In Part 1 of this discussion we covered the increasing emphasis on Arc Flash hazards in relation to working on your facility’s electrical distribution systems. We also emphasized the importance having an Arc Flash Hazard study performed if you do not yet understand the hazard ratings of your gear. Now we want to discuss mitigation techniques that can be considered.
The incident energy of an arc flash event is a function of available fault current squared multiplied by the duration of the arcing current. Thus solutions to reducing the hazard to personnel include:
Reducing the available fault current Reducing the clearing time for protective devices to clear the fault
The first of these two methods addresses the current part of the equation. The second reduces the duration part of the equation. A common way to address available fault current levels is the specification of transformer impedances. This value has a direct affect on the level of fault currents available to a particular system.
The second bullet item is the reduction of clearing time, thus addressing the duration component of the equation. The 2014 NEC addresses this in Article 240.87 for feeders with breakers 1200 A or greater. In this case, the NEC requires one of the following:
Zone-selective interlocking Differential relaying Energy-reducing maintenance switching with local status indicator.
The purpose of this discussion is not to review the details of these methods, but to make owners aware that these are engineering/design issues that have great potential to reduce the arc flash hazard levels in their facilities. Another item that can reduce the duration is the specification of the type of molded-case circuit breakers. Often, breakers with fixed instantaneous settings are specified in PDU sub-feed breakers and/or panelboard mains. If these breakers are specified with adjustable instantaneous settings, it provides an opportunity for the engineer to provide breaker settings that reduce the Arc Flash PPE Category. TechSite recently performed a study for a client that has PDUs with sub-feed breakers that supply remote distribution centers. By changing the sub-feed breakers from a GE type THQD to a GE type TFJ the Arc Flash PPE Category went from 3 to 0.
Another design issue that can affect arc flash hazards addresses moving people further away from the source of incident energy. Methods that address this are locating breaker controls away from switchgear/switchboards where breakers are located and utilizing remote racking for breaker removal from switchgear. These methods get personnel out of and beyond the arc flash boundary.
Redirecting arc blast energy is also a technique that can be implemented. This is the concept with specifying arc-resistant switchgear. Arc resistant switchgear, designed to IEEE standards, can contain a fault within the switchgear and re-direct the blast energy through designed ducts. Meeting the IEEE standards assures that an internal arcing fault will not: cause doors or covers to blow open during the event, fragment and eject parts within the protected area, allow the arcing fault to burn through the enclosure, or have any of its grounding connection become ineffective.
The key point is that the application of engineering controls are a very effective way to enhance safety for electrical workers and anyone who interfaces with a facilities electrical equipment. Ideally, arc flash safety is addressed when first designing the electrical distribution for a facility. However many of the techniques discussed here can be retrofitted into existing systems.