TABLE 1.16. Measures for Mitigating Arc Flashes at Key Locations in Data Centers
| System Location | Issue | Recommendations |
| LV service entrance equipment | Protection on HV side of transformer does not respond quickly for LV faults | Transfer trip, differential protection along with ZSI |
| UPS input/output switchboards | Selectivity requirements lead to extended clearing times | ZSI or optical detection |
| PDU secondary (480 V to 208/120 V) | Low fault levels and P combined with transformer inrush currents extend arc-clearing times | Maintenance switch, compartmentalization of PDU with administrative controls |
| Generator paralleling equipment | Low fault levels combined with multiple sources extend arc-clearing times | Bus differential or optical detection, adaptive relay settings |
| MV distribution equipment | Multiple utility sources and/or generator sources result in high fault currents | High-speed shorting switch, bus differential, and optical relaying |
Figure 1.11. An outfit for PPE HRC4.
Figure 1.11 shows a PPE outfit for HRC=4. It looks like a space suit and hampers the mobility of a worker to render fine tasks. The arc flash energy must be reduced to no more than 8 cal/cm2 with appropriate measures.
1.15.2 Arc Flash Labels
Computer-based software can generate a variety of arc flash labels. The format, size of the label, and even colors can be modified according to user’s choice. NFPA specifies minimum data that should be included in an arc flash label. Generally, the labels are laminated to withstand weathering effects. Figure 1.12 shows a specimen arc flash label.
Figure 1.12. A specimen arc flash label.
REVIEW QUESTIONS
1 The calculated symmetrical three-phase bolted fault in a 480-V switchgear assembly is 40 kA. Using IEEE equations, calculate the arc fault current. Assuming that the fault is cleared in 0.1 seconds, both for Ia and 85% Ia, calculate the incident energy release and arc flash boundary. Use the gap length and working distance as per IEEE tables. The 480 V three-phase system is high resistance grounded.
2 Repeat with Lee’s equations.
3 Repeat using Equation (1.9) of the text.
4 Consult NFPA 70 E and the text in this paper and specify the PPE outfits for category 3 and 4 hazard levels in detail.
5 What is the IEEE intent of calculating a second arcing fault current at 85% of Ia?
6 List five points in order of their importance for worker’s safety.
REFERENCES
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9 IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, 2002.
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