Keep the following in mind when working on exposed, energized electrical conductors or circuit parts:

3 Rules-of thumb for arc flash

1.) Incident energy decreases by the inverse square of the distance
2.) Incident energy is proportional to time
3.) Each layer of arc-rated clothing reduces the heat to the body by approximately 50%

Remember that if the incident energy decreases by the inverse square of the distance, it will also increase by the square of the distance as you approach an arc source.


Electrical systems fed by a transformer rated less than 240 volts with a capacity of less than 125 kVA pose an arc flash hazard.

IEEE 1584 says that calculations may not be necessary for these systems but that doesn't mean there is no hazard concerning arc flash. The IEEE 1584 guide only applies to three-phase power systems fed by a transformer rated less than 125 kVA and less than 240 volts.

A single-phase transformer can have more arc energy. Additionally, there is a hazard of flying molten metal that could be ejected from the equipment. The hazard is reduced, not eliminated.


Arc flash protective clothing ratings

According to ASTM F1959, arc-rated clothing and equipment are rated to allow a 50 percent probability of a second-degree burn if that clothing is exposed to the rated incident energy for 1/10th second. Some clothing could experience break open at its rating.


Correct body position for operating electrical equipment while wearing an arc-rated face shield

Face the hazard so the face shield does not become a heat scoop. For example, stand on the side with the hinge, extend whichever arm is appropriate, face the hazard and operate the equipment.

Some people fault that approach, reasoning that the PPE is designed to protect you, but arc-rated PPE is designed to protect you if its rating is not exceeded. It's impossible to know if the OCPD is going to function correctly. The door may blow open and could strike the operator, causing injury.


Serious injury can result from arc flash, even at lower voltages

OSHA has documented at least two instances where people suffered serious injury at 120/240 volts. One worker received significant third-degree burns to his chest, hand, and arm.


Reference: NETA World Spring 2011 Tech Quiz