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:

$log~{I_a}~=~K~+~0.662~log~I_{bf}~+~0.0966~V~+~0.000526~G~+$
$~~~~~~~~~~~~~~~~~~~~0.5588~V~(log~I_{bf})~-~0.00304~G~(log~I_{bf})$ …..[1]

where:
log = log₁₀
I_a = arcing current (kA)
K = –0.153 for open configurations or
K = –0.097 for box configurations
I_bf = 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:

$log~I_a~=~0.00402~+~0.983~log~I_{bf}$ …..[2]

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

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