Example Calculation

A single line diagram is provided below to illustrate how to calculate arc flash arcing current. The bolted fault current is calculated using MVA method for simplicity.

Given:

K = -0.097 (Box Configuration)
Ibf = 34.73 kA
log Ibf = 1.540704783
V = 0.46 kV
G = 25 (LV MCC from Table 2)

log Ia = -0.097+0.662log(34.73)+0.0966x0.46+0.000526x25+0.5588x0.46xlog(34.73)-0.00304x25xlog(34.73)
log Ia = 1.259474086
Ia = 18.17498605 kA

For low-voltage applications, under 1 kV, a reduced arcing fault current which is 85% of the arcing fault current Ia is required to be calculated to allow a second calculation needed due to variation in arc current. For cases where the arc current falls on the steep part of the time-current curve or falls near a step change, the variation in arc current could cause a significant error in protective-device tripping or operating time.

Ia = 85% x 18.17498605 = 15.4487 kA

On the next post, we will be calculating the Incident Energy and PPE using the provisions of NPFA 70E.

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