The other electrical hazard; AEDC's arc flash safety program gets high marks Published May 3, 2010 By Philip Lorenz III AEDC/PA ARNOLD AIR FORCE BASE, Tenn. -- An arc flash can happen in less than a thousandth of a second, causing substantial damage, fire and death. A dramatic example of this phenomenon is a lightning strike, as often seen during a thunderstorm. Equally dangerous is the type of arc flash that can occur at industrial complexes like AEDC. An arc fault happens when electric current flows through air gaps between conductors. Equipment failure and accidents, like touching a test probe to the wrong surface or a slipped tool, are the most common causes of an arcing fault. Arc flash is caused by uncontrolled electrical current traveling through the surrounding air which is then ionized into plasma. The heat energy and intense light at the point of the arc is called arc flash. Arc faults are extremely dangerous and potentially fatal to personnel. The product of arc fault current and voltage concentrated in one place, results in enormous energy released in several forms. Arc fault generates large amounts of heat that can severely burn human skin and set clothing on fire. Temperatures at the arc can reach higher than the temperature of the sun's surface. The arcing faults also produce large shock waves that can blow personnel off their feet. The other exposure risks to arcing faults include flying debris, severe sound waves, shock hazard due to touching energized conductors, and etc. Tom Carpenter, Manager of ATA's Electrical Systems Engineering section, said it was only within the last few years that the danger posed by this phenomena was fully recognized and an effort put forth to address it throughout the electrical industry. Approximately six years ago, John Miller, ATA's Deputy General Manager at the time, tasked Carpenter and others on his team to put together a plan for an arc flash safety program for ATA and later the Air Force's upper management to assess. Tommy Kirk, an electrical system lead with the 704th Civil Engineering Squad, said Carpenter and the others on the team have accomplished a lot since the program began. "We have one of the best arc flash safety programs in the Air Force," Kirk said. "It's still ongoing. We're making improvements in the system, we're lowering hazards everywhere." Kirk said AEDC is unique due to its electrical system requirements for ground testing and the infrastructure that supports the mission. "We probably have the largest 480-volt system in the Air Force," he said. "We have several thousand 480-volt breakers on base." Since he came on board at AEDC in 2005, Kirk said many of the 480-volt circuit breakers on base have been replaced with modern versions that reduce the threshold of arc flashing. The goal is to have breakers that will trip upon detecting a fault while simultaneously avoiding tripping circuits due to relatively minor surges and power fluctuations. "The modern circuit breakers that we're putting in react quicker to a fault than some of our older equipment did," he said. "They'll clear a fault quicker, so the energy won't have a chance to build. A lot of the older 480-volt breakers use electro-mechanical trip units and can take two to three seconds to clear a fault. A modern breaker with a microprocessor based trip unit can clear a fault in three thousandths of a second. "In many cases this will effectively eliminate the arc flash hazard," he explained. "Replacing equipment is not the only way to reduce a hazard. We can install an electronic trip unit in an old breaker, which is a cost effective option if the electrical equipment is otherwise serviceable. We can also place operational constraints on electrical equipment that has a high arc flash hazard rating." He acknowledges that keeping these systems safe, but also fully functional is an ongoing balancing act. However, his team not only passed the Unit Compliance Inspection in 2008, but Carpenter and the team received recognition for the manner in which they have operated their arc flash safety program. "As I recall, what made the program note-worthy during that inspection was how mature it was compared to other bases in AFMC," said Lt. Col. Robert Voegtly, who led the inspection team and is currently the 36th Wing inspector general at Andersen AFB, Guam. "It was a fairly new and stringent safety requirement and we assessed that Arnold really took it seriously. There was a well documented program which was being executed at every level." Kirk said the first step in the arc safety program literally involved an in-depth study of the 480-volt electrical substations on base, starting with those in the test areas, like the Propulsion Wind Tunnel, Engine Test Facility, Aeropropulsion Systems Test Facility, the von Kàrmàn Gas Dynamics Facility and Space and Missiles. "The first thing that has to be done during an arc flash study is to verify the physical configurations," Kirk said. "We take the as-built drawings you have available, then physically walk down the system, verify it, the conductor sizes and lengths, equipment ratings and so forth." Carpenter said the next part of the program was broken into three main phases. "One is to gather the data that we need to build an electrical model," he said. "That includes cable sizes, lengths, breaker types, sizes, transformer sizes; other things that you need to model the electrical systems in a computer. "The next phase was to build the model - an engineer sits down, takes all that data, and builds the model in the computer. And that computer model then calculates arc flash incident energy and boundary. "The final phase is actually putting the labels out on the equipment to identify what the hazards are and the PPE required. However, we are also looking at ways to reduce the incident energies associated with an arc flash, e.g. changing protective device settings, replacing equipment, etc. Carpenter described the process undertaken by his team. "We couldn't do the whole base at one time, so we worked an area at a time," he said. "We started in the ETF area and we have primarily finished those. We completed the walk down in ETF and while we were creating the model for that, we were doing walk downs of PWT. "We're working our way through the base area," he continued. "The next area we're doing walk downs in are the general office facilities. On top of that, we're updating [electrical substation and related systems] drawings. The bulk of money we're spending right now is on refurbishment of electrical equipment." The amount of energy generated in an arc flash determines the PPE required. This could range from a situation where a low level of protection is needed to a powerful enough flash in which no degree of PPE would protect a worker.