Distance is an important factor in arc flash incident energy: halving the distance from the arc will increase the energy by 4 times.
The working distance is the shortest distance a worker is expected to be in relation to the electrical equipment when an arc flash occurs. This distance is based on the voltage and type of equipment. Because radiated energy will be lower when moving further away, it will be at its highest at the working distance. For this reason, the incident energy is calculated at this point. It is recognized that hands could be closer than the working distance and higher protection than calculated incident energy is sometimes chosen for gloves for this reason.
Moving further away, the energy is reduced until at some point 1.2 cal/cm² is reached: this is the arc flash boundary. When standing outside of this boundary, no hazard mitigation is necessary. In the picture shown above the worker is standing within the arc flash boundary but not quite at the working distance, in this case personal protective equipment (PPE) is still required to protect against the worst-case incident energy.
The table below shows some typical values for the arc flash boundary for low voltage equipment:
|Incident Energy||Arc Flash Boundary|
|1.2 cal/cm²||0.5 m (working distance)|
|4.5 cal/cm²||1.0 m|
|9.0 cal/cm²||1.5 m|
|18 cal/cm²||2.0 m|
|36 cal/cm²||3.0 m|