The price for safety is small, but the costs are enormous

The Wolf Creek plant completed its 15th and shortest refueling and maintenance outage last November, after completing a plant-record continuous run of 506 days on October 7. One of the tasks completed during this outage was the retrofitting of several 13.8-kV and 4,160-V switchgear cubicles with breaker racking systems to protect personnel from the effects of an arc flash incident. Many arc flash accidents involving medium-voltage switchgear are caused by mechanical failure of the breaker racking mechanism.

In the old days

If you've been in the power generation industry for more than 20 years, you may remember using very little safety equipment—perhaps a pair of rubber gloves, overalls, hard hat, and safety glasses—to rack a breaker. Thermal protective suits with a hooded helmet were adopted in the late 1990s, but the air supply was limited to the volume of the suit. Today, although our understanding of the dangers of arc flash has improved, the quality and quantity of safety gear haven't kept pace in many plants. But there are new options many plants are adopting for worker safety.

Members of the Electrical Safety Program at Wolf Creek have studied the arc flash phenomenon for several years and adopted all the standard protective gear available on the market—including bulky 65 calorie/cm2 suits. Staff engineer calculations based on the IEEE Std 1584-2002 and NFPA 70E tables (at the time, the tables stopped at 40 calories) showed that this suit was an appropriate choice for working on 13,800-V metal-clad switchgear. To put these arc flash energy readings into perspective, realize that standard cotton apparel will ignite at about 4 cal/cm².

Defining the problem

The problem facing Wolf Creek's plant safety committee was that no one person or organization really understood the blast effects or the amount of thermal energy released during a high-voltage switchgear arc flash. In fact, plant engineers, when accounting for the relay and breaker response time, recalculated the worst-case energy release at 67 to 77 calories for the 13.8-kV switchgear. That was significantly more than any protective suit could handle—not to mention the equally deadly blast effects for which there is no protection.

Plant management realized that safer work practices and buying higher-rated safety gear was no longer the answer: Somehow, technicians and bystanders had to be safely separated from the source of the arc flash. The calculated blast/flash distance ranges from 112 ft to 215 ft with the switchgear cabinet door open, but it is just 30 feet with the door closed. Remember to think in three dimensions—the boundary extends from the switchgear cabinet front inside a cone-shaped impact zone. If your switchgear cabinet is sitting on the turbine deck supported by metal grating, then the blast zone could conceivably extend down or up one or more levels and would certainly extend into normal work areas.

Keeping the maximum distance from the source is the best safety practice when racking or working on live switchgear. The Electrical Safety Program staff looked at a variety of options and settled on the latest remote breaker racking option that gave technicians the option to move well outside the "danger zone" when racking in a breaker. The next step was to convince the plant's Leadership Council that spending several hundred thousand dollars to virtually eliminate the vulnerability to breaker arc flash would be worth it.

Estimating the cost

The $300,000 price tag for upgrading ninety 13.8-kV and 4,160-V breakers is peanuts compared with the human and lost-generation costs of a single arc flash incident. There may be no better way to loosen the purse strings of a stingy accountant than to have him or her watch a video of the surprising toll of an arc flash incident.

With approvals in hand, the Electrical Safety Program staff began their evaluations of alternative remote racking systems. As an intermediate measure, technicians are donning flash suits rated for the calorie rating of the work activity and establishing a flash boundary when any breaker is racked in or out. In addition, all operators at the Wolf Creek Plant are now required to wear fire-retardant work clothes during any electrical work on equipment 50 V or greater.

Making the upgrades

A comprehensive review of available remote-racking technologies revealed the usual range of similarities and differences. All of the designs investigated use some variation of a motorized racking mechanism, which allows the technician to remain well outside the arc flash and blast hazard boundary. Some designs require the cubicle door to remain open while racking the breaker. This approach provides minimal protection for anyone in the arc flash blast boundary but no added protection against mechanical malfunctions such as breaker alignment or shutter problems. Another design allows "door closed" remote racking but does not provide protection for a misaligned breaker or the failure of shutters to open—either of which could produce a fault that could trigger an arc flash blast.

After completing its evaluations, Wolf Creek chose the Safe-T-Rack (STR) remote racking system (Figure 4) from Switchgear Solutions (www.safe-t-rack.com). One of the key selection criteria was that the system required no internal switchgear cabinet rewiring. Because those cabinets are part of the plant's safety systems, the time and cost to design, approve, and update hundreds of drawings could exceed the capital cost of the STR system. At Wolf Creek, it had already taken several months to accumulate the data and twice that long to complete all the calculations and evaluations. Adding another year or two to the schedule by making changes to the plant's safety systems was out of the question.

4. Easy storage. A single tether and controller can be used for multiple STR-equipped switchgear cubicles. Source: POWER magazine


5. More than arm’s length. Wolf Creek’s Craig Barbee demonstrates how to rack a breaker with the Safe-T-Rack system while keeping a safe distance. Source: POWER magazine


6. Safety in a box. The Safe-T-Rack system control panel has overcurrent protection for the racking motor should the shutters not open. As the breaker stabs make contact with the malfunctioning shutters, motor current increases and the control panel trips power to the racking motor. Source: POWER magazine
 

7. Square deal. The LimiTilt is a device that attaches magnetically to the front of the breaker cover to detect if the breaker is out of level. This protects the worker should the ground stab hang up or if one side of the racking mechanism isn't working properly. Source: POWER magazine

The STR system is operated by a remote controller tethered to a small junction box mounted on the inside of the cubicle (Figure 5). The full-function controller keeps all of the needed controls at the operator’s fingertips, including open and close as well as breaker charge. Aircraft-style fittings (Figure 6) ensure longevity in the harsh power plant environment and prevent connections from being made incorrectly. A LimiTilt attached to the breaker (Figure 7) monitors the pitch and roll of the breaker during racking.

The STR's controls tap directly into the limit switch circuits, eliminating the need to modify control wires or original breaker controls. Unlike competing systems, this one doesn't require the cubicle door to be opened to attach or operate the remote controller. A typical STR cubicle kit can be installed in a few hours or less by an experienced electrician without having to de-energize the bus.

By the end of the 15th refueling outage, 38 of the Wolf Creek plant's 90 high-voltage switchgear units had been retrofitted with the STR; the rest are scheduled to be upgraded by the next refueling outage. Low-voltage switchgear (rated at 480 V and lower) will be upgraded for safety in the future.