The timing of this installation was incredibly fortunate. Over the years, sand used in the manufacturing process had accumulated around the cables in the pit below the cabinet; the cable insulation had become brittle as a result. It was just a matter of time before something happened, and only one week after the PGR-8800 was installed, the plant experienced an arc-flash incident. Had the PGR-8800 relay not been there, this incident would have caused catastrophic damage to the facility and months of downtime. It all started as a phase-to-ground fault in one of the cables below the cabinet. The arc tracked up the cable to the main bus inside the cabinet, where it threatened to escalate into a three-phase fault, drawing more energy and potentially destroying the cabinet, and possibly, the plant—not to mention the effect on any of the people who were usually nearby.

As the arc reached the cabinet, all six of the light sensors reacted to the bright arc-flash light, even the ones that didn’t have a direct line of sight. The arc-flash relay responded immediately, sending a trip signal to the main breaker in less than 1 ms. Deschamp was able to confirm this as each sensor shows its trip status. Without the PGR-8800, the main breaker would not have reacted fast enough to stop the catastrophic arc flash. Instead, damage was minimal: a thumb-sized piece of metal was burnt off one of the bus bars. The cabinet and the switchgear inside suffered only minor damage.

“Had this relay not been there, they were looking at between $800,000 and $1 million of cost, considering downtime and equipment replacement,” Deschamp said. “Besides the lead-time for delivery and the installation, they would have had to cut a hole in the outside wall of the plant to install it.” Deschamp estimates that lead-time on electrical equipment of that size, with 4000-amp capacity, would be anywhere from 10 to 24 weeks— and that’s just for delivery, not installation.

The actual cost of the replacement switchgear would have been a small portion of the overall loss for the plant. Instead, the entire cost to the end-user was only $6000, which included replacing a few insulators and a portion of the bus bar. Fortunately Evans Enterprises, Tulsa, Oklahoma, was able to obtain the 1970s-vintage equipment and the plant was back up and running within 24 hours. Immediately after the incident, the customer requested that Deschamp install Littelfuse PGR-8800 Arc-Flash relays in all of their larger electrical cabinets.