Arc-Flash Protection Saves Lives

An arc-flash occurs when the air surrounding a conductor becomes ionized, allowing the air itself to become an electrical conductor. The resulting fault current creates a dangerous and potentially fatal release of thermal energy.

One of the root causes of an arc-flash is air, which is an unreliable insulator. There are 2 inputs for the event to occur:

  1. Ionized air, which is an effective electrical conductor
  2. Fault current, which generates enough heat to keep the air ionized

The energy of the arc-flash is proportional to



is a voltage drop across the arc.
I is an arcing current.
t is a duration of a fault.


The duration of a fault is the sum of a relay trip time and breaker clearing time. As a protection engineer, we will focus on how do we detect an arc-flash condition rapidly and trip right away.


Arc-Flash detection is triggered from the light flash and the overcurrent occurring at the same time. The fault current can be detected, using conventional current input into a protective relay. Moreover, the flash form the arc produces a huge magnitude of light. Light Magnitude is measured in a standard unit of lux. A 2000 Amp arc fault, which is a very small one, can cause up to 2,000,000 lux. (Basically 20 times the light as a direct sunlight in a sunny day)


SEL has 2 types of arc-flash sensor:

Arc-Flash Point Sensor

1. Point Sensor

Arc-Flash Bare-Fiber Sensor

2. Bare-Fiber Sensor


Every sensor has 2 fiber optic cables. One of the fibers is used for sensing the arc-flash light. Another one is used for a loopback test from a visible red LED, which flashes every 10 minutes in a special pattern.

Arc-Flash Application

Example of using mixed arc-flash sensors

Incident Energy



This graph shows the reduction in incident energy from the test corresponding to the video, which results in calories per square centimeters. The relay can greatly affect the duration of an arc-flash and bring the incident energy way down. (Hazard risk category is based on NFPA 70E, a Standard for Electrical Safety in the Workplace.)


Arc-Flash Test


For more information about SEL arc-flash protection products, click here.