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.

03/28/2013


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

Fig. 1: Damage from an arc flash increases rapidly with time, and the faster the current can be shut off, the less damage there will be. Courtesy: LittelfuseBecause 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.


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