How to specify an arc flash relay

Engineers must consider arc flash prevention in the electrical systems that supply power for HVAC, elevators, plant machinery, and other high-power equipment.

By Justin Mahaffey, Littelfuse, Chicago March 28, 2013

As awareness grows of the extreme danger of arc flash hazards for electrical and maintenance workers, and in response to greater focus on arc flash from OSHA and NFPA 70E, building designers are being asked to consider arc flash prevention in the electrical systems that supply power for HVAC, elevators, plant machinery, and other high-power equipment. 

One approach is to specify arc flash relays to be installed inside electrical cabinets. These relays detect the light of an emerging arc flash in <1 ms and send a trip signal to the shunt trip of an upstream device such as a circuit breaker. This article will explain arc flash relays and the considerations for their selection, such as fault current at the panel, trip time, sensor placement, and zones.

Arc flash defined

An arc flash is a sudden release of energy caused by an energized conductor shorting to either ground or another phase. It can be caused by a dropped tool, something as apparently harmless as a misplaced test probe, or by a ground fault that escalates into an arc flash. 

Arc flash events can also be prevented by ground fault relays and resistance grounding systems, which will protect against faults resulting from a phase coming in contact with ground, but they will not protect in an event where a phase comes in contact with another phase. 

Generally speaking, an arc flash is possible on systems operating at voltages from 300 V and above. However, arc flash incidents at 208 V have occurred when the available fault current was very high, including in high-rise buildings and older commercial buildings where 208 V is used instead of 480 V. An arc flash that lasts for 10 ac cycles on a 480 V system with 25 kA available fault current releases as much energy as detonating 2 lbs of TNT. 

It produces a blast wave that can smash equipment cabinets, damage or destroy a person’s hearing, collapse lungs, and in some cases fling a victim across a room. It can propel debris and blobs of molten metal at ballistic speeds. It also produces an intense flash of light—ranging from ultraviolet through infrared—that can cause third-degree burns on exposed body parts within a fraction of a second. After the blast, wiring insulation and other components may be on fire, creating toxic smoke. The danger of arc flash is the reason that personnel working on energized electrical panels are required to wear cumbersome flash-resistant personal protective equipment (PPE) and electrical panels must be carefully labeled with information on safe approach distance and level of PPE required.

Preventing arc flash

Because workers cannot be counted on to de-energize equipment or follow all safety procedures, it falls to the system designer to mitigate arc flash hazards. The key is to minimize the available energy. As shown in Figure 1, damage from an arc flash increases rapidly with time, so the faster the system can clear the fault, the less damage there will be. 

One way to defend against arc flash is to retrofit electrical cabinets with arc flash relays, which reduce arc duration by sending a trip signal to the upstream device faster than conventional over-current relays, thus limiting the incident energy and protecting workers from hazards. In many cases, the protection provided by an arc flash relay can reduce the level of PPE required for compliance with NFPA 70E safety standards and OSHA workplace safety requirements.

How an arc flash relay works

An 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

In 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.

What to look for in an arc flash relay

The most important selection criteria are reaction time, trip reliability, avoidance of nuisance tripping, sensor design and installation, ease of use, and scalability and flexibility. 

  • Reaction time: The fastest possible trip times for available arc flash relays vary from less than 1 ms to about 9 ms. Because the idea is to limit energy by shortening the duration of the event, faster is better. The total time it takes for the fault to be cleared is the sum of the response time of the arc flash relay and the time it takes for the circuit breaker to open on a relay-trip input, which may be 50 ms.  
  • Trip reliability: The two main aspects of reliability are trip redundancy and system-health monitoring. An arc flash relay with redundant tripping has a secondary backup for its primary trip path logic that takes over in case of failure. The backup function is not influenced by any time delay programmed for the primary trip path. Since it is not dependent on input power to the relay, it can respond to an arc flash fault that occurs when powering up the system after a plant shutdown even before the unit’s microprocessor is fully operational (which may take 200 ms). To maintain full arc flash relay operation despite power interruptions, some arc flash relays have provision for battery backup. 
    Health monitoring makes sure the system is in good operating condition and should extend all the way from the light sensors to the output of the trip circuitry.
  • Avoidance of nuisance tripping: Most arc flash relays have fixed light thresholds between 8000 and 10,000 lux. Bright light—opening a switchgear cabinet to strong sunlight, a camera flash, or nearby welding—could cause a nuisance trip. A programmable time delay might prevent this from happening, but, as shown above, it is better to avoid delays as much as possible. A better alternative is to use an arc flash relay that accepts current transformer inputs; this way the unit can ignore a bright light that is not accompanied by a sudden current increase. The current inputs can also be used for overcurrent tripping without an arc, if desired. 
  • Sensor design and installation: Most arc flash relay installations use multiple fixed-point light sensors. It’s best to install enough to cover all accessible areas, even if policy is to only work on deenergized systems. At least one sensor should have visibility to an arc fault if a person blocks another sensor’s field of view. Light sensors may also be installed in other electrical cabinets and on panels that are subject to routine maintenance and repairs, such as those associated with MCCs. 
    Some arc flash relays can also work with fiber optic sensors that can range in length from 26 to 65 ft. This is a good way to make sure all areas are covered. However, long “open” fiber strands designed for light reception over their entire length should be used with caution. If an arc flash occurs at the far end of such a strand, then the light arriving at the detector end may not be sufficiently bright to cause a relay trip due to attenuation (db loss) along the fiber length. Some manufacturers avoid this problem by using interconnecting hardware with hardwired outputs from the interconnection and detector points back to the arc flash relay. Some products allow up to six light sensor inputs per relay, and up to four relays can be interconnected to act as a single unit. This means up to 24 light sensor inputs can be used to monitor an electrical equipment configuration.
  • Ease of use: It’s generally better to choose an arc flash relay that does not require field assembly, calibration, or advanced configuration before installing, simply to prevent errors in setup or configuration. Event-logging software, which is provided in some relays, also helps to make troubleshooting easier.
  • Scalability and flexibility: Some arc flash relay designs allow the interconnection of multiple devices, such as multiple relays, each with several sensors. This can be useful in situations such as an MCC that does not have a main breaker that can be relay-tripped. Here an arc flash relay in the MCC can trigger an arc flash relay in an upstream feeder cabinet to shut off the power. 

By understanding the application of arc flash relays and their selection considerations, building designers can design safer electrical systems and respond to OSHA and NFPA’s increased focus on arc flash hazards.

Justin Mahaffey is sales engineer for Littelfuse, where he helps customers improve uptime and worker electrical safety. His experience includes heavy industrial applications such as oil and gas drilling and electric power utilities. Early in his career, Mahaffey worked as both a test and product engineer.


White, James, 2010. Exploding the Myths about Arc Flash, Plant Engineering, April 8, 2010