When it comes to calculating the prospective incident energy from an arc flash, two variables stand out as the most critical: short-circuit current and arc duration. The calculated incident energy, expressed in calories per square centimeter (cal/cm²), defines the severity of an arc flash and can be used in arc flash risk assessments. In addition, the value can be used for the selection of personal protective equipment and may also be listed on arc flash labels.

Have you ever encountered an arc flash label with a very high incident energy listed? Usually, this is the result of a long arc duration. The longer the arc lasts, the more energy is released and the more dangerous the situation becomes. Since incident energy is directly proportional to arc duration, if the duration increases, the incident energy increases proportionally. Conversely, if the duration decreases, so does the incident energy. Time is everything!

Arcing fault current

A traditional short-circuit study involves calculating the maximum available fault current at a specific location. This is known as the “bolted fault current,“ referring to a fault without added impedance—imagine the fault as being tightly “bolted“ together. This current value is used to verify the interrupting or withstand rating of protective devices and equipment.

However, during an actual arc flash, an electrical arc introduces additional impedance into the circuit. This reduces the fault current to what’s known as the arcing fault current. If this lower current is below the threshold of the overcurrent protective device’s (OCPD) instantaneous trip setting, it may not respond quickly. Instead, the OCPD may operate in its time-delay region, significantly increasing arc duration and, consequently, the incident energy.

IEEE 1584, Guide for Performing Arc-Flash Hazard Calculations, provides a detailed model for calculating both the arcing fault current and the resulting incident energy. 

Arcing upstream

Arc duration is normally based on the time it takes an upstream OCPD to interrupt the arcing current. In the case of a circuit breaker, there are at least two basic operational regions: a time-delay region and an instantaneous trip region.

In the time-delay region, the OCPD allows a temporary overload to persist for a short time. This is useful for normal operations such as motor starting or transformer inrush currents. However, for higher-­magnitude faults—such as those caused by insulation failure—the device should ideally trip in the instantaneous region, clearing the fault quickly within a few cycles.

Problems can occur when devices are set for selective coordination with the goal of only the device closest to the fault tripping to minimize the extent of the outage. While this approach improves reliability, it may also require higher settings with longer delays for upstream devices. Depending on the magnitude of the arcing current, a device may trip in the time-delay region and significantly increase the incident energy.

NEC 240.87: Reducing arc energy

Recognizing this issue, the National Electrical Code addresses it directly in Section 240.87. According to the 2023 edition, if a circuit breaker has a continuous trip rating of 1,200A or higher, it must include a method to reduce the clearing time and, as a result, the arc energy. The NEC states that one of the acceptable means is an energy-reducing active arc flash mitigation system. These systems are designed to significantly reduce arc duration, which in turn significantly reduces incident energy.

Active arc flash mitigation systems

At the core of these systems is an optical sensor that detects the intense light produced by an arc flash. The system can use light and current sensing to confirm an arc flash event.

The system can respond in several ways. One method sends a trip signal to the upstream OCPD for an instantaneous trip. Another method, known as an arc quenching device, creates a controlled, bolted, three-phase fault inside a specially designed enclosure. This method eliminates the arc by removing the gap that allows it to form, rapidly diverting current away from the arc path, which effectively neutralizes the hazard before it fully develops. Manufacturers state the operation can occur within 4 milliseconds (ms).

Proof in the numbers

Let’s look at an example with an available bolted fault current of 40,000A at 480V. The calculations are based on IEEE 1584 and include a vertical electrode configuration, a 32-mm gap and a 24-inch working distance. If the protective device is set to trip in 30 cycles—a delay that is sometimes used for coordination—the calculated incident energy is 20.74 cal/cm². 

An arc energy reduction system with an instantaneous tripping of three cycles could reduce the incident energy to 2.07 cal/cm². If an arc quenching system is used, that operates in 4 ms, reducing the energy to 0.17 cal/cm². Proof that time is everything! 

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