We Have Produced Electricity for Decades, and Every Year We Learn More about the hazards and how to protect ourselves from those hazards. Failure of a medium-voltage breaker produces an arcing fault that causes an arc flash blast. This sounds pretty simple, almost innocuous. But, the phenomena and accompanying consequences are anything but simple.

WHAT IS AN ARC FLASH BLAST?

An arc flash blast is produced by an arcing fault, during which substantial electric currents pass through the air, typically containing vaporized arc terminal material such as copper. The arcing involves high temperatures of up to or beyond 35,000°F (19,427°C), which is approximately four times the surface temperature of the sun. Explosive pressures are created by an arc flash blast. Pressures are generated by expansion due to metal vaporization and the thermal expansion of the air that occurs as the arc passes through it.

As an example of the potential for damage in this situation, the U.S. military has developed a bomb that uses an explosively formed penetrator (molten copper) that can cut through the armor plating on tanks. These penetrators are fired into the target vehicle at high speed, penetrating armor plating and destroying what is underneath the plating. We do not want to find ourselves on the receiving end of our own arc flash-generated, explosively formed penetrator.

The air in the arc stream expands as it is heated up from ambient temperatures to that of the arc, to about 35,000°F. This expansion of copper and air results in a large pressure blast. Records show that such blasts have knocked workers off ladders or thrown them across rooms. Laboratories measuring the concussive forces associated with arc flash have found sound levels of 1400 dB and pressure levels of 2160 lbs/ft² (103 kPa) at 2 ft (0.6 m) from the flash on 480-V load center breakers. Eardrums rupture at around 720 lbs/ft² (35 kPa), and lung damage occurs at 1728 lbs/ft² to 2160 lbs/ft² (83 kPa to 103 kPa).

CHANGES IN PROTECTION

For years we stood in front of the breaker cubicle with nothing to protect us. Then, as we learned more about arc flash blasts, we began to provide personal protective equipment (PPE). The PPE has evolved from wearing leather gloves to wearing electrical-rated gloves and arc flash suits.

The National Fire Protection Association (NFPA) Standard 70E has been the bible for electrical safety for many years. Hazards were measured in calories per square centimeter. NFPA 70E ratings only went up to 40 cal/cm² and workers were not to perform work on live buses above that rating.

Recently, we learned the calculations used in NFPA 70E were missing some key components. The original calculations did not take into account the multiple components required to cause a breaker to trip. A fault-detecting device (which adds its time), a trip relay (which adds its time) and the breaker itself should all be taken into account. When you add the times together and compute your energy values, you will find that the calorie ratings can be double or even triple what the original calculations estimated the hazard to be.

With 5 to 10 arc flash blasts occurring in the United States every day, Wolf Creek Nuclear Operating Corp. realized it needed to do more to protect its employees. The company began looking into better arc flash suits. There are suits rated at over 120 cal/cm². At first look, these seemed to meet Wolf Creek's needs, since most large 13.8-kV to 15-kV bus arc flash energies are calculated to be around 50 cal/cm² to 80 cal/cm². This was the company's thinking until it realized what the 40-cal/cm² limit for live bus work was all about.

When there is an arc flash incident above 40 cal/cm², no thermal suit can protect the worker from a concussive blast. At an Institute of Electrical and Electronics Engineers (IEEE) conference, a doctor/scientist explained that in an arc flash blast above 40 cal/cm², an arc flash suit would only provide your family with a body to bury. The blasts described earlier in this article show that just the pressure to lungs and ears would cause life-threatening injuries and permanent damage to any worker directly in the blast. Also, the vaporization of the copper and the fragments from the explosion could kill a worker, even if the person had been wearing the best arc flash suit on the market.

THE ONLY SOLUTION

Wolf Creek has always racked (operated) medium-voltage breakers from right in front of the cubicle, ensuring that the shutter opened and that the breaker was rising or lowering level. The solution is to remove the worker from that blast zone right in front of the cubicle. Several companies are now offering remote breaker racking tools or systems.

Wolf Creek began looking into what was available on the market. The modifications to the breakers, the engineering costs and the potential downtime made the safety equipment seem expensive. In fact, many companies are allowing their employees to face this hazard simply from a dollars and cents position.

