Exploding the misconceptions and myths about arc flash issues

As a member of the NFPA 70E Committee and an instructor, I hear it all. There seems to be some common misconceptions among people that are like urban legends; they just don’t go away. One of my favorites is that “You don’t have to worry about arc flash below 240 volts.” While it may be true that an arc is more difficult to sustain at lower voltages, there have been some ...

04/01/2010


 

 

As a member of the NFPA 70E Committee and an instructor, I hear it all. There seems to be some common misconceptions among people that are like urban legends; they just don’t go away.

 

One of my favorites is that “You don’t have to worry about arc flash below 240 volts.” While it may be true that an arc is more difficult to sustain at lower voltages, there have been some well-documented incidents at 208 V that have resulted in large, sustained arc flashes. This usually requires a very large short circuit available current to sustain the arc.

 

These types of short circuit currents may be seen in high rise buildings or older commercial buildings where 208 V is used instead of 480 V. Florida Power and Light has a video of one such accident titled, “Once the Arc Begins.” It’s a real eye-opener for people who think there’s no arc flash hazard below 240 V.

 

Another one I often hear is, “Arc-rated PPE is required to walk or pass through an area containing electrical equipment.” If the equipment is operating normally, the enclosure doors are properly secured and there are no exposed energized conductors or circuit parts, no such PPE is needed.

 

One proposal that was introduced at the last NFPA 70E Report on Proposals meeting was to include a statement that would make it clear that no PPE was required to just pass through an area containing normally operating electrical equipment.

 

It was rejected, mostly because many committee members did not think it was needed. As often as I’m asked about this situation, I think it is, so I will get another shot at reviving it at the Report on Comments meeting.

 

Incident energy explained

Many people in classes I conduct believe that “The arc rating on their PPE and equipment provides full protection at the amount of incident energy the PPE is rated for.” Not according to the ASTM standards. The arc rating on PPE and clothing is the incident energy, for 1/10 of a second that could result in a second-degree burn to bare skin underneath the clothing or PPE.

 

The rating is the same for clothing and face shields or windows in hoods. The following rules-of-thumb may help clarify the incident energy concept.

 

Incident energy is proportional to time %%MDASSML%% If exposure time is doubled, the incident energy received by a surface doubles. We are only talking cycles per second on this; time adds up quickly. One cycle is 0.0167 seconds. A molded- or insulated-case circuit breaker usually has a maximum total clearing time (the time from the start of the arc to when it is extinguished) of about 2 cycles (0.03 seconds). A low-voltage draw-out circuit breaker has a typical maximum total clearing time of about four cycles (0.07 seconds).

 

If through aging and/or lack of maintenance these devices slow down %%MDASSML%% even just a few cycles %%MDASSML%% the result on incident energy can be dramatic. If the circuit breaker does not open at all, which I’ve seen many times over the years, the incident energy can increase by 10 times or more. It really depends on how the next overcurrent protective device (OCPD) upstream sees the fault.

 

If the next upstream OCPD sees the fault as a short circuit, then incident energy will probably double or triple. If that OCPD sees the fault as an overcurrent (due to its settings), it may take several seconds to operate. There are 60 cycles to one second, so you can see that the incident energy could be very low if the circuit breaker trips properly, or it could be pretty tremendous if everything goes wrong.

 

Incident energy decreases by the inverse square of the distance %%MDASSML%% Sounds complicated, but its not. As I move away from an arc source, the heat received by my body decreases very rapidly. This is good news, as distance is our friend.

 

If you have a choice between working close or moving back %%MDASSML%% even if it may be less comfortable %%MDASSML%% if there’s an arc flash, you will receive less heat from the arc. The opposite is also true, though. If you have to get right on top of something to work on it, the incident energy is increasing rapidly as you get closer to that potential arc source.

 

Each layer of clothing under arc-rated clothing reduces the heat received by the body by about 50% %%MDASSML%% Let me be very clear on this: Adding flammable layers under arc-rated clothing does not increase the arc rating of the clothing; it just reduces the heat that your body would receive. Rule 410(A)(3) in the NESC states:

 

“NOTE 1: A clothing system (multiple layers) that includes an outer layer of flame resistant material and an inner layer of non-flame resistant material has been shown to block more heat than a single layer. The effect of the combination of these multiple layers can be referred to as the effective arc rating.”

 

Sorry guys, you didn’t quite get it right on that. The note starts out okay, as multiple layers will block more heat, but if the arc rating of the outer layer is exceeded, it could go into “break-open.” Break-open is when the arc-rated fabric chars, crumbles and falls apart. Once that happens, the flammable layers underneath will ignite. You can figure out the rest. There are other statements in Rule 410 that makes me scratch my head and wonder what they were thinking.

 

Facing reality

Another thing I hear all the time is, “We’ve never had an accident here, so why worry about it?” This goes along with people saying “We’ve done it this way for (20, 25, 30) years, why should we change now?” and “Arc flashes are so rare, chances are we’ll never see one.” You really have to love the optimism in these statements. I also love people who are willing to roll the dice, especially with other people’s lives and well-being.

 

Yes, arc flashes are relatively rare events, thankfully. However, when teaching electrical safety, I often ask how many people in the class have been involved with, or know someone who has been involved with an arc flash. In every class, 90% to 95% of the students raise their hands. The difference is, they haven’t been in a big one %%MDASSML%% yet. Most of us have had little screwdriver-melters, or maybe lost an eyebrow or two, but nothing that has really injured us.

 

Large arc flashes are rare, but when they happen, they are life-changing events. Your life is forever changed in a split second %%MDASSML%% less than 1/10 of a second actually %%MDASSML%% and it can never be the way it used to be. The way you feel about your self-worth, the way you interact with your wife (or husband) and family may never be the same again.

 

Dr. Mary Cappelli-Schelpheffer once said that about 70% of people who survive a major arc flash event have to receive long-term counseling, and almost 60% of those same people will have failed marriages. If the prospect of multiple surgeries, constant pain and debilitating injuries is your cup of tea, roll those dice. Not me, brother. I’ll take a few minutes of inconvenience and discomfort, rather than the alternative.

 

Energized conductors, equipment

Another myth is, “There’s no arc flash hazard if there’s no exposed energized conductors or circuit parts.” This one is almost true. For most types of equipment, the probability of an arc flash is very, very low %%MDASSML%% but not impossible, by the way %%MDASSML%% if it is operating normally and the enclosure is secured.

 

However, inserting or removing draw-out circuit breakers, bus plugs and MCC buckets can cause an arc flash where there would not normally be a perceived hazard.

 

“Perceived” is a critical word in this statement because normally operating electrical equipment has been known to fail. I guess it’s safe to say that it was not really normally operating, but we don’t know that until it fails.

 

Energized electrical equipment can fail at any time without notice. People can make mistakes %%MDASSML%% even if they have many years of experience. Even the most qualified person can make a mistake.

 

We may be distracted by personal problems (kids, spouses, finances, etc) or physical issues such as lack of sleep, sleep apnea and other sleep disorders, or due to medications. For this reason, OSHA directs us to de-energize equipment before we work on or near it: 29CFR1910.333(a)(1) states:

 

“Live parts to which an employee may be exposed shall be de-energized before the employee works on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations.”

 

The regulation is that live parts “shall be de-energized.” The word “unless” was an unfortunate choice, as it implies we can seek other means to work on energized circuits or equipment. That is not true, as OSHA takes a very narrow view of when work is allowed to be performed energized.

 

If there is an electrical accident, be prepared to defend your decision to work on energized equipment or circuits. Chances are you won’t be able to.

 

However, there are times when it would be considered infeasible to work de-energized such as voltage testing or other diagnostic work or work on batteries that cannot be isolated. In these rare events, NFPA 70E directs us to use an “Energized Electrical Work Permit.”

 

This permit ensures that the proper steps have been taken to assess the hazards, select the proper PPE and obtain the necessary management approvals to perform hazardous work. The NFPA 70E, Annex J has an example permit form that can be used. Since it is in the Annex, it can be modified to fit the needs of almost any workplace.

 

People using the tables in the NFPA 70E have a few of their own misconceptions. First, they don’t bother with the notes for the tables. They do know that the tables factor in hazard and risk; they know how to find the type of equipment they’re working on; and they do know how to choose the hazard/risk category, but things stop there. Why is it that we don’t want to take the time and effort needed to adequately protect ourselves?

 

Many workers rely on someone else to make decisions that directly affect their lives and health. Here’s a hint: Table 130.7(C)(9) factors in hazard and risk for equipment that was properly engineered, properly installed and properly maintained. Period! When using the tables in NFPA 70E or arc flash labels, you must also perform a risk analysis %%MDASSML%% regardless of what others may say. If the equipment you’re about to work on is decrepit, hasn’t had maintenance in years or is in any way suspect, the tables and the labels may not be adequate to protect you.

 

Beware of this myth: “I’m a P.E., so I am qualified to do your arc flash study.” Hold on just one second. Have an arc flash study done by a company that understands electrical power systems and the nuances involved in arc flash studies. Just because a person has a P.E. after his or her name does not mean he or she is competent to do your arc flash study.

 

Here at Shermco, we are in the process of redoing two arc flash studies that were originally performed by P.E.s, but who obviously did not understand how to do a proper arc flash study. It is way more than just crunching numbers. Those customers are paying twice for the same project.

 

Conclusions

 

My Dad would always say, “You gotta be smarter than the nail you’re hitting.” This is true, especially when working around energized electrical equipment. Take training from reputable, qualified people and companies.

 

How can you tell if they are qualified? If they advise you how to “get around” the OSHA regulations, or say that NFPA 70E is too restrictive, you probably want to go somewhere else.

 

There are a lot of wolves lurking around in sheep’s clothing. It’s like everyone who can walk and talk can do training. Hold their feet to the fire and make them prove what they say by using the OSHA regulations or NFPA 70E. Take the initiative to learn how to protect yourself %%MDASSML%% for you and your family. And don’t believe everything you hear on the grapevine.

 

This photo shows the results of an arc flash event on 480-V switchgear.

 

Appropriate personal protective equipment must be worn during panel cover removal of energized 480-V switchgear.

 

This worker is testing for the absence of voltage on 13.2 kV switch. Personnel must wear proper PPE %%MDASSML%% even when equipment is thought to be de-energized.

 

These workers are applying temporary personal protective grounds on a medium-voltage switch lineup.

 



 



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