How an arc flash relay reduces costs
Conaway Electrical Service, Lagrange, Ga., conducted an arc flash study for a major manufacturer in the southeastern United States; its engineering team knew that there were areas in the plant where electrical arc-flash safety was a concern. The best way to resolve those concerns was not immediately apparent.
Conaway’s customer manufactures electrical wire and cable ranging from data cables to heavy power conductors used in buildings, mining and general industrial applications. Much of the plant’s electrical distribution equipment is housed in electrical houses (e-houses).
One of those e-houses is a 20-foot-long trailer mounted up a flight of stairs on a mezzanine. It is filled with breakers, PLC panels and had a an arc flash hazard rating of Category 3, which corresponds to an incident energy of 8 to 25 cal/cm2. The Category 3 rating requires anyone working inside it to wear personal protective equipment (PPE) consisting of safety glasses or goggles, hearing protection, hard hat, cotton underwear, fire-resistant (FR) shirt and pants, FR coveralls (in addition to FR shirt and pants), arc flash hood, leather gloves and leather shoes.
Wearing this level of PPE was a real burden in terms of time and money for the manufacturer and its employees. The PPE was time-consuming to put on and take off, uncomfortable and confining.
The manufacturer had arc flash labels in place and had the proper PPE on hand, but it looked for a way to reduce the Category 3 hazard rating. On three other e-houses the company had paid Conaway to install a new circuit breaker between the e-house and the four breakers in question. Because of tight working spaces and large cumbersome existing wires, the labor cost was high. Each installation, including circuit breakers and parts, cost roughly $60,000. In addition, the reduction in arc flash hazard was insufficient.
The plant engineering team learned about arc flash relays from its electrical distributor, Mayer Electric, who suggested that an arc flash relay might provide a solution. This relay uses light sensors to detect the light of a developing arc flash and sends a signal in less than one millisecond to open the upstream power breaker. By interrupting the power quickly, it dramatically limits the amount of incident energy, preventing a small arc from growing into a dangerous and destructive event.
Representatives of Littelfuse, which manufactured the relay, demonstrated the arc flash relay. After Conaway approved the solution from a technical standpoint, Conaway was chosen to handle the project. Technicians installed the arc flash relay in an enclosure mounted to the outside wall of an e-house (see Figure 1), near one of the entrances. Figure 2 shows the installation.
On the front of the enclosure they installed a pair of lights that indicate the status of the arc flash relay (on or tripped). They also put in a small box with duplicate indicators next to the other entrance of the e-house, as shown in Figure 3. The idea was that workers would be able to see, before they entered the e-house, if the relay protection was not working. The relay had the ability to communicate via Modbus.
For sense input to the relay, they installed four point-light sensors in strategic locations (as shown in the center of Figure 4) and two fiber-optic light sensors, covering the entire top level cable tray.
Because there were four lines feeding the e-house, it would be necessary for the arc flash relay to trip four breakers simultaneously, which was initially a concern. Littelfuse application engineers Cory Anderson and Alex Kalinski worked through the calculations on the given circuit breakers; they found that the trip coils on the four breakers drew less than 200 mA, which combined were well below the 5.75-amp output trip current available from the arc flash relay, and they approved the installation. As expected, all four tripped during tests with no issues.
The relay instantaneously tripped the circuit breakers on each and every test. Installing the arc-flash relay saved $30,000 on this installation, about half the cost of the previous attempt, and the company acknowledges that it is a better solution than what they had tried in the past.
The interior of the e-house now officially has no arc flash hazard. Under last year’s classification system it would have been rated as a hazard risk Category 0, but that category has been eliminated as superfluous. No hazard now simply means no hazard, and workers no longer need to don PPE before they enter the e-house.
There are nine other e-houses in this plant, and there are 20 or more other plants around the country; the people involved in this project hope to be able to expand it to cover all those other e-houses, improving worker safety and protecting uptime.
How an arc flash relay works
An arc flash occurs when an energized phase conductor with sufficient current available faults to ground or another phase conductor. The result is essentially an electrical explosion, as metal vaporizes to form a cloud of ionized gas that radiates intensely across the electromagnetic spectrum, including visible, ultraviolet and infrared light sufficient to damage eyes. The radiated energy strikes anyone in the vicinity and can deposit enough thermal energy to cause first, second or third-degree burns in a fraction of a second. It can also ignite any burnable material in the vicinity, including clothing and hair. The arc flash also creates a high-pressure blast wave that can damage hearing, crush a person’s chest, throw him or her across the room and smash equipment.
All this is accompanied by shrapnel—bits of metal, both solid and molten, that are flung at ballistic velocities. The longer the arc continues to burn, the greater the damage. There are several methods for reducing the energy available to an arc flash, including the use of current-limiting fuses. An arc-flash relay uses light sensors to detect the intense light given off as an arc flash begins and within a few milliseconds sends a signal to an upstream breaker to open and shut off the power. This stops the arc flash in its tracks.
Use of an arc flash relay greatly reduces the hazard risk category of the panel or enclosure that it protects and can even (in some cases) eliminate it altogether. A qualified engineer should make that determination.
Bryan Waldrop is director of field operations for Conaway Electrical Service.