There are several companies that have a seemingly simple remote control for the racking and charging operation. Some companies have a “robotic” racking motor unit that removes the operator from the breaker area. And while some companies offer remote breaker racking that requires the cubicle door be open, others offer it with the door closed. When looking at these devices, Wolf Creek began asking questions about what is happening to the breaker and, if something did happen, how we would know about it since we would be 30 ft (9 m) away and would not be able to see the racking process. Also, what about the arc flash boundary, would having the door closed be better than having the door opened?

A REVIEW OF SYSTEMS

Over the years, we have witnessed a couple of breaker incidents where the breaker became hung up in the cubicle. Since we were watching it, we were able to stop and prevent it from continuing into the rosettes. There also have been incidents where the chain drive for the shutters malfunctioned. In each of these incidents, the racking process was stopped because we were watching the breaker. Had the operator not stopped the breaker, racking-in the misaligned breaker could have caused equipment damage or possibly created an arc flash blast.

With that understanding, we reviewed the available systems again. We wanted to have some equipment protection along with the personnel protection. Since we would not be able to see the breaker during the racking process, we would need the system to help us.

We found a system that includes a device that can detect if the breaker becomes out of level either fore and aft or side to side. Should it detect this out-of-level situation, the device will shut off the racking motor. The system also provides an overcurrent protection for the racking motor should the shutters not open or should the breaker become bound in the racking process. If this situation occurs, the overcurrent protection opens the breaker on the control panel, shutting off the racking motor.

This protection for the equipment gives the operator some knowledge of what is happening inside the breaker cubicle. This system also allows the door to be closed, which reduces the arc flash blast boundaries.

When we calculated our arc flash energies, we found that the arc flash boundaries had expanded to approximately 215 ft (66 m) with the cubicle doors open. With a boundary of approximately 215 ft, we would have to evacuate most of our turbine building every time we racked a breaker up or down. Normally that wouldn't cause a problem, except if we were off-line for repairs. Every breaker manipulation during an outage adds about 30 to 45 minutes to all work going on in the turbine building. With the doors closed, we regain a significant amount of the building, which eliminates the delays associated with evacuations.

The only system we found on the market that provides equipment protection and employee protection was Switchgear Solutions' Safe-T-Rack system. Together, IEEE and NFPA are performing studies, and testing and evaluating the hazards associated with arc flash blasts. At this time, we do not know how these studies will affect requirements or provide guidance for future energy hazards. But Wolf Creek believes remote breaker racking is the only way to safely rack medium-voltage breakers.

A BEST PRACTICE

Wolf Creek is installing the Safe-T-Rack system on all 90 13.8-kV and 4.16-kV breakers at its plant. This investment will not only save lives and prevent possible equipment damage, but also will pay for itself by reducing the arc flash boundary zones. The initial cost of the investment will be realized by the reduction of lost generation and will pay continued dividends from the initial installation through decommissioning. When we are in a refueling outage, if we can prevent losing an hour of critical path time, we will have paid for one-third the cost of the installation.

Most all other solutions fail to take into consideration the mechanical failures of the racking mechanisms and the aging process of equipment. Also, we realize if you are not able to watch the breaker and are completely removed from the process, you will be unable to prevent a mechanical failure from turning into an arc flash blast incident.

During an industry review in the summer of 2007, Wolf Creek was noted as having a “strength” in electrical safety, and the remote breaker racking project was listed as a best practice. Since that rating, several plants have contacted Wolf Creek to make benchmarking trips to find out how easy it is to make electrical safety a priority.

Arc flash blasts are real; they happen every day. We can't keep our heads in the sand any longer. Just because they haven't happened at your plant yet, does not mean an arc flash blast can't happen to you tomorrow. The price for safety is small, but the costs are enormous.

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Craig Barbee has been in the utility industry for 22 years. He has worked for Wolf Creek Nuclear Operating Corp. as a senior nuclear station operator, as an assistant to the superintendent of operations and as project manager for the remote breaker racking project. crbarbe@wcnoc.com

Bob Skidmore has worked in the utility industry for 22 years. He has worked for Wolf Creek for 17 years as the master electrician and then for five years as a maintenance specialist, in which he headed the Electrical Safety Program and served as co-project manager for the remote breaker racking project. roskidm@wcnoc.com