An arc flash is the light and heat from an electrical arc fault, a discharge through air to ground or another phase. Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit. NFPA 70E and IEEE 1584 guide protective practice, and PPE is the last line of defence, not the first.
What happens in an arc fault
The arc heats, its resistance falls and current rises until something melts, trips or vaporises. Metal conductors vaporise and expand explosively, with copper expanding about 67,000 times in volume according to the reference reviewed. The companion effect, arc blast, is a pressure wave. Radiant output runs from far infrared to ultraviolet.
The same article gives an example: a phase to phase fault on a 480 V system at 20,000 A is about 9.6 MW, and over 10 cycles at 60 Hz that is about 1.6 MJ.
The two main references
NFPA 70E guides safe work practices around exposed or potentially energised conductors. Its hazard category table maps tasks and voltage levels to recommended PPE categories. OSHA's electric power standards reference it.
IEEE 1584 gives a calculation method for incident energy from the maximum fault current, fault duration and equipment data. The same equations give flash boundaries. It is one of two ways to select PPE, the other being the NFPA 70E table.
Arc rating is the maximum incident energy a PPE material resists, in cal/cm2, tested under ASTM F1506. The flash protection boundary is the distance at which an unprotected person has a 50 percent chance of a second degree burn.
Reducing exposure by design
PPE protects only after an arc flash has happened. Engineering controls come first:
- Lower upstream protective device settings temporarily during maintenance, to shorten arcing time.
- Use remote operation and remote racking, so workers stay outside the hazard zone.
- Consider zone selective interlocking, arc light detection, ultra fast earthing switches, current limiters and high resistance grounding.
Fault duration is the lever software can touch. Protective relays are programmable, so settings, firmware and setting groups are safety relevant. A wrong setting group left active can lengthen the arc.
Where electrical safety meets software
Modern switchgear and relays have network interfaces and settings files. Treat those as part of the safety system. Control who can change protection settings, record changes, and keep known good copies. If a remote command can open or close a breaker, include it in the work procedure, alongside the lockout in our LOTO checklist.
OSHA on electrical hazards
OSHA lists main hazards including contact with power lines, lack of ground fault protection, missing or discontinuous grounding, equipment not used as prescribed, and improper use of cords. It recommends insulation, guarding and grounding, protective devices, safe work practices and training.
Quick check
Is PPE enough on its own?
No. The reference describes PPE as the last line of protection, to be used with engineering controls and safe work practices.
Does de-energising remove the need for lockout?
Working de energised without locking out is a hazard OSHA asks employers to consider. Follow the lockout procedure for the equipment.
Sources
Secondary summary; consult NFPA 70E and IEEE 1584 for current requirements.
Last reviewed: 19 October 2026.
