Arc flash hazard is calculated as incident energy in cal/cm² and represents the radiant energy that contacts a worker’s skin when an arc flash occurs. Note that this is radiation only, other hazardous effects such as pressure waves are not expressed in this number. A minimum of 1,2 cal/cm² is defined as the point where incident energy can lead to serious injury when not mitigated. On the so-called Stoll curve this is the point where, in 50% of the cases, exposure leads to a second-degree burn.
Hazard calculation of incident energy takes a lot of factors into account, the main factors include:
The time it will take to interrupt the arcing current is determined by how fast the feeding fuse or circuit breaker operates. The poor electrical conductance of air means that the arcing current is less than the short-circuit current – sometimes only 35% of it – and it is not immediately obvious what this operating time is. For example, a fuse chosen for its quick operation under short-circuit fault conditions may take up to 1 second to interrupt the arc. Similarly, the magnetic trip on a circuit breaker may not operate, leaving the thermal element to interrupt after a much longer delay.