target="_blank" rel="nofollow" href="#ulink_10f3d229-9c7b-54f9-ac51-996c5fde305b">(1.12) 
The equation is based upon data normalized for an arc time of 0.2 seconds, Where:
En = Incident energy (J/cm2) normalized for time and distance
K1 = −0.792 for open air and −0.555 for arcs in a box
K2 = 0 for ungrounded and high resistance grounded systems and −0.113 for grounded systems. Low resistance grounded, high resistance grounded, and ungrounded systems are all considered ungrounded for the purpose of calculation of incident energy.
G = conductor gap in mm (Table 1.5).
Conversion from normalized values gives the equation:
where:
E = incident energy in J/cm2
Cf = calculation factor = 1.0 for voltages above 1 kV and 1.5 for voltages at or below 1 kV
t = arcing time in seconds
D = distance from the arc to the person, working distance (Table 1.6)
x = distance exponent as given in Reference [9] and reproduced in Table 1.7.
A theoretically derived equation can be applied for voltages above 15 kV or when the gap is outside the range in Table 1.5 (from Reference [9]).
(1.14)
TABLE 1.6. Classes of Equipment and Typical Working Distances
Source: IEEE 1584-2018 Guide [9]. © 2002 IEEE. Also see Chapter 3.
| Classes of Equipment | Working Distance |
| 15-kV switchgear | 36 |
| 15-kV MCC | 36 |
| 5-kV switchgear | 36 |
| 5-kV switchgear | 36 |
| 5-kV MCC | 36 |
| Low voltage switchgear | 24 |
| Shallow low voltage MCCs and panel boards | 18 |
| Deep voltage MCCs and panel boards | 18 |
| Cable junction box | 18 |
TABLE 1.7. Factors for Equipment and Voltage Classes
Source: IEEE 1584 Guide [9]. © 2002 IEEE.
| System Voltage, kV | Equipment Type | Typical Gap between Conductors | Distance × Factor |
| 0.208–1 | Open air | 10–40 | 2.000 |
| Switchgear | 32 | 1.473 | |
| MCC and panels | 25 | 1.641 | |
| Cable | 13 | 2.000 | |
| >1–5 | Open air | 102 | 2.000 |
| Switchgear | 13–102 | 0.973 | |
| Cable | 13 | 2.000 | |
| >5–15 | Open air | 153 | 2.000 |
| Switchgear | 13–153 | 0.973 | |
| Cable | 13 | 2.000 |
For the arc flash protection boundary, defined further, the empirically derived equation is:
where EB is the incident energy in J/cm2 at the distance of arc flash protection boundary.
For Lee’s method:
(1.16)
Due to complexity of IEEE equations, the arc flash analysis is conducted on digital computers. It is obvious that the incident energy release and the consequent hazard depend upon:
The available three-phase rms symmetrical short-circuit currents in the system. The actual bolted three-phase symmetrical fault current should be available at the point where the arc flash hazard is to be calculated. In low voltage systems, the arc flash current will be 50–60% of the bolted three-phase current, due to arc voltage drop. In medium and high voltage systems, it will be only slightly lower than the bolted three-phase current. The short-circuit currents are accompanied by a DC component, whether it is the short circuit of a generator, a motor, or a utility source. However, for arc flash hazard calculations, the DC component is ignored. Also, any unsymmetrical fault currents, such as line-to-ground fault currents, need not be calculated. As evident from the cited equations, only
