pad mounted equipment that they have PPE once the cabinet is open, prior to doing any sort of work or voltage testing.

Alan

Thank you Alan.

In my case, I'm trying to determine what the AFH boundary should be for items like air break switches where you are 20 feet down from the switch (exposed energized components). Using the methodology in NESC and the MAD, then it would be 8 cal system but with a 20 ft distance, I would say it is 0 cal system.

I have a number of these switches that on poles in fields and can be operated by trained personnel other than lineman. These other personnel have insulated gloves for shock hazard but no FR.

Comments?

Switches

If you start with let's say 20 cal/cm2 energy level at 18", by the time you get to 20' the level would be about .1 cal/cm2. The energy level will be proportional to the inverse of the square of the distance change.

Alan

This situation may be a little different than an electrician standing in the same plain as the blast wave front. What if you are operating a switch or fuse via handle on one side that feeds an oil filled xfrm on the other side. If the transformer blew, gravity would pull the flaming oil down on top of you even if you are 20 feet away. Same could be true arrestors, or just molten switch metal, it could fall down on top of you. So in this case I don't think you can go by incident energy falling to 1.2 cal/cm2. You have to draw a gravity 'cone' at the base of the pole where flaming debris could hit. So if it was oil filled xfrm maybe 40ft radius, for a plain air break, may 10ft radius.

This is just off the cuff, I don't have a specific distance in mind. Just that we should consider gravity and the debris field pattern in this case and not necessarily AFB.

???

Haze, you left me behind on that one! Just can't follow you on that point.

Alan

Haze,

Not sure where you going with that last reply. I'm talking about an air break switch, no transformers but I suppose the same could be asked about opening a transformer cutout with an extendo stick or reseting the secondary breaker on a pole mount txf.

Either way, I think one must maintain some common sense here. If you take the what if's to an extreme, one shouldn't walk down a street where pole mount txf's are, avoid sidewalks that have network vaults and manholes with underground cables.

What I am saying is that the risk here has to be evaluated in a different way. Under normal circumstances the person and equipment are on the same horizontal plain, and the face/chest is the area that would be hit hardest with an arc flash blast. So if a person is without PPE, we have to move him to a distance away from the source so that he will not be hit with more than 1.2 cal/cm2 - the AFB.

Your question was how to establish the AFB for a person on the ground, I assume that person to be doing some task like operating an overhead pole device. I thought I read 'air break' in one of the replies - I added in other pole mounted devices.

The fact that you ask implied to me that you have some concern. That the person on the ground needed to be out of the AFB, or whether or not an AFB even existed. So my mind took me to worse case accidents and a thought of what they might be. These would be person at bottom of pole operating devices over his head at top of pole.

1) Little real concern - operating a rod operated air break switch which is the only device on the pole. What would be the risk here? The switch shorts and explodes. Molten metal could be sprayed radially. Insulator material could be fragmented and sprayed as projectiles.

2) Lot more concern - same switch but with a oil filled transformer on the same pole, or, remote operating an oil filled recloser. Device faults and explodes - we now have flaming oil falling down - some radially but a larger accumulation falling to the base of the pole.

So would a person benefit from being in FR clothing in either of these cases? Definitely in #2, maybe not so much in #1. If there is benefit to being in FR within some distance of the pole. That distance I will call the AFB line.

I assume you ask your question about the AFB because the people outside of the AFB don't have FR clothing. My point is that we can NOT use the IEEE or NFPA formulas for calculating an AFB with something like a recloser or xfrm on a pole. Yes the Incident Energy from the blast will drop below 1.2 cal some 10 feet down the pole - but the heat from the burning oil is above 1.2 cal all the way to the ground. Hence the AFB would be a circle of diameter 'x', with the pole in the center, and the source is the device at the top of the pole - thats how you get the cone shape.

Now can we extract the same logic to devices that are not oil filled, like air breaks, fused disconnects, lightning arrestors, etc. I have never in my 30 years witnessed a switch blow on a pole. But I was standing some 40 feet away when a lineman threw in a gang operated air break down stream, that caused all the brand new arrestors that the lineman just installed - to explode (actually only two out of the three exploded - they were all the wrong voltage). It was like 4th of July with one of those white 'air burst' star shells going off. If I had been under the pole, and had my choice between my FR Level 2, or my 60/40 Wool/Polyester suit - I would have chosen the FR. If I am wearing my FR for my own safety then I must be within the AFB.

Maybe I am not answering the question to the specifics of your situation but I just thought I would inject this in for thought as it is another phase of work we could consider for FR.

I am NOT saying it is necessary, just throwing it out for thought. Unless I completely misread the question.

The AFB

You say you witnessed one. Was anyone hurt as a result. I can assure you that with my 8 years as a lineman working everything from 120 volts to 115 kV HOT, that I have I have seen alot. I had the "priviledge" of taking 2 of my friends down from poles, and it was not fun....sad days. Since that time as an engineer involved with the same types of systems, I have accumulated much practical eperience. Enough to keep me from jumping to conclusions with areas outside my level of experience or expertise. I am very careful about that.

First off, the AFB does not exist in the NESC! Do we need one? Maybe in some cases but our experience tells us to be careful with it. It is not about how the sparks fly, but when and where is the hazard. You mention the light show from the arrestors but don't mention the injuries.

You also talk about the "burning oil" at the ground. In most of the cases I have seen where burning oil was an issue, it was at the pole top level and not at the ground.

Haze, I hope you will take this as constructive, but you and I both have issues with the standards. One of your biggest concerns appears to be that the committees hand out regs without the proper experience. Surely we will not make the same mistake.

Just my thoughts,
Alan

Believe me, I am no expert when it comes to line work. My experience is industrial. I am not suggesting anything here at all, other than to be aware of dangers where they exist and to think if FR clothing can help. I would not want the NFPA regulations getting involved in utilities. Could you imagine how bad that would be.

Maybe I completely misread the question as I thought the author was looking for a method of how to establish an AFB around the pole. But I guess that was not was he was asking.

With regard to the arrestors blowing, no, no one was injuried. For one there was no one near the pole. That was by coincidence. Work was inside the plant and the utility sent only one truck. The truck and crew had to relocate two pole upstream to operate the air break after finishing the work on the meter pole that had the arrestors. If a person had been standing directly below the pole, in tee shirt and shorts, I doubt he would have been hurt seriously, at the most one or two spot burns from some molten metal, like welding splatter. Maybe not even that as it probably would have cooled on the way down.

Again, my comments were just to provoke thought. Which I think is valid for any member here, even those no expert in a field.

Haze,

While discussion and questions to provoke thought are meant to be beneficial, one thing we all must keep in mind is that this is an open forum and that the audience will be varied. With that I think we have some obligation to keep scenarios and what if's in the realm of the real world, as there may be readers of the forum that aren't technical and that may lead to misunderstandings.