Case study: Arc energy protection schemes

While the above discussion provides a general overview of 240.87 and its methods, it is beneficial to understand how the different methods may be included in an arc flash study. This section presents multiple examples as case studies of various arc energy reduction methods on a standardized simple electrical system. Table 1 provides a summary of the arc flash calculation results for each example.

Table 1: Arc flash calculation results — case study of common scenarios/overcurrent protection schemes. Courtesy: CDM Smith

The case study electrical system consists of a utility at 480 V with an available three-phase available fault current of 30 kA and a switchgear having one main circuit breaker of 4,000 A and several feeder circuit breakers of smaller rating. For this case study, the bolted-fault current of 30 kA yields an arcing fault current of approximately 20.8 kA. All calculations were performed using SKM PowerTools software. For this case study, we will assume the feeder circuit breakers are appropriately sized and selectively coordinated with downstream equipment. Standard circuit breakers and relays from major manufacturers were simulated; for methods, such as the arc flash relay or UFES system, manufacturer-reported operating times were utilized.

All calculations are based upon a simplified system; as such, only broad generalizations should be taken from the results. Arc flash calculations specific to a given system should be performed where necessary per NFPA 70: National Electrical Code, OSHA, NFPA 70E and other standards or requirements. In these case studies, the UFES performed best, producing an arc flash of almost negligible value. The arc flash relay and differential relaying methods produced similar results, with the arc flash relay performing slightly better thanks to the inclusion of light-sensing. The ZSI system for the circuit breaker used in these case studies had a slight delay compared to the minimum instantaneous delay, which resulted in it having a slightly higher incident energy than the maintenance switching option. All methods simulated, except for the base instantaneous pickup, produced incident energy results below the relatively common rating of regular/daily arc flash personal protective equipment (12 cal/cm²).

Figure 3: Instantaneous method — sacrificed coordination — curves overlap creating race condition for faults above ~12 kA. Courtesy: CDM Smith

In this example, the “base” method is built upon selective coordination with the feeder circuit breakers only, as a time delay is introduced, this would not satisfy the requirements to use an instantaneous pickup setting to satisfy 240.87. The “instantaneous pickup” scenario satisfies 240.87 but sacrifices selective coordination, potentially creating a race condition for high-level faults downstream of the feeder circuit breakers. Figure 3 shows an example of the coordination of the instantaneous pickup scenario with sacrificed coordination; Figure 4 shows an example of the “coordination” of the overcurrent protective devices when the maintenance switch is engaged.

Figure 4: Maintenance switching method — if not disabled after work, miscoordination for faults above ~7 kA. Courtesy: CDM Smith

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