Personal Protective Equipment (PPE) is often mistakenly viewed as the solution to arc ﬂash hazards. In reality, properly selected PPE does not guarantee freedom from injury. NFPA 70E only makes the claim that injuries sustained during an arc ﬂash event would be reduced and survivable due to arc-rated PPE. Incorporating arc flash mitigation strategies lessen the potential for an arc flash event.
The most effective arc ﬂash safety programs look to incorporate “safety by design,” where mitigating techniques, including Engineering Controls, are employed to reduce risk.
The engineering controls covered in this two-part blog series will mitigate risk by either: 1) Reducing the available arc flash energy level (the focus of this blog), OR 2) Reducing the exposure of the worker so that he/she is not subject to harm.
The Role of the Circuit Breaker or Fuse in Lowering Arc Flash Energy Levels: Arcing time is one of the key determining factors for arc ﬂash energy. Arc flash incident energy varies linearly with time (source: IEEE 1584-2002). If the duration of the arcing fault doubles, the available energy doubles; if the duration is reduced by half, the energy is also cut in half. Since incident energy is proportional to arcing time, the proper selection of faster-acting overcurrent protective devices is a powerful mitigation strategy.
Over-Current Protective Device (OCPD) Coordination Study: Settings of adjustable OCPDs must be chosen carefully to maintain coordination among devices while also properly protecting equipment and allowing for normal load currents and routine temporary overcurrents (e.g., motor starting current) to ﬂow without causing a trip. If arc flash levels are not specifically considered as part of the study, opportunities to make adjustments to breaker and relay settings that can lower incident energy levels may be overlooked. While a coordination study is not a requirement of an arc flash risk assessment, it is recommended to have this study completed as part of the assessment in order to allow for determination of “optimal” OCPD settings that can meet these competing requirements.
Specialized Relaying, Such as Optical Technology: A key to arc ﬂash mitigation is quickly clearing faults. Circuit breaker or relay settings near the source of power may have signiﬁcant time delays to allow for coordination of downstream devices. Specialized relays detect arcing faults by looking for the ﬂash of light associated with the arcing fault in addition to the characteristic current ﬂow. However, to be detected, both the high current and a burst of light must exist. When both conditions are present, the optical relay can operate very quickly to clear the fault, which typically occurs through the operation of an overcurrent protective device.
Virtual Main Arc Flash Mitigation System: When fed directly from a transformer, switchgear and switchboards can be subjected to dangerous levels of arc ﬂash incident energy. Adding a virtual main system reduces the arc ﬂash energy on the entire switchgear, including the main incoming section. A digital relay and overcurrent sensing is added to the low-voltage side of the service transformer and is designed to trip an existing upstream fault breaking device, often a medium-voltage circuit breaker or other vacuum interrupter.
There are more arc flash reduction solutions available, and even more considerations that must be evaluated in order to develop an effective arc flash mitigation strategy. In the second of this two-part blog series, we will discuss the strategy of removing workers from harm’s way.