70E defines arc flash hazard as interaction with equipment, if they are not doing anything to the equipment and there are no exposed parts, no PPE is required to be in the room.

Edit, never mind, already addressed.

I think it is silly to assume that manufacturers would sell equipment that cannot contain an arc within the AIC rating of the equipment. Just my opinion, you mileage may vary

Well it won't. Why do you think every OEM is designing, testing, and offering Arc Rated switchgear now? Because they know the non arc rated stuf won't contain an arc.

Take a look at the operation manual for any manufactures (Non arc rated) switchgear. See all of those danger and warning statements regarding arc flash? The OEM's call that CYA.

The AIC is the short-circuit (high, abnormal) current in the conductors that the equipment can safely interrupt (open the circuit) without blowing up. Usually that's the job of a circuit breaker or a fuse. The AIC is also limited by the capacity of components to sustain that current for a couple seconds (don't remember the exact definition, but basically the components won't blow up if sujected to such current before a circuit protection opens timely).
The fault causing the short-circuit current can be farther in the distribution network, not necessarily in that equipment, so the energy released within the equipment is smaller than in an arc flash (think Joule's effect: the heat is released along all the length of the conductor rather than at one point).

An arc flash happening in an equipment releases much more energy, all concentrated in the same place. It's similar to an explosion. Think dynamite. How many pounds of dynamite going off can your equipment contain? If it's not designed to contain an explosion, it probably won't contain one. And it's safer to assume it won't and act accordingly.

Switchgear is often rated in MVA withstand, a measure of energy not unlike dynamite. It is not only an interupting rating but also withstand for the bus bracing. If a vessel is rated to safely control a certain amount of energy (think dynamite), should not that control be extended to integrety of the vessel.

You guys are preaching to the choir here, I understand what the rules of Arc Flash protection include. But having that understanding doesn't prevent me from also having the oversight that we are in a silly posture.

The key thing is that switchgear interrupting or withstand ratings are for external faults, not faults in the switchgear. The energy of the arcing fault itself for the design fault is not in the switchgear so the switchgear does not have to contain it.

Gee, my head is spinning.

It was perfectly clear when I wrote it. It didn't become unclear until I read it.

Try this:

The switchgear has to withstand current going through it without being damaged. It has to interrupt current without being damaged. It doesn't have to withstand an arcing fault inside the switchgear.

Ahhh, much better

TRUE But...





This is true I am trying to recall from memory need to dress accordingly. NFPA 70E 2009 90.2 "during activities s such as the installation, OPERATION, maintenance, and" I cannot remember the rest but the key work here is operation to me that means automatically or manually. door open or closed does not matter. why even be worried about door open or closed calcualtions I would be more worried about survivability if it's a three four whatever why take the chance do the benefits really outweigh the risks, to me it's a no-brainer dress for the hazard.

Like I said earlier, I'm looking for the requirement, not the safest. But given the direction of this discussion, I'll play devil's advocate here.

How about this scenario:

You are pulling 5 KV cable into an existing cable tray which contains 3/C 500MCM tray cable feeding switchgear. The cable is relatively new and in good condition. Since you are pulling cable into an existing tray and that cable is energized, you are interacting with the energized cable and therefore must wear the appropriate PPE which may well be HC4 but at a minimum HC2. This is not based on the tables but on the calculated values for the secondary side of the equipment feeding this cable so the tables don't apply.

Comments?

Article 100 Arc Flash Hazard FPN No. 2 references Table 130.7(C)(9) for examples of activities that could pose an arc flash hazard. The last two items in the table indicate that an arc flash hazard would exist while examining the cable (HRC 4 or 2 depending on whether it is in confined or open space). I'd say that pulling cable into a tray with energized existing cable is at least as hazardous as examining the cable.

My recommendation would be to require PPE for the calculated IE at the closest distance the worker gets to the energized cable.

You bring up a good point however; could not 'fault withstand' also include an internal fault?

Where we are most concerned with these withstand ratings is usually at a main switchboard or switchgear where fault current is the highest near the supply transformers. In my mind this is where a disaster would happen of greatest consequence and it is for these events that I am mostly concerned about adequate withstand ratings. I can imagine a faulty circuit breaker for which the main would need to trip.

Most faults down stream will have diminished current flow because of the cable and arc resistance typical of a distribution system. Where we have 21kA of current available at a bus, typical feeder fault a few hundred feet away is already diminished to something like 6kA.

