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Repeat Example 1.4 with rigorous calculations.

Table 1.8 equations do not specify the gap length and also the system grounding. As the equations are for the maximum incident energy, it can be assumed that the system is ungrounded. Also for Table 1.8 equations, a working distance of 610 mm (2 ft) is specified in 1584 Guide. Thus, there are qualifications and compromises in using these equations.

To start the rigorous calculations, a time–current plot of settings on 800-A LVPCB and 400-A MCCB is shown in Figure 1.10. The maximum time on the trip band is used, shown by dots in this figure. This figure is based on circuit breakers of a certain manufacturer. This shows that the LVPCB maximum ST delay band is used, as the approximate equations are based upon this assumption. The opening time of the low voltage circuit breakers is built in the time–current plots supplied by the manufacturer. Thus, we consider the maximum operating times as shown by bold dots in Figure 1.10. These arcing times are 0.5 second for 800-A LVPCB and 0.017 second for the 400-A MCCB.

The results of the calculations are in Table 1.13. The calculations have been made for grounded and ungrounded systems, and with gaps and working distances as shown. A LVPCB will be provided in a lineup of LV switchgear, and a working distance of 610 mm and gap of 32 mm are applicable as per IEEE 1584 Guide. A 400-A MCCB may be installed in a MCC, where the working distance is reduced to 455 mm and the gap is 25 mm. The variations in the results are apparent and significant for an application.

Figure 1.10. To illustrate the arcing time for low voltage circuit breakers, at the calculated arcing current.

TABLE 1.13. Arc Flash Calculations, Example 1.5

Breaker	Bolted Fault (kA)	Arcing Fault (kA)	Trip Time (sec)	Opening Time (sec)	Arc Time (sec)	Ground	Gap (mm)	Working Distance (in)	Arc Flash Boundary (in)	Incident Energy Cal/cm2	PPE
800-A	36.8	19.95	0.5	0		No	32	610	198	27	4
800-A LVPCB	36.8	19.95	0.5	0		No Yes	32 32	610 610	198 166	27 21	4 3
400-A	36.8	19.95	0.017	0		No	25	455	21	1.5	1
MCCB						Yes	25	455	18	1.2	0
(MCC)