How Is an Arc Flash Detected?

Three variables are known to govern the amount of energy released in an arc-flash event:

  1. The magnitude of fault current.
  2. Working distance.
  3. Duration of fault.

The energy released in an arc flash directly correlates with the duration of the fault. It is obvious that the goal should be to reduce as much time as possible for the protective devices since a time reduction by half will reduce the energy released by half. The typical clearing time for breakers varies depending on the device responsible for clearing faults, ranging anywhere from 1.5 cycles (1.5 cycles = 0.025 seconds) to eight cycles (0.13 seconds). Other methods are available, including differential relays and zones interlocking. However, the quickest way to detect a fault, up until now, is with an arc-flash detection relay (AFDR), as it can detect an incident within 0.15 cycles (0.02 seconds).

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What are they?

A large amount of energy is released in the form of heat and light when an arc flash occurs. Optical sensors placed in the equipment can detect this light almost instantly. The sensors transmit light from the flash through a relay to start a trip using solid-state devices. The optical sensors constantly monitor the ambient conditions, and a sudden change could cause a false-positive to be sent to the relay. For example, if the sensor was being used to monitor conditions in a bucket at a motor control center (MCC), and the maintenance worker opened the door, the sudden increase in light might trigger the relay to interpret it as an arc flash. To prevent shutting down a site unnecessarily, AFDR systems incorporate instantaneous overload elements to detect faults. The relay will only intervene when the light level increases and a fault is detected.

Arc Flash Event:


AFDRs are not a new technology and have been used since the early 1990s. The optical cables used back then were insulated, and they only detected light on the cable tip, known as point sensors. Today, a new type of uninsulated fiber optic cable allows light to be received in 360 degrees along the length of the cable. These cables can be routed into a loop to test the system’s health and ensure everything is working properly. Moreover, these sensors can detect ultraviolet light, allowing them to react to the highest wavelengths of radiated energy (200 nm-600 nm). Both technologies can be combined depending on client needs; the fiber optic cable runs along the switchgear busbars, while point sensors monitor cubicles and other items such as buckets, panels, and disconnects, etc.


The best equipment for AFDRs is that which has interrupters made of SF6 or vacuum. These devices will clear the fault on their own under “normal” fault conditions. In this case, the sensor would not detect any light, and any detected light would indicate an arc flash. The arc is broken up in air when used with equipment and lower voltages that utilize arc chutes. The ambient light will be altered under normal fault clearance. The positioning of sensors is crucial for the system’s proper functioning.

Some manufacturers have integrated this technology into their devices to provide added protection. For instance, Mersen manufactures a medium voltage fuse that can communicate with AFDRs. The traditional fuse in the transformer’s primary winds takes time to react when an arc flash occurs on the secondary. The fuse controller receives input from the AFDR, which monitors low voltage conditions. When an event occurs, the fuse controller sends a signal to the fuse via fiber optics. This command tells the fuse to switch the current supply to a smaller fuse, which opens quickly and in a controlled way. This rapid-action process reduces the time needed to fix the fault and, in turn, the energy released by an arc flash.

In conclusion:

The devices constantly monitor light and current, so they only trip if an event occurs, and there is no need for coordination with existing electrical systems. The relays work in a reactive mode, and their main goal is to shut down the source of the event. It is also difficult to test the reliability of the AFDR system if the optical cable does not form a loop. The relay is designed to stop an arc flash. The AFDR is a wonderful addition to any electrical system, whether it’s new or old, as it adds a level of safety in the event of an accident. The quick response of an AFDR could save thousands in equipment costs and millions in downtime. It could even be the difference between life and death.

This post was written by Justin Tidd, Director at For nearly half a century, Swartz Engineering has been at the forefront of industry safety. They are a family-owned company specializing in power distribution for the electrical industry. Our design ensures maximum flexibility for excellent reliability and a high return on investment.

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