A Committee Members View

I have been in contact with one of the members of the 70E Committee to help understand what their thinking was as to open door/closed door. His response helped me considerably in understanding why it is written the way it is. I’ll attempt to summarize his response here:

In the 2004 70E, the arc flash boundary applied only to exposed energized components. For the 2009 cycle, a number of proposals were made to change the limitation of exposed conductors. A lively debate ensued and the committee compromised to amend the definition of Arc Flash Hazard as found in Article 100. The key points are in the FPN No. 1 which added “…or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc.� It also added “Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.� The definition of Arc Flash Protection Boundary was changed and includes the phrase “When an arc flash hazard exists�.

It is his interpretation of the standard (and he is on the committee) that the determination of arc flash boundary applicability is left up to the user to determine “when an arc flash hazard exists�. In other words, there is no blanket rule that can be applied when there are no exposed energized components. The user must make a determination of the hazard.

His response was a great help to me in understanding this. It is very clear that when there is exposed energized conductors or circuit parts, an arc flash hazard exists and everything applies. Using his interpretation and the text of the code, if the enclosed equipment is being used under normal operating conditions and is properly installed and maintained, then it is not likely to pose a hazard. So what is normal? A circuit breaker in a panel has been designed to switch a load. It is my opinion that this is a normal operation and therefore does not pose an arc flash hazard. If no arc flash hazard exists, then PPE is not required. I may be interacting with the switch, but not in a manner outside of the design nor in a manner that should cause an arc. But if the switch is not moving properly and I have to use excessive force to move it, it is no longer a ‘normal’ operation. In that case, some PPE would be required. This thought process would then apply to any application. You evaluate the risk and hazard and make appropriate determinations.

The downside of this interpretation is that there is no clear singular set of rules that can be painted. Things have to be evaluated. Rules can be set for a given location or facility, but to make blanket statements about enclosed equipment operations really isn’t possible. A good solution might be to use the tables for closed door operations since they address the open door/closed door issue in the tables.

I apologize for the long post, but I felt the information provided by this committee member was important.

Finally glad to see some sense of reason to enclosed equipment!

Thanks

TxEngr, Thanks for information. Like I say a definitive answer does not exist. Only the company’s electrical safety plan or policy (user) must determine the hazard like you said and I loved the example you gave.

TxEngr, Thanks also from me. The info helps.

Good stuff Tx, if only all of the people making these decisions at facilities had your sense of reasoning. I am in an unique position where I see the aftermath of arc flash enclosure failures on a regular basis, so I get more exposere to it than most people do. Seeing what I see every week, I would bever personally do any interaction with equipment without PPE, I have just seen way to many enclosures fail.

As you pointed out, the thinking behind the 70E verbage leaves many grey areas, but if the hazard level (Ei) is identified, and a failure occurs while switching that causes the operator to be injured because they were not wearing PPE, I would bet OSHA would find the company negligent.

Zog, your right. I believe people are looking for black and white answers regarding arc flash. Your right the Ie will be your building block. Fortunately, there is some statistical data and some standards. Unfortunately, many times I believe it still comes down to the persons reasoning when doing the analysis of the IE and the risk hazard associated with a specific task for specific equipment. Some may be over conservative and some less conservative in recommendations. Many times the person performing the study has to understand and reason through the hazards. The process of making hazard analysis decisions seems to not always be based on statistical data or standards so some analytical reasoning is required to make the best hazard evaluation decision based on the data we have today.

Therefore, is the concern of what if something happened that the person performing the analysis did not consider. I suppose the lawyers work on that one. I believe that is why people want black and white answers to help eliminate the legal issues.

I believe the electrical hazard analysis statistical data process and standards is still in infancy and will develop greatly in a few years.

If I was an engineering consulant I would lean to be very, very conservative with my customers.

At a recent conference I attended one of the speakers was discussing arc flash related litigation issues. There was a case recently in South Carolina where a person was injured (No PPE, working live) from a phase to ground arc flash.

Now here is the scary part, the defendant (Engineering firm) was found liable because the GF settings were not set to the minimum settings!!! The prosocution argued that the arc flash would have been limited if the setting were set to minimum.

To me, that is just crazy thinking, never mind all the coordination issues, or the fact the person injured was working on live equipment without justification or any PPE. I guess doing coordination studies just got alot easier for all the PE's out there, just set them to minimum to CYA.