Mitigation is defined as, “to make milder, less severe or less violent.” When applied to electrical workplace safety, arc flash mitigation involves taking steps to minimize the level of hazard and/or risk associated with an arc flash event. ANSI Z10-2012, Occupational Health and Safety Management Systems, released a hierarchy of arc flash mitigation controls, as shown below.
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 mitigating effects of arc-rated PPE.
The most effective arc flash safety programs look to incorporate “safety by design,” where effective mitigating techniques, including engineering controls, are used to reduce the risk to the worker. The engineering controls covered in this article will reduce risk by either:
- Reducing the available arc flash energy level, or
- Reducing the exposure of the worker, so that he/she is not subject to harm.
Solution Group 1: Reduce Arc Flash Energy Levels
The Role of the Circuit Breaker or Fuse in Lowering Arc Flash Energy Levels
Why is a circuit breaker or fuse always considered in arc flash analysis? Because arcing time is the key determining factor for arc flash energy. Per the equations in IEEE 1584-2002, arc flash incident energy varies linearly with time. If the duration of the arcing fault doubles, the available energy doubles; half the duration and the energy is 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.
OCPD Coordination Study
An Over-Current Protective Device (OCPD) coordination study optimizes the protective device setting for reliability and arc flash protection. While an OCPD study is not a requirement of an arc flash analysis, it is recommended to have this study completed as a component of an arc flash analysis.
The OCPD coordination study will determine if minor adjustments in circuit breaker (or other over-current protective device) settings can lower incident energy levels. However, settings must be chosen to properly protect equipment while also allowing for normal load currents and routine temporary overcurrents (e.g., motor starting current) to flow without causing a trip.
Specialized Relaying — Optical Technology
Quickly clearing faults is a key to arc flash mitigation. Circuit breaker or relay settings near the source of power may have significant time delays to allow for coordination of downstream devices. A relatively new way to address this issue is to use relays that detect the presence of arcing faults by looking for the flash of light associated with the arcing fault in addition to the characteristic current flow.
However, for an arcing fault to be detected, both the high current and a burst of light must exist. When both conditions are present, an optical relay can operate very quickly to clear the fault. This typically occurs through the operation of an overcurrent protective device. Alternatively, an optical relay can activate a shorting switch that creates a bolted fault that clears the arc more quickly than a circuit breaker could operate. Optical relays can also be used as the protective relay in a virtual main configuration.
Virtual Main Arc Flash Mitigation System
Switchgear and switchboards can be subjected to dangerous levels of arc flash incident energy when fed directly from a power transformer. The addition of 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. It designed to trip an existing upstream fault breaking device, often a medium-voltage circuit breaker or other vacuum interrupter. This mitigation solution could also incorporate other technologies, such as:
- A maintenance selector switch, which temporarily lowers the instantaneous short circuit current setting. The maintenance setting lowers the available arc flash incident energy and temporarily forfeits selective coordination.
- Zone-selective interlocking with downstream branch circuit breakers in the switchgear eliminates the need for the maintenance selector switch. Arc flash energies can be permanently reduced with zone-selective interlocking.
Solution Group 2: Remove Workers from Harm’s Way
De-energizing equipment does not absolve the facility from the responsibility of performing an arc flash analysis or providing the necessary personal protective equipment (PPE). The following arc flash mitigation solutions remove a worker from the location of or place a barrier between the workers and exposed energized parts.
Infrared Viewing Windows
Having infrared (IR) windows installed in electrical equipment panels enables IR scans to be performed without exposing the worker to hazardous energy. Transparent to infrared rays, IR windows allow hot spots to be registered by a thermographic camera. They also facilitate permanent access for inspection of electrical components without disturbing operations.
Online Temperature Monitoring
Online temperature monitoring, via wireless sensors installed during a planned outage, provides continual access to critical connection points, where traditional thermography cannot be used. This technology evaluates the equipment’s current condition without exposing workers to energized parts, since equipment covers do not have to be removed.
Remote Racking System
A remote racking system (RRS) allows circuit breaker racking operations to be performed via a control panel located away from the cell, removing the operator from manual contact with the circuit breaker. If the operator controlling the RRS is located outside the arc flash boundary, the need for PPE is eliminated.
Conclusion
Electrical hazards are a significant safety and financial risk for electrical workers and their employers. OSHA mandates that work on electrical equipment must be performed in a manner that does not expose the worker to undue risk of injury. Complying with the safe work practices dictated by NFPA 70E and implementing arc flash mitigation strategies through engineering controls will enhance workplace safety for employees and reduce the financial risk for your company.
Antony Parsons, Ph.D., P.E. is a technical consultant in Schneider Electric’s Power Systems Engineering group. He is responsible for providing power system analysis, troubleshooting, and design consulting services for Schneider Electric’s customers, as well as engineering support for Schneider Electric’s field services operations. Dr. Parsons received the BSEE degree from the University of Houston in 1995, as well as MSE (1996) and Ph.D. (1999) degrees in Electrical Engineering from the University of Texas at Austin. He is a member of the IEEE 1584 working group on Arc Flash Calculations and represents Schneider Electric as a member of the Technical Advisory Committee to the IEEE/NFPA Arc Flash Collaborative Research Project.