A shunt trip on the circuit breaker is activated if the current falls out of equilibrium. If the arc-fault current is below the local breaker’s instantaneous setting, ZSI still will trip it in its instantaneous time but with an additional processing and communications delay that typically is 50-100 ms.ĭifferential relaying uses current transformers on both the line and load sides of the protected equipment that monitor current to ensure it is equal on both sides. ZSI allows the local circuit breaker to operate instantaneously when the arc-fault current is above the breaker’s instantaneous setting. ![]() The goal is to limit the power outage to the area of the fault. If the local protective device does not clear it, the upstream device attempts to clear it. Zone-selective interlocking (ZSI) will detect a fault and block an upstream protective device from operating until the local protective device has a chance to clear the fault. Some background on each approach may be helpful when choosing the best one for a specific situation. Clearing Reduction MethodsĮquipment designers, controls engineers and plant engineers share a common interest in keeping their approaches to arc-flash mitigation NEC-compliant, but the best approach will depend on the particular application. Unless the fuse’s clearing time is shorter than 0.07 seconds, one of the following methods, some of which are the same as those for reducing that clearing time with circuit breakers, must be used: differential relaying, energy-reducing maintenance switching with local status indicator, an energy-reducing active arc-flash mitigation system or an approved equivalent means. The underlying goal is to reduce the clearing time at the available arcing current. In much the same way that Article 240.87 covers methods for arc energy reduction for circuit breakers, Article 240.67 covers requirements for arc energy reduction for fuses and fusible switches rated 1,200 A or higher using the same incident energy reduction methods as in 240.87. The 2017 edition adds Article 240.67 Arc Energy Reduction (for Fuses). Although it always is preferable to de-energize electrical equipment before examining it or working on it whenever feasible, when a circuit breaker is used without an instantaneous trip (or with an instantaneous trip above the prospective arc-fault current), the means to reduce the incident energy must be provided when energized work must be performed. The update adds two new methods: an instantaneous trip-setting or instantaneous override. These methods include the use of zone-selective interlocking, differential relaying and an energy-reducing maintenance switch. If a circuit breaker can be set to trip at 1,200 A or higher, then one of a number of methods to reduce clearing time must be employed. Given that incident energy is a product of current and time, fault clearing time is a special concern wherever significant currents are present. What’s new in NEC 2017? One thing that’s new is an update to Article 240.87, Acceptable Methods for Arc Energy Reduction, which defines when arc-flash mitigation must be applied. Incident energy is calculated based on overcurrent device clearing times and available fault current at that equipment location. The 2011 version included a requirement to label equipment for arc-flash hazards, and this requirement was refined further in NFPA 70E in 2015 to add the arc-flash boundary and incident energy to the labels. The first discussion of arc flash appeared in the 2002 version. To provide some perspective on the latest changes, here’s a brief review of the NEC standard’s history as it relates to arc-flash protection. The newest edition of the standard, NEC 2017, includes some important changes that will affect how electrical equipment is designed, installed and maintained. In recognition of the dangers these events pose, the code-making panels and committee that develop the National Electrical Code® (NEC) standard have continued to refine the requirements related to arc flash protection. By Dean Katsiris and Dave Scheuerman, Littelfuse Inc.Īs any experienced industrial electrician can attest, arc-flash events can be destructive, costly and even fatal.
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