The industry has relied on a variety of active and passive technologies to decrease arc-flash risk. Passive solutions, such as arc-resistant switchgear, contain energy from an arc flash and redirect it up and away from the user. Active systems use a variety of arc-flash detection techniques to trip an upstream circuit breaker and reduce switchgear incident energy. They include zone-selective interlocking, bus differential relays, arc detection relays and energy reducing maintenance switches — all of which can be used to comply with National Electrical Code (NEC) Section 240.87, which lists methods designed to reduce arc energy in systems of 1200 amps or more.
However, both passive and active systems have drawbacks. Passive solutions enhance user safety, but do not protect equipment. And the incident energy reduction achieved via active solutions is limited by the clearing time of the upstream breaker, which can exceed four cycles. With these limitations, some manufacturers pursued high-speed arc mitigation solutions to offer enhanced safety, superior incident energy reduction, advanced equipment protection and minimized downtime. It was out of these improvements that UL 2748 was born. Published in August 2017, this standard does more to improve safety and protect equipment because compliant devices no longer depend on the clearing time of upstream breakers or other overcurrent protective devices (OCPDs).
UL 2748 covers devices intended to quench arcing faults by creating a lower impedance current path, located within a controlled compartment, to cause the arcing fault to transfer to the new current path. Since current always follows the path of lowest impedance, a lower impedance current path causes current to stop flowing through an uncontained arcing fault and instead into the arc quenching equipment, thereby instantly extinguishing the arc flash.
There are two ways to create a lower impedance current path: apply a bolted fault to the switchgear or create a parallel but lower impedance arcing fault that’s fully contained inside arc containment vessels. Though systems that apply a bolted fault to mitigate an arc flash have existed for decades, few if any solutions in the field have been tested to the UL 2748 safety standard as of this writing. One of the downsides of bolted fault, or earthing switch solutions, is they draw peak available fault current during operation. This produces enormous electromagnetic forces on upstream equipment and can lead to damage or even secondary, uncontained arcing faults upstream.
The second method, creating a lower impedance current path via a parallel arcing fault, provides a current-limiting benefit that is not possible in a bolted fault solution. Such systems draw significantly less peak fault current when they operate and, therefore, induce much less stress on upstream equipment. Certain current-limiting arc quenching systems were developed in conjunction with UL 2748 and now comply with the Standard.
Because UL 2748 devices essentially transfer arc energy to a contained location until an upstream OCPD opens and de-energizes equipment, they tend to have a much faster operation time than circuit breakers. And when used in conjunction with an arc detection relay, their use results in much lower incident energy than other methods described in NEC 240.87.
This standard does more to improve safety and protect equipment because compliant devices no longer depend on the clearing time of upstream breakers or OCPDs.
Regardless of the method used to create the lower impedance current path, devices must pass thorough performance tests to comply with UL 2748, including:
The first three tests are typically performed on low-voltage metal-enclosed switchgear per ANSI/NEMA C37.51 and are conducted again with the arc fault quenching equipment installed and operating in the switchgear. The tests result in a continuous current and withstand rating specifically for the arc quenching device.
The fourth test involves performing IEEE C37.20.7 arc fault testing to “demonstrate that the quenching device does not create an arc fault hazard during the quenching operation.” While this proves that an arc quenching device doesn’t create an arc fault hazard during operation, it does not mean that the switchgear is arc resistant when an arc quenching device is installed. However, some manufacturers have done additional testing to prove that the arc quenching device, when used in conjunction with a suitable arc detection system, can reduce incident energy to such a low level that standard NEMA 1 switchgear can pass the C37.20.7 test guide to attain an arc-resistant rating.
The fifth test is very specific to UL 2748 and determines the “maximum time for an arcing fault to transfer to the intended lower impedance fault.” In other words, this test determines the time between when the arc quenching device receives a signal to operate and when the unintended arcing actually ceases. This time can be combined with the time to detect the arcing fault (typically the speed of the arc detection relay) and used to calculate the incident energy of the switchgear with the arc fault quenching equipment installed.
UL 2748 devices are a significant advancement over traditional active and passive means of reducing arc-flash risk. When used in conjunction with a means of arc detection, they can dramatically reduce incident energy, enhance safety and equipment protection, and minimize downtime — often without the need for ducts, plenums or special enclosures. And while a UL 2748 device may require replacement after an arc-flash event, the switchgear protected should sustain little or no damage.
Arc quenching switchgear is a “big-picture investment.” The technology is about six to 10 percent more expensive than arc-resistant switchgear when comparing total installed costs. But, should an arc flash occur, an arc-resistant configuration’s cost of recovery often far exceeds arc quenching switchgear’s price tag. In short, the cost of UL 2748 device replacement is much lower when compared to repairing or replacing a switchgear lineup.
Furthermore, it can take anywhere from three to six months to fully replace an entire line of equipment. And if the switchgear in question enables revenue generation, those dollars are lost during the prolonged downtime. In contrast, replacing an arc quenching device often takes less than a day — substantially increasing the likelihood of faster revenue recovery.
UL 2748 sets the groundwork for faster technologies than those that rely on upstream OCPDs to clear faults and supports the development of equipment that exceeds the safety provided by many traditional methods used to meet NEC 240.87 requirements. My hope is that more manufacturers and end users will realize the potential of this standard and implement arc quenching switchgear to increase safety, save equipment and substantially reduce the financial impact of arc-flash events.