# Calculations in a Flash

One sentence in the IEEE 1584 Standard, IEEE Guide for Performing Arc-Flash Hazard Calculations, frequently has people scratching their heads: “Equipment below 240V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply.” What does this sentence mean? What is so significant about 240 volts (V) and 125 kilovolt-amperes (kVA)?
IEEE 1584 and arc flash calculations

The IEEE 1584 standard provides equations used for estimating the prospective incident energy of an arc flash. Defined in terms of calories per centimeter squared (cal/cm2), the incident energy is largely dependent on the available short-circuit current and the duration of the arc flash. Duration is typically determined based on how long it takes an upstream circuit breaker or fuse to operate and clear the fault. The longer the device takes to operate, the longer the arc flash lasts and the greater the total incident energy will be.

One exception to the duration issue is if the arc flash occurs at 208V. It is widely believed in the industry that, if an arc flash occurs at 208V with a lower short-circuit current, it is unlikely to sustain long enough to result in any significant amount of incident energy. In fact, during the original arc flash tests used to develop the IEEE 1584 standard, only one test at 208V resulted in an arc flash sustaining itself using a standard test enclosure.

These results lead to defining a lower limit where circuits do not need to be considered in the study. IEEE 1584 defines this lower limit in terms of circuits less than 240V (implying 208V circuits) and served by a transformer smaller than 125 kVA (112.5 kVA is the next standard size). The smaller the transformer, the less short-circuit current will be available on the secondary side.

Incident energy, transformer kilovolt-amperes and duration
As an example, if an arc flash occurs on the 208Y/120V secondary terminals of a 112.5-kVA transformer, the IEEE 1584 calculations indicate the equivalent arcing short-circuit current would be 3,982 amps (A). This results in an estimated incident energy exposure of only 0.20 cal/cm2 for each electrical cycle or 1/60th of a second. Considering the benchmark for arc flash protection is 1.2 cal/cm2, frequently referred to as the onset of a “just curable” burn threshold, one cycle of incident energy is minimal by comparison.

In order to have more incident energy, the arc must sustain itself longer, at lower fault currents at 208V; this is normally considered unlikely. The table below provides a comparison of the prospective incident energy at 208V based on standard transformers size less than 125 kVA with durations from one electrical cycle up to 30 cycles.

For now, the 240V/125-kVA exception still defines the lower limit. However, as more becomes known, the 125-kVA lower limit may change and perhaps even be defined in terms of amps. This would make it easier to apply to all 208V circuits instead of only those served by transformers. Recent research indicates that under certain conditions, such as the bus terminating into a barrier, a 208V arc flash could sustain down to lower levels of short-circuit current than previously expected. I witnessed just such a test earlier this year in a lab. Until more is known, many people have begun using 75 kVA and even 45 kVA as a more conservative lower cutoff limit.

It is still a hazard
This exception is often interpreted to mean “no arc flash hazard exists.” This interpretation is not only incorrect, it’s dangerous. Simply because the exception excludes these circuits from the study does not mean a hazard does not exist. It just means it may not be as large of a hazard.

Caution must always be used because it is possible to sustain an arc at 208V at higher levels of short-circuit current as well as at lower levels of current given the right conditions. Remember, it is always better to place equipment in an electrically safe working condition!

PHILLIPS, founder of www.brainfiller.com and www.ArcFlashForum.com, is an internationally known educator on electrical power systems. His experience includes industrial, commercial and utility systems, and he is a member of the IEEE 1584 Arc Flash Working Group. Reach him at jphillips@brainfiller.com.