Controlling and containing arc flash with maintenance, technology
Brent Henry, director of marketing for power distribution and control assemblies at Eaton, talks with Plant Engineering about the continuing issues around arc flash and how plant managers can address these issues.
Plant Engineering (PE): Talk about the arc flash issue itself. We’ve been concerned about arc flash for decades, and we still have many arc flash incidents every day. Why?
Henry: First, it’s worth noting that arc flash events do not occur spontaneously in electrical equipment. They result from factors that can, for the most part, be controlled—correct installation, environment, regular maintenance, proper maintenance practices, and protecting the switchgear from foreign matter such as conductive dust, moisture, and rodents.
That said, arc flash continues to be a concern in the industry, and enhancing safety in the workplace continues to be critical. When regular, proper maintenance of electrical equipment is not performed, the risk of arc flash increases; with limited resources, there are competing priorities between expanding production, maintaining existing equipment, and replacing outdated equipment. Furthermore, installing or retrofitting equipment with the latest arc flash safety technology can strain budgets.
Second, I think it’s fair to say that many arc flash safety solutions available today present tradeoffs, which can make the buying decision difficult. The least expensive solutions tend to have the least effect on reducing risk, potentially leaving equipment vulnerable to arc flash damage and having little impact on safety. Other, more expensive solutions may enhance safety but may not protect the electrical equipment from damage. For example, arc-resistant gear is designed to redirect arc energy away from personnel, but the equipment itself would still sustain internal damage from arc faults, which could take months to repair or replace.
PE: How has technology adapted to the arc flash issue to date? Can you talk about the progress safety officials have made in recent years?
Henry: Broadly speaking, electrical equipment manufacturers have approached the issue of arc flash safety from two angles. The first is arc flash containment. Since safety is the number one priority for every organization, the thinking goes, a product that can redirect the arc energy away from the user, regardless of what occurs internally to the equipment, was a gigantic step forward. And that is the philosophy behind arc-resistant equipment. When all the doors are closed and latched and panels are in place, arc-resistant equipment contains and redirects the energy from an arc blast away from the user. Arc-resistant equipment enhances personnel safety, but often there is catastrophic internal damage to the equipment.
Manufacturers also have looked to reduce incident energy to address arc flash safety. Incident energy is the amount of energy, at a prescribed distance from the equipment, generated during an electrical arc event. It increases as available fault current and clearing time increase. So manufacturers have developed products that are capable of quickly detecting and clearing arc faults inside electrical equipment. The faster a given product clears the fault, the lower the incident energy will be, the less the equipment will be damaged and the safer it will be. Unfortunately, in many cases, these products have effectively hit a wall in terms of how quickly they can clear an arcing fault. This is because many incident energy reducing technologies rely on opening an upstream over current protective device to clear the fault. Power circuit breakers, such as those found in low-voltage switchgear, can take up to four cycles or 67 milliseconds to clear a fault after receiving a trip signal. This means that no matter how quickly an arc flash can be detected, it will take a few additional cycles to actually clear the fault. This creates a lower bound for the incident energy. Attempts to reduce it further require utilizing a device that can act faster than a power circuit breaker.
In power systems with higher available fault current, multi-cycle clearing times can result in unacceptably high levels of incident energy, which can require special PPE and can fail to prevent extensive equipment damage and process downtime.
PE: Why is Arc Quenching Magnum DS Switchgear different?
Henry: Eaton’s Arc Quenching Switchgear eliminates the tradeoffs between protecting equipment and providing enhanced safety. Arc Quenching Switchgear relies on a specially designed current limiting Arc Quenching Device, which can transfer and contain an arcing fault in less than one quarter of a cycle, or less than 4 milliseconds. By reducing the clearing time by a factor of 10, the incident energy is also reduced by an order of magnitude. This means that with 85,000 A of available fault current at 480 V, the incident energy will be well below the 1.2 cal/cm2 threshold for which second degree burns occur.
When the Arc Quenching Device receives a trigger signal from the Eaton Arc Flash Relay, it produces a lower impedance arc in a contained micro-environment within the Arc Containment Vessels located in the Arc Quenching Device. The lower impedance arc collapses the voltage and immediately extinguishes the unintended arcing fault as the current begins to flow into the Arc Quenching Device. This quenching operation occurs in less than 4 milliseconds. The arcing continues safely contained inside the AQD until the upstream power circuit breaker trips.
PE: Equipment damage and the resulting downtime is one concern from an arc flash incident. How does Arc Quenching Magnum DS Switchgear address this issue?
Henry: All of the benefits of Arc Quenching Switchgear stem from its incredibly fast arc quenching time, which results in dramatically reduced incident energy. The incident energy is reduced to such a low level that Arc Quenching Switchgear can pass the ANSI/IEEE C37.20.7 arc-resistant test guide in standard NEMA 1 construction, without the need for ducts, plenums, thicker gage steel, or any of the other enclosure reinforcements used in traditional arc-resistant equipment. And Arc Quenching Switchgear can exceed the protection provided by traditional arc-resistant equipment by passing the C37.20.7 test guide even when doors are open or panels and breakers are removed.
In fact, the incident energy is reduced to such a low level that damage to the switchgear is minimized or completely eliminated. So this means that the expensive switchgear assets are protected from damage and that downtime from an arc flash event can be drastically reduced, since minimal or no repairs will be necessary to bring the switchgear back on line. Recovery from an arc flash event used to consist of replacing the damaged switchgear
PE: What more should manufacturers do to prevent arc flash incidents?
Henry: Manufacturers need to continue to drive innovation in the field of arc flash safety to advance safety and protect equipment and processes. Furthermore, it’s critical that the most effective technologies to enhance safety and operational reliability are implemented and we participate in various industry organizations and standards committees to advance the adoption of the most effective solutions. Additionally, we must work closely with engineering consultants and our customers to advance safety where we work and in our homes so that the technology that meets the highest standards is specified and becomes standard.
Always remember to never work on energized equipment!
Learn more about the Arc Quenching Magnum DS Switchgear below.