How to specify an arc flash relay
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.
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