How to specify an arc flash relay


How an arc flash relay works

Fig. 2: An arc flash relay uses light sensors and current sensors to detect the beginning of an arc flash, and then sends a signal to an upstream circuit breaker to shut of the power. Courtesy: LittelfuseAn arc flash relay (Figure 2) uses light sensors to detect the beginning of an arc flash and then sends a signal to an upstream device (typically a circuit breaker) to shut off the power in less than 1 ms. The benefit is in the disparity between typical overcurrent devices that can take up to two cycles (32 ms) to detect a fault and send a trip signal to the upstream device, and the arc flash relay that reacts in fractions of that time, thus lowering the incident energy. The disparity is even greater for some circuit breakers that have longer clearing times of more than 8 cycles/100 ms.

Where to install arc flash relays

In commercial buildings, arc flash relays are typically mounted inside the 480 V, 3-phase switchgear in the maintenance room. They are generally not specified for other panels because the power at those panels is typically below 300 V. 

In an industrial facility, arc flash relays are installed both in the main switchgear and in smaller switchgear distributed across the plant, as well as in motor control centers (MCCs) and other electrical panels, assuming voltage is above 300V. 

The arc flash relay should not be connected to the breakers on the branch circuits coming from the bus bar, but rather to a breaker upstream. If an arc flash occurs on the bus bar, then tripping downstream breakers won’t help.

Arc flash relays and power analysis software

Power system analysis software packages like SKM, ETAP, and EasyPower are starting to include arc flash relays in their component libraries. This allows users to perform what-if analysis for a variety of relay configurations, circuit variables, and fault conditions, although there are limitations: Current power system analysis software can model the overcurrent setting of an arc flash relay but not the light detection, which can lead users to increase the pickup value or time delay to avoid nuisance tripping; this could lead to arcing faults going unnoticed until it is too late. With some arc flash relays this is not an issue as the light is used to prevent nuisance tripping on electrical noise or momentary overload conditions while still allowing for very fast tripping, and in most cases there are work-arounds for the limitations of the software.

Arc flash relays and zone identification

Fig. 3: Generally, it is recommended to mount 1 or 2 sensors per cubicle to cover all horizontal and vertical bus bars, breaker compartments, drawers, and anywhere that there is potential for an arc fault. Threading a fiber-optic sensor through the cabineIn zone-selective interlocking, an electrical system is divided into zones, with each zone extending from the output of the breaker at its input to the input of the next breaker downstream. An overcurrent protective device that detects an overcurrent or ground fault in its own zone will trip instantly while simultaneously signaling the breaker upstream to go into delay mode. This maintains selective coordination and at the same time minimizes fault duration and the damage it causes. The arc flash relay can also be linked in a similar manner to provide backup protection. For example, an arc flash event in a MCC will cause the arc flash relay to send a trip command to the main circuit breaker in the MCC. If that breaker does not trip, the arc flash relay will send a signal to the linked arc flash relay upstream to trip the upstream feeder circuit breaker. Because an arc flash relay accepts multiple light sensor inputs, separate sensors can be installed in each cubicle, compartment, or bucket. Protection zones can be implemented by connecting several arc flash relays to the same breaker. If individual circuit protection is desired, it’s possible to install separate arc flash relays in each cubicle and connect them to the associated feeder circuit breaker.

Zone protection generally is not used in a commercial building application because safety is paramount, and if there is an arc fault then the entire switchgear cabinet should be de-energized.

How long a delay

Many arc flash relays have programmable time delays on their light sensor inputs whose purpose is to reduce nuisance tripping from such causes as flash photography. Typically, a programmable time delay filter can be set between zero (instantaneous light detection) and one or two seconds for special application conditions. As a general rule, the programmable delay should be set to the minimum value consistent with nuisance tripping avoidance 

Arc flash relays with current inputs can be set up to trip on either light input alone, light input inhibited by current input (no trip on light unless there is sufficient current), and on overcurrent. The overcurrent trip delay can be used for selective coordination. Coordination for arc flash protection is not practical due to the fast reaction time required and utilizing light as a fault detection method.

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