The 2014 edition now lists five clearing time reduction means. The 2011 edition of the NEC listed four means to reduce clearing time. Clearing time refers to the length of time necessary for an overcurrent protective device to extinguish the arc completely. Incident energy is a function of current and time, so if the arcing time is reduced, the incident energy will be reduced. One of the units used to measure incident energy is calories per centimeter squared (cal/cm 2).” A method used to calculate incident energy may be found in IEEE 1584, Guide for Performing Arc Flash Hazard Calculations. Incident energy, as defined in NFPA 70E, Standard for Electrical Safety in the Workplace®, is, “The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. ![]() What does clearing time have to do with arc energy reduction? To answer this question we need to understand incident energy. If the overcurrent device in the circuit breaker meets this criterion, then documentation and a method to reduce the clearing time must be provided. ![]() This means that even though an electronic trip circuit breaker with a 1200 A sensor has its current rating switch set to, for example, 0.5 (600 A), it will still require an arc energy reduction means (see Figure 4). You may recognize this wording as it is similar to that used in Section 230.95 to define when ground-fault protection of equipment is required. These questions were answered in the 2014 edition of the NEC which requires arc energy reduction where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher (see figure 3). But what about a circuit breaker that has an adjustable instantaneous trip function that has been set to OFF (see figure 1)? And what about a circuit breaker that has a fixed instantaneous override (see figure 2)?įigure 1. But what does that mean? It obviously refers to a circuit breaker with no instantaneous trip function at all. In the 2011 edition of the NEC, arc energy reduction was required whenever a circuit breaker does not have an instantaneous trip function. Since the reason for the requirements is improved worker safety, to be accomplished by reducing the arc flash hazard, the new title makes better sense. Why the title change? The former title referred to when the requirements in the section applied while the new title refers to what the requirements in the section are intended to accomplish. In the 2014 edition, it has been changed to Arc Energy Reduction. The title of 240.87 in the 2011 edition was Non-instantaneous Trip. Quotations from the NEC are shown in italics. Let’s take an in-depth look at the changes and what they mean. This section has changed significantly in the 2014 edition. The motivation was to protect electrical workers better from the arc flash hazard when a short time delay is used to slow down the operation of a circuit breaker to achieve coordination in an electrical system. ![]() It required the use of Zone Selective Interlocking (ZSI), differential relaying, energy-reducing maintenance switching or an approved equivalent means where a circuit breaker without an instantaneous trip function is used. This section was added to Article 240 in the 2011 edition of the NEC. Now that the 2014 edition of the National Electrical Code ® ( NEC ®) has been published let’s look at what’s new in the requirement for arc energy reduction in Section 240.87.
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