How Much Is Too Much? Short circuit (fault) current

An arrow shaped like a lightning bolt next to an illustration of an arc flash

The 2020 Edition of the National Electrical Code and the 2021 Edition of NFPA 70E define the term available fault current as: “The largest amount of current capable of being delivered at a point on the system during a short-circuit condition.”

The terms “fault current” and “short circuit current” are interchangeable, and both terms are used depending on which standard is being referenced. The NEC and NFPA 70E use fault current, and calculation standards such as IEEE 3002.3-2018 and IEC 60909 use short circuit current.

The calculations are performed to predict how much current could flow to electrical equipment under evaluation, such as panelboards, motor control centers, switchboards and similar equipment, to determine if the equipment has an adequate interrupting or withstand rating. If it does not have an adequate rating, the equipment is considered to be overdutied. Short circuit calculations are more important than ever because the results are also used as part of an arc flash study.

Although the calculations may seem complicated, they are really just an application of Ohm’s Law—as in I = V/Z, where I is the current in amperes, V is the system voltage and Z is the impedance. I will discuss other details in subsequent columns. Fortunately, power system software makes performing the study much easier—as long as you know what you are doing. Otherwise it will be the proverbial “garbage in, garbage out.”

Begin at the beginning

Short circuit studies can require a staggering amount of data. One of the first steps should be to contact the electric utility company to determine how much short circuit current they have available—typically at the service entrance. A simple way of thinking is this: If the dam breaks, how big is the flood?

Another important part of the study is to create or update the facility’s one-line drawing. This provides the road map of the electrical system. The impedance data is at the heart of the study and determining the impedance of transformers and conductors is very important.

Bolted short circuit current

Section 110.9 of the NEC states: “Interrupting Rating: Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.”

This requires verifying the equipment can adequately interrupt or withstand the available fault current. The equipment in the study has an upper limit of how much current it can interrupt or withstand that is defined by standards such as UL and ANSI. If the available fault current exceeds the interrupting or withstand rating, the equipment could fail, sometimes catastrophically.

Short circuit studies must be based on an appropriate standard such as “IEEE 3002.3-2018 - IEEE Recommended Practice for Conducting Short-Circuit Studies and Analysis of Industrial and Commercial Power Systems.” The calculations are based on a “bolted” fault, which means the short circuit current has no additional impedance at the contact point. Only impedances such as the source, transformers and conductors are considered.

Arcing short circuit current

Beginning with the 2000 edition of NFPA 70E, Hazard Risk Category tables were introduced for use in selecting arc rated clothing and PPE. The tables contain notes that define the maximum available fault current where the PPE selection is valid. Why is short circuit current so important with PPE selection? Because short circuit current is one of two main variables used in calculating the severity of an arc flash known as incident energy. While there are many other secondary variables, the short circuit current defines the intensity or explosiveness of an arc flash.

The other important variable is the duration of the arc flash, defined by how long it takes an upstream overcurrent device to operate, which is also dependent on the available fault current.

When “IEEE 1584 - Guide for Performing Arc-Flash Hazard Calculations” was first published in 2002, it introduced a method for calculating the arcing fault current during an arc flash. The most recent 2018 edition has improved upon the first edition. When an arc flash occurs, the short circuit current jumps across an air gap between the conductors, which introduces additional impedance. This impedance reduces the current, so the arcing short circuit current will always be lower than the bolted current. This lower arcing current may result in an upstream overcurrent device taking longer to operate and creating a longer arc duration. This means the total incident energy exposure could be greater and more hazardous.

My next column will explore the details of short circuit studies.

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