240 V or less.
The circuit is supplied by one transformer.
The transformer supplying the circuit is rated less than 125 kVA.
This qualification has now been removed in 1584, 2018 edition.
The user is referred to IEEE Guide 1584 for three-phase systems rated less than 240 V.
1.8.1 Ralph Lee’s and NFPA Equations
Ralph Lee equations from Reference [11] are as follows:
Maximum power in a three-phase arc is:
(1.5)
where MVAbf is bolted fault mega-volt-ampere (MVA).
The distance in feet of a person from an arc source for a just curable burn, that is, skin temperature remains less than 80°C, is:
where t is the time of exposure in seconds.
The equation for the incident energy produced by a three-phase arc in open air on systems rated above 600 V is given by:
where:
D = distance from the arc source in inches
F = bolted fault short-circuit current, kA
V = system phase-to-phase voltage, kV
tA = arc duration in seconds.
For the low voltage systems of 600 V or below and for an arc in the open air, the estimated incident energy is:
(1.8)
where EMA is the maximum open air incident energy in cal/cm2, F is short-circuit current in kA, range 16–50 kA, and DA is distance from arc electrodes, in inches (for distances 18 in and greater).
The estimated energy for an arc in a cubic box of 20 in, open on one side is given by:
where EMB is the incident energy and DB is the distance from arc electrodes, inches (for distances 18 in and greater).
1.8.2 IEEE 1584 Guide Equations
This is based on IEEE 2002 Guide. Included here for reference and completeness.
The IEEE equations are applicable for the electrical systems operating at 0.208 to 15 kV, three-phase, 50 or 60 Hz, available short-circuit current range 700–106,000 A, and conductor gap = 13–152 mm. For three-phase systems in open air substations, open-air transmission systems, a theoretically derived model is available. For system voltage below 1 kV, the following equation is solved:
TABLE 1.5. Classes of Equipment and Typical Bus Gaps
Source: IEEE 1584-2018 Guide [9]. © 2002 IEEE. Also see Chapter 3.
| Classes of Equipment | Enclosure Size (in) | Typical Bus Gaps (mm) |
| 15-kV switchgear | 45×30×30 | 152 |
| 15-kV MCC | 36×36×36 | 152 |
| 5-kV switchgear | 36×36×36 | 104 |
| 5-kV switchgear | 45×30×30 | 104 |
| 5-kV MCC | 26×26×26 | 104 |
| Low voltage switchgear | 20×20×20 | 32 |
| Shallow low voltage MCCs and panel boards | 14×12×≤8 | 25 |
| Deep voltage MCCs and panel boards | 14×12×>8 | 25 |
| Cable junction box | 14×12×≤8 or14×12×>8 | 13 |
(1.10)
where:
Ia = arcing current in kA
G = conductor gap in mm, typical conductor gaps are specified in [9] (see Table 1.5)
K = −0.153 for open air arcs, −0.097 for arc in a box
V = system voltage in kV
Ibf = bolted three-phase fault current kA, rms symmetrical.
For systems of 1 kV and higher, the following equation is solved:
This expression is valid for arcs both in open air and in a box. Use 0.85 Ia to find a second arc duration. This second arc duration accounts for variations in the arcing current and the time for the overcurrent device to open. Calculate incident energy using both 0.85 Ia and Ia and use the higher value.
Equation (1.11) is a statistical fit to the test data and is derived using a least square method; see Appendix A for a brief explanation of least square method.
Incident energy at working distance, an empirically derived equation,
