The National Fire Protection Association standard for "Electrical Safety in the Workplace" (NFPA 70E) calls for an arc-flash hazard analysis. To comply you must determine both a "flash protection boundary" and the personal protective equipment (PPE) requirements for people working within that boundary. Last week, we explained what a boundary is and why it's necessary. This week, we'll discuss how to calculate a flash protection boundary.

How to Determine the Boundary

The arc-flash boundary can't be determined by a casual survey of electrical equipment. The only practical way to determine this boundary is to:

1. Calculate the magnitude of the arc (a function of the available short circuit current);
2. Estimate how long the arc will last (a function of the interrupting time of the fuse or circuit breaker); then
3. Calculate how far away an individual must be to avoid receiving an incident energy of 1.2 cal/cm².

Requirements for Small Facilities

For small facilities that primarily use 240 volts or less and have minor power requirements (e.g., mainly lighting and receptacle loads), it may not be practical or economical to calculate arc-flash boundaries. For these workplaces, 70E provides a default flash boundary that can be used without calculations. The default boundary extends 4 feet from the energized exposed components, and any individual inside this boundary must wear proper PPE to avoid a permanent injury in the event of an arc-flash.

In most small facilities, however, the 4-foot boundary is probably overly restrictive and individuals may resist using PPE. This could result in enforcement issues.

Sometimes, though, the opposite may be true: The 4-foot boundary may be inadequate. NFPA 70E states in a footnote that the 4-foot boundary only applies where the available short circuit current doesn't exceed 50,000 amperes and the clearing time of the fuse or circuit breaker doesn't exceed 0.1 seconds, or any combination not exceeding 5,000 ampere seconds.

Does this footnote put small facilities back in the position of collecting data and conducting calculations to justify using the 4-foot boundary? Probably not. In most small facilities, if the electrical system was properly designed and has been properly maintained by competent electricians (always installing properly sized fuses and circuit breakers), the 4-foot boundary should be more than adequate to avoid any permanent injury from an arc-flash.

Requirements for Large Facilities

For other facilities, especially those having employees, contractors or service personnel that perform functions exposing them to energized components, the 4-foot default boundary is probably not practical or appropriate. My experience is that most equipment operating at 480 volts and less will have an arc-flash boundary of less than 12 inches. This means that flame resistant (FR) clothing for the face/chest area is not required when near that equipment.

However, my experience is also that most large facilities have some equipment where even the 4-foot default boundary is not adequate to avoid permanent injury in the event of an arc-flash. Consequently, when the limitation of 5,000 ampere seconds is exceeded or when realistic flash boundaries are desired, 70E provides a formula (based on IEEE Std 1584) to be used under engineering supervision (see table below). To use the formula requires knowing the available short circuit current and corresponding clearing time.

Getting the Data

To determine the short circuit current, ask the electric utility company about its delivery capability at the service entrance/meter point. (Getting this information can be as easy as a phone call or as difficult as pulling teeth.)

Proceeding from the service entrance to the equipment to be worked on, you must record the length, size and type of every conductor and the nameplate information of every transformer in that path. With this data and the right software, you can calculate a reasonable estimate of the available short circuit current for use in the flash boundary formula.

To determine the arc-flash clearing time, you must collect data on every fuse and circuit breaker in the circuit between the utility service entrance and the equipment where the flash boundary is to be established. Based on this data, time versus current interrupting information is then acquired from the protective device manufacturer.

Using the short circuit current and time-current data, a reasonable estimate of the time required to interrupt the arc-flash can be determined for use in calculating the arc-flash boundary.

Calculating the Boundary

Where only a few circuits need to be evaluated, it may be feasible to use the 70E formula to determine the arc-flash boundary. However, if many circuits are involved, you should consider using software or hiring a consultant

Some software perform all the calculations required, including determining:

• available short circuit current,
• fault clearing time, and
• arc-flash boundary.

The price for this software can exceed $10,000 and should be used under engineering supervision. Remember that you still need to collect the data before it can be entered into the software program.

Next week, we'll discuss what needs to be done once the arc-flash boundary has been determined.

Table: NFPA 70E formula to calculate arc-flash boundaries

WHAT I LOVE ABOUT SAFETY

By Woodrow Slone, Jr.

I grew up watching my dad come home each night from his shift in the mines. Many nights he would come home with some sort of minor injury. One night in particular (I was 5 or 6 at the time), dad came home with a very bad cut next to his left eye. I remember how upset my mother was because the hydraulic hose just missed dad's eye by less than half an inch. He could have lost his eye.

Another memory burned into my mind is the newspaper spread out on the floor beside dad's bed. Each day mom would replace it with a "clean" one. The paper was there so that at night when dad was sleeping he could spit out the black wads of coal dust that he would cough up. My dad always told me, "Son, never go to work in the coal mines".

I followed his instructions and went to college, then began a career as a grade school teacher. Two months after I graduated college, dad was rushed to the hospital because he could not breathe. He was diagnosed with severe black lung and was told he could not work anymore. Over the next couple of years, I watched dad and mom cope with the loss of income and the uphill struggle dad faced trying to get his life together. I can still see him going to the store with his nasal cannula and his oxygen bottle.

Eventually, I left the school system and went to work for a local coal company. Third shift night clerk, safe job, better pay than teaching. Four years later, I was promoted to Safety Tech and spent 9 years getting the "Work Safe" message out to each employee. I became an EMT, and because we kept a company ambulance on site, I began administering first aid to injured coal miners. I dealt with everything from people dying from heart attacks to being killed in on-the-job accidents (I won't list the details).

Before Thanksgiving in 1993 mom called me. She was very, very upset. I rushed over to the house and found her holding dad upright in a chair. He was unconscious. After a quick assessment it was painfully clear that my dad had suffered a major heart attack. After 45 minutes of performing CPR on him the ambulance arrived and the EMTs took over and transported my dad to a local emergency room where he was pronounced dead. The cause of death was ruled as complications due to black lung. Performing CPR on a close member of your family will leave a lasting impression.

The next year I was transferred underground and became a coal miner. I took my safety message with me and tried to be an example of how to work safe. (How well I did this or if I had any positive response, I do not know.) I am now back in the field of "Safety" with a different company. I have two mines with 375 employees that I am responsible for. In the 5 years that I have worked for this company, I have helped deliver the message that a "Safe Mine is a productive mine." We have won 2 inter-company awards for being the most improved mines in our company and three inter-company awards for having the best I.R. in the company.

I believe that if I can keep one miner from developing black lung or keep one miner from experiencing a disabling injury, then I have been a success in my career in safety.

By the way, I took your test. The first sheet of paper was filled, the second sheet of paper was blank. I will make a difference to someone. That was my promise to my dad when I put him in the ground. As long as I live, the message "Safety First" will always be heard in the hills of Eastern Kentucky.

Thanks

Woodrow Slone, Jr
Second Generation Coal Miner
Safety Coordinator
Excel Mining, LLC

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