How to Perform an Arc Flash Study with the changes to IEEE 1584 takes it to a new level

An arc flash study can be hard enough to do on its own, but the latest version of IEEE 1584—IEEE Guide for Performing Arc-Flash Hazard Calculations has made it even harder.

Since arc flash studies became popular in the early 2000s, a lot has changed. Electrical safety has gotten a lot better thanks to the continued development of standards like NFPA 70E—Standard for Electrical Safety in the Workplace and the latest edition of IEEE 1584, as well as improvements in arc-rated clothing and personal protective equipment (PPE) and changes in how electrical systems are built.

An arc flash caused damage to equipment.

In an arc flash study, the calculations of the arcing fault current, the incident energy, and the arc flash boundary are the most important parts. The