I am often asked, “What determines the severity of an arc flash?” Incident energy is the most straightforward answer, but what factors determine incident energy? Most people assume that it is all a factor of voltage and current, but there is much more to it. It is not as simple as saying, “Higher voltage means bigger arc flash.” While several factors determine the energy level of a potential arc flash inside an electrical device, three main factors contribute to the severity:
Traditionally, we think of current rating as the rating on the nameplate. For example, a 400-amp panelboard is rated to handle 400 amps of current, the full-load current rating of the equipment. When it comes to arc flash severity, however, the concern is not with the full-load current rating, but with the short-circuit current rating. Short-circuit current and full-load current are different.
Full-load current ratings are traditionally found on the equipment nameplate. The short-circuit current rating (SCCR) is usually found on a sticker somewhere inside the equipment. SCCR is the amount of current the equipment can withstand during a short-circuit condition before the equipment is destroyed. All electrical protective devices are also rated for short-circuit currents, with most referred to as AIC rating. A protective device often limits the equipment duty of the overall cabinet or panelboard, but this is often overlooked by personnel when replacing components or adding circuitry.
To compare, a panelboard rated at 600 amps full-load current may have a SCCR of 22,000 amps. Why is the SCCR rating so much higher than the full-load current rating? With a short circuit, current takes an unintended path, usually to ground. When this occurs, there is little resistance in the path. This low resistance leads to high current that can rise to many thousands of amps instantaneously. If a panel has a SCCR of 22k, this panel is built to withstand 22,000 amps before it is destroyed.
When we perform an arc flash analysis, we calculate the available fault current, which is the possible current that would pass in the case of a short circuit and compare it to the SCCR of each electrical device throughout the system. If the available fault current exceeds the SCCR of the equipment, it presents a severe arc flash hazard and potential destruction of electrical components.
Time is another major factor in arc flash severity. The old saying, “Time is of the Essence”, is true with arc flash severity. The longer the fault continues, the more severe the arc flash. The most effective way to reduce the fault time is to ensure that breakers, fuses, and other interrupting devices open as quickly as possible to clear the short. If the breaker protecting the shorted panel is not set properly or does not operate as intended, the results could be disastrous. Overcurrent Protective Devices, such as breakers and fuses, are the first line of defense once the event is put in motion.
An arc flash risk assessment accounts for the trip characteristics of breakers and determines adjustments that might be necessary for quicker reaction time when possible. Let’s tie this in with the discussion on current from above. In the event of an arc flash, there is a short circuit. The short-circuit current can reach thousands of amps. However, a correctly-sized and set breaker that is operating properly will interrupt that current before it reaches those dangerous levels. For example, assume there is 30ka of available fault current at a particular point in the electrical system and the panel at that location has a SCCR of 22ka. This means that the current in an uninterrupted short circuit would exceed the rating of the panel. It would destroy the panel and create a very severe arc flash. However, assume that the breaker protecting the panel is rated at 400 full-load amps and is working properly. In this case, the breaker may clear the fault in just a few cycles before the short-circuit current can reach those dangerous levels. This could prevent equipment destruction and turn a major arc flash event into something relatively minor.
The third major factor determining arc flash severity is the distance of the worker from ground zero. When an arc flash risk assessment is performed, an arc flash label is placed on the equipment with a value for incident energy. Incident energy determines the severity of an arc flash. The higher the incident energy, the more severe the event. However, this number is represented at a working distance, which is typically 18 inches from live parts.
The relationship between distance and incident energy is an inverse squared relationship. In other words, if the working distance is doubled, the incident energy at that distance reduces by a factor of 4. If the working distance is tripled, the incident on the worker is reduced to 1/9th the original value. Conversely, if the working distance is cut in half, the incident energy is raised by a factor of 4.
Arc flash events can be dangerous and deadly. Many factors determine the severity of the event, but three values—current, time, and distance—largely determine the severity. The severity of an event can be reduced when (1) the short-circuit current is reduced, (2) the reaction time of the overcurrent protective devices is increased, and (3) the working distance is increased.