New Technology to Trap Killer Sparks
General Electric engineers invent an arc-flash absorber that can consume 5000 amperes and trip in mere microseconds
PHOTO: Salisbury by Honeywell
4 March 2009—On the morning of 14 April 2006, an engineer was working on a 480-volt disconnect switch in the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory, in New York, when a blinding flash of heat and light left him seriously burned. The cause, an electrical hazard called arc flash, rattled the lab and set off a chain of investigations.
The engineer was lucky to escape with just first- and second-degree burns on his face, chest, arms, and hands. Arc flash is an explosion that happens when the electrical resistance of air breaks down, connecting conductors with an arc of hot plasma. The temperature of an arc can exceed four times the surface temperature of the sun, and a 10 000 ampere arc at 480 V packs the equivalent power of about eight sticks of dynamite.
It is estimated that at least five to 10 serious arc-flash incidents occur every day in the United States alone, according to a report by engineering services company ESA. The loss in time and money amounts to about US $1 billion annually. The U.S. Department of Labor’s Bureau of Labor Statistics shows that arc-flash injuries account for 77 percent of all recorded electrical injuries. But, as the Brookhaven investigation revealed, a lot of arc-flash events are preventable.
At General Electric’s John F. Welch Technology Centre (JFWTC) in Bangalore, India, a team of engineers has developed what it believes is the best way yet to prevent arc-flash hazards. It’s a device called an arc-flash absorber. It uses ionized gas to transfer the open arc into a contained electrical system in less than 200 microseconds. In the process, it absorbs 20 percent of the electrical energy, thereby reducing the damage caused by the short-circuited current and lessening the overall stress to the electrical system. The GE engineers plan to report details of their work at the IEEE Petroleum and Chemical Industry Committee Technical Conference, in September, in Anaheim, Calif.
To combat arc flash today, power grids deploy either energy-limiter or energy-diverter systems. As the names suggest, the former cuts off the supply of electricity, taking about 10 milliseconds to react. The latter is more commonly used and diverts the arc by deliberately shorting the circuit, taking about 5 ms to react. However, the diversion can stress an industrial plant’s or distribution grid’s entire electrical system. And the longer it takes for these circuits to react, the greater the damage to upstream equipment, such as the transformers. Since the total electrical energy is a function of voltage, current, and time, dramatically reducing the arc-transfer time from 5 to 10 ms to less than half a millisecond minimizes the damage significantly.
”Our technology is fundamentally different from the existing ones and will pave the way for safer power distribution in the near future,” says T. Asokan, senior scientist at JFWTC. One big difference is that, unlike other existing technologies, the arc absorber doesn’t involve any moving parts, thereby improving its reaction time.
In the event of an arc fault, the GE device, which is connected in parallel to the electrical system, triggers a plasma gun that produces a current pulse 10- to 20-microseconds long and as large as 5000 A through the air inside the arc absorber. The plasma transfers the arc flash to a double-layered safe-containment vessel, allowing the electrical system to trip. The hot, ionized vapor produced during this operation evaporates a layer of ablative polymer, instantly cooling it, so the gas can be vented out safely.
”Depending on the ultimate cost of the device, it could be a quantum leap in electrical safety,” says John P. Nelson, president of NEI Electric Power Engineering, in Arvada, Colo. Even though this would be used only in new and retrofitted systems, Nelson thinks there are still a lot of electrical systems that could use this device.
The developers have tested the arc absorber for a short-circuit level of 65 kiloamperes, that is, for low- (600 V) to medium-voltage (15 000 V) electrical systems but say it can be scaled up to 100 kA. Nelson believes that even the 65 kA rating will handle the vast majority of electrical systems. ”It would appear that the theory would work for practically any system, so there is a high probability that it could be developed for higher-rated equipment.”
The prototype that GE has built can be fitted in a regular control panel and, depending upon the magnitude of the current, can withstand six to 10 operations, says Asokan. The device is being prepared for global safety certification from organizations such as Underwriters’ Laboratories and the Canadian Standards Association.
About the Author
Seema Singh is a Bangalore, India–based writer. In the June 2008 issue of IEEE Spectrum she detailed Tata Sons’ scheme to turn a profit selling supercomputing services.
To Probe Further
The GE engineers plan to report details of their work at the IEEE Petroleum and Chemical Industry Committee Technical Conference, in September, in Anaheim, Calif.
IEEE and the National Fire Protection Association are involved in a collaborative research project to better understand and prevent arc flash.