Arc Flash – Calculating Arcing Current

Arcing current is used to calculate the incident energy from an arc flash. The predicted three-phase arcing current, which is lower than the bolt fault current, must be determined so that operating time for protective devices can be set.

For a system with a voltage under 1000 V, equation (1) will be used:


log = log10
Ia = arcing current (kA)
K = –0.153 for open configurations or
K = –0.097 for box configurations
Ibf = bolted fault current for three-phase faults (symmetrical RMS) (kA)
V = system voltage (kV)
G = the gap between conductors, (mm) (see Table 2)

For system voltage of 1000 V and higher, equation (2) is applicable:


Table 2—Classes of equipment and typical bus gaps
Classes of equipment Typical bus gaps (mm)
15 kV switchgear 152
5 kV switchgear 104
Low-voltage switchgear 32
Low-voltage MCCs and panelboards 25
Cable 13
Other Not required

Source: IEEE 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations