Occupational Health & Safety

Arc Flash Training & PPE Protection

For the most effective worker protection, you're going to have to go beyond standard compliance.

I received a call recently from a large electric utility. A line worker, we'll call him Bill, had been badly burned while performing work near a 69kV piece of equipment. This incident illustrates several issues that a safety professional or engineer must consider in development of an electrical safety training and PPE program, two of which are not directly addressed in regulations or the consensus standards. Thinking through a clothing system and electrical safety program can save money and make compliance easier and more effective.

The worker was in a 50 cal/cm2 rated winter suit. The undergarments and outer garments were provided by the company, but workers were given a lot of latitude on undergarments as long as the outer shell was compliant with the company's clothing policy.

The worker chose not to wear an arc-rated sweatshirt even though this was commonly available in the company's clothing catalog. Instead, the worker was wearing a non flame-resistant T-shirt under the sweatshirt. Though this system was in compliance with NFPA 70E, NESC, and OSHA 1910.269, it was not the best choice for the worker. Commonly when doing assessments for electric utilities or any electrical installation, companies will consider the OSHA standards and the latest consensus standards such as NFPA 70E and the NESC; in addition, the most diligent companies will have their engineers or a contractor perform detailed arc flash studies and label equipment. Yet many companies do not thoroughly think out their full program. This article will introduce you to a few new things you can consider to help go beyond standard compliance to a better-thoughtout approach to arc flash hazards.

Understanding Arcs

When engineers talk about electric arc nowadays they teem with enthusiasm. Many have popped up shingles on the Internet touting their expertise in arc flash. Most are not experts and often have little more than exceptional knowledge of electrical theory and software predictors. Clothing companies consider themselves experts, too, but few really are and the vast majority are blinded by their product mix focus. Having internal experts is a great idea, and such professionals should direct the internal game; getting training and having periodic external expert program audits and periodic system assessment audits will bring much to improve even the best program. Equipment and clothing vendors are a nice free source, but an independent auditor will see things these folks will miss and his value often will be a life-saving program tweak that may have little or no cost.

An electric arc is a complex phenomenon that is just being better understood in the electrical world. The effect on clothing is interesting, and the field of research is growing in many ways that are not getting much publicity but that can affect the effectiveness of your arc flash program. Usually in the popular literature you will hear of two types of arcs, but there are actually four types of arcs and the last two are rarely considered in arc hazard assessments and only cursorily addressed in the arc flash calculations.

The four types of electric arcs are:

    1. Open air arc (this is the primary arc used in arc testing)
    2. Arc-in-a-box (this is used in one form for arc testing in the EU)
    3. Ejected Arc (when arc plasma hits the worker)
    4. A tracking arc (most common at higher voltages, arc plasma conducts on skin or through clothing)
Only the first two are considered in the calculations and standards. The open air arc is well understood. In lab testing, we control movement of the arc for the sake of repeatability, but in real life from 480V higher the open air arc can quickly turn into an ejected arc or a tracking arc. The tracking arc is most common at very high voltages or during an electrical contact. This is the arc type that caught Bill. The standards and calculations didn't offer Bill the best protection, which would have been arc rated garments over his whole body. Obviously, Bill broke several company rules that would have prevented this injury entirely and the company isn't to blame, but to make a program more "forgiving" of worker mistakes, arc rated clothing on the whole body makes the most difference.

Toward Better PPE

In doing arc flash calculations, some of the effects of the ejected arc will be accounted for by default. The IEEE 1584 equipment configurations account for some of this type of arc, but the tracking arc is totally unaccounted for, even in the literature. I recently had a utility company safety person in a heated meeting tell me that NESC doesn't require them to consider it at all. This is a misunderstanding of how standards work and their intent. Certainly one would like simple, definable compliance to a known standard, but OSHA standards do not require you to only consider certain defined hazards but to attempt to protect from all "recognized hazards" even if they are not easily definable.

Dr. Tom Neal of Neal Associates Ltd. and Michael Lang of Ferraz-Shamut have introduced us to "ejected arcs" in several IEEE papers in the past two or three years, but this hasn't gained much momentum (Dr. Neal and I have an ASTM F18 taskforce working on a test to measure this effect). The new ASTM F2676, of which I am taskforce chair, uses an ejected arc to rate blankets for their resistance to arc plasma in a defined current over time. This revolutionary standard, developed by the ASTM F18 committee, might enliven research for better arc flash clothing, protective blankets, shields, and even dramatically safer equipment. Research using this model could also lead to a more realistic model of arc flash calculations (it is not the intent of the author to disparage current calculation techniques but to help spur research using ideas gleaned from the leading edge of clothing testing). Dr. Neal and Lang's papers have opened a new insight into PPE that could lead to much better PPE and less arc flash energy.

Researchers in ASTM have long known PPE performs differently at different amperage exposures. Lower amperages and longer exposures create the same total energy, but the PPE response is dramatically different. Plasma ejection is usually a lower part of the total energy in a lower amperage arc such as the 8000 amp arc used in the ASTM arc testing. Recent testing using a high-speed camera and the ASTM test set up indicate that the bulk of the plasma doesn't reach the mannequin/panel until the arc energy reaches 10-15 cal/cm2, and as the energy increases more and more plasma reaches the test surface. In higher current arcs more of the plasma reaches the test surface faster. Recent tests have shown this at work.

At 10kA an ejected arc plasma cloud, as picked up on a high-speed camera (see Figure 24), has reached a worker 34 inches away in 23 microseconds, while a 20kA arc has reached a worker at 34 inches in 11 microseconds and ignited a cotton T-shirt at that distance in about 15-25 microseconds.

Because of the newer information coming out all the time, we recommend including the following steps when performing an arc flash hazard assessment and building an arc rated clothing program that will give better protection from all four types of arcs:

    1. Mandatory clothing for electrical workers should offer choices, but it should be clear, enforceable, and adequate for protection.
    2. No melting "flame-resistant materials" anywhere in the system.
    3. Use (non-melting) flame-resistant materials, especially for the upper torso, including undergarments. Underwear is a huge issue. I have seen three underwear ignitions under arc rated outer garments.
    4. Think about moisture management underneath the clothing for worker comfort and reduction of the risk of tracking arc. Arc rated T-shirts may eliminate the use of 7-9 oz. outer shells in shirts in the next few years. Two layers are often more protective and more comfortable to wear.
    5. Use clothing systems that provide arc rating in multiple layers rather than just a thick, 8 cal/cm2 shirt. Though compliance is easier with a single garment, making all the garments arc rated will almost always provide better protection.

This article originally appeared in the August 2009 issue of Occupational Health & Safety.

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