A Calculated Introduction

This article is the first in a series that provides a step-by-step approach for performing arc flash calculations.

Arc flash studies and software

If you ask someone who just performed an arc flash study how they did it, a common response is to name a particular arc flash software package. Ask what calculation method the software is based on, and many will likely answer, “IEEE 1584,” which is known as IEEE Guide for Performing Arc Flash Hazard Calculations. However, if you probe a little deeper and ask if they have ever used the IEEE 1584 equations to manually calculate values such as the incident energy or arc flash boundary, most will say, “no.”

Like other topics in power-system analysis, to have a deeper understanding of the subject, it is important to know how to perform the actual calculations and not just rely on results from software. So, let’s dig deep into the calculations and see how to use the equations for arc flash studies.

Arc flash studies and IEEE 1584

The equations and methods defined by IEEE 1584 are at the heart of major arc flash programs for systems up to 15 kilovolts (kV). Test data from hundreds of arc flash experiments were used to empirically derive the equations that are valid for the following conditions:

• Systems operating from 208 
volts (V) to 15,000V

• Three-phase arc flash

• 50 and 60 hertz (Hz)

• Bolted short-circuit current from 700 amperes (A) to 106,000A

• Conductor gap distances from 13 millimeters (mm) to 152 mm

It may be hard to imagine, but arc flash studies up through 15,000V are based on only four different types of calculations:

• Arcing short-circuit current for systems from 208V to 1,000V

• Arcing short-circuit current for systems from 1,000V to 15,000V

• Incident energy calculations for systems from 208V to 15,000V

• Arc flash boundary calculations for systems from 208V to 15,000V

Subsequent articles (starting in March) will explore each one in detail using real-world calculation examples.

Step 1: arcing short-circuit current

A traditional short-circuit study is commonly used to verify the interrupting rating of electrical equipment. It assumes the short circuit is “bolted,” meaning no additional impedance is introduced at the fault. During an arc flash, the short-circuit current will flow across an air gap and result in an arc impedance that reduces the current to something less than what flows during a bolted condition.

The first step in the calculation process is to determine the magnitude of the arcing short-circuit current from an arc flash. There are two different calculations depending on the system voltage: one for lower voltages from 208V to 1,000V and one for systems from 1,000V up to 15,000V. Each type of calculation requires a known “bolted” short-circuit current, and the lower voltage equation requires the gap distance, voltage and whether it is in an enclosure or open air.

Once the arcing short-circuit current has been calculated, it can be used in the next step to determine the incident energy.

Step 2: calculating incident energy

An arc flash study’s main focus is to determine the prospective incident energy to select the appropriate arc rating of personal protective equipment (PPE) and clothing. Incident energy is the amount of thermal energy impressed on a surface (person’s body) at a specified working distance from the arc source. It is normally provided in terms of calories per square centimeter (cal/cm2) and requires many different factors, such as the arcing short-circuit current, the type of equipment, working distance, whether the system is effectively grounded, the gap distance and the arc duration.

Step 3: arc flash boundary

The final step in the arc flash calculation process is to determine the arc flash boundary (AFB). This calculation uses a known (calculated) incident energy at a specific distance to determine the new distance (AFB) where the incident energy drops to 1.2 cal/cm2. This value is generally considered the threshold where arc flash protection is required.

Incident energy, arc flash boundary, PPE and other requirements?

Compliance with several NFPA 70E requirements can be achieved with the results of the arc flash calculations. Incident-­energy calculations can be used to determine the arc rating of protective clothing and PPE, and as part of an arc flash label program. The AFB calculation can be used to satisfy the AFB requirement in addition to listing it on the arc flash label.

This article is only the beginning of understanding arc flash calculations. In March, the series will continue with solved example problems to illustrate each type of calculation. Click here for part two.

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