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:
.....[1]
where:
log = log_{10}
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:
.....[2]
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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