Avoiding arc flash in the nick of time

Arc flash relay saves glass plant almost immediately after installation.

By Tim Deschamp, Evans Enterprises

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A U.S. manufacturer of refractory materials recently averted catastrophe by a slim margin. An arc flash began in a switchgear cabinet near the control room, but a just-installed arc flash relay shut off the power before major damage could occur, saving the plant from expensive downtime and equipment replacement costs.

The company operates a plant that makes fiberglass insulating blankets and glass insulating bricks. It is an older facility, and much of its electrical equipment dates to the 1970s. The plant had been having major issues with utility power; there were voltage dips, for which the utility refused to accept responsibility, that caused product quality problems. The refractory company then called on Evans Enterprises to install power quality metering that would detect any anomalies in the incoming power. Evans also did a number of other things around the plant, including installing shunt trips on all the circuit breakers.

An imminent danger

Prior to bringing Evans in, the refractory company had hired an engineering firm to conduct an arc flash analysis in an effort to reduce the arc flash hazard categories of equipment throughout the plant. While this might seem routine, the analysis had uncovered a dangerous situation: One 480 V cabinet, which was fed from a 3,500 kVA transformer, exceeded arc flash hazard Category 4, and there was no level of personal protective equipment (PPE) that would allow a worker to approach the cabinet if it were open. The door to the cabinet was therefore kept closed, and everything related to it was done remotely, but the cabinet, along with the heavy transformer associated with it, is located in a room next to the control room within feet of the operators. This was clearly an untenable situation.

An arc flash is a sudden release of energy caused by a fault between an energized conductor, either to ground or to another phase. It creates intense light that can cause third-degree burns, a high-pressure blast wave, and flying debris.

Protecting against arc flash

A circuit breaker — even a fast-acting one — by itself offers little to no protection against an arc flash, because it cannot operate quickly enough to stop the arc before it reaches dangerous levels. The same is true of a breaker operated by an overcurrent relay. Another issue is that a high-impedance arc fault may not draw enough current to cause a breaker to trip instantaneously, which requires a high-value overcurrent. An arc welder with a 1/8-in. rod, for instance, may use arc current of just 120 A, which is less than the normal trip current of many power-panel breakers. Ground-fault relays and resistance-grounding systems help to protect against arc flash caused by ground faults, but not in the case of a phase-to-phase fault, which is more likely to be caused by electrical worker error and therefore put human life in danger, not to mention the damage to equipment and downtime. One way to protect against all arc flash incidents regardless of the cause is an arc flash relay, which responds to the light of the arc flash itself and shuts off power before the arc grows to a catastrophic level.

How an arc flash relay works

Figure 1. Minimal damage occurred to the bus bar. The whole cabinet would have been destroyed had the arc flash relay not been installed.
Figure 1. Minimal damage occurred to the bus bar. The whole cabinet would have been destroyed had the arc flash relay not been installed.

An arc flash relay works by detecting the bright light emitted by an arc and activating the relay trip input to the main circuit breaker upstream, generally in 1 ms, although this time is adjustable in some relays. Most main power breakers will open in 50 ms or less, greatly limiting the damage (Figure 1). The light is detected with a set of sensors, either individual point type or a distributed fiberoptic cable type that is sensitive over its length. Some arc flash relays can also use current transformers on the power inputs to the cabinet or enclosure to be protected. The idea here is to help the relay distinguish between a real arc flash and a sudden bright light that might be caused by nearby welding, flash photography, or even sunlight falling on the open door of the cabinet. If the current transformers are in use, then the arc flash relay looks for a sudden increase in current coinciding with a bright light; if the current is unchanged, then the relay will not trip, helping to prevent nuisance trips. Figure 2 shows a typical connection. In this particular case, current transformers were not needed because the cabinet was in a room by itself with no bright light sources near it.

Figure 2. The electrical cabinet shows arc flash and the arc-flash-relay installation.
Figure 2. The electrical cabinet shows arc flash and the arc-flash-relay installation.

When the refractory company asked Evans Enterprises to see what could be done to reduce the arc-flash hazard category, Evans decided to install a PGR-8800 arc flash relay from Littelfuse, which could be retrofitted in the existing panel.

Speed of response is critical; the whole idea of an arc flash relay is that it limits the energy released during an arc flash by “seeing” the light emitted from the flash, interpreting it as a fault, and triggering the upstream breaker to clear the fault in a time that is much faster than a typical overcurrent relay can react. An examination of the specifications for several arc flash relays on the market shows that the fastest possible trip times vary from less than 1 ms to about 9 ms. Trip reliability in the relay can be ensured by redundant trip circuits (both analog and digital) and self-diagnostics, including continuous closed-loop checking of the condition of the sensors.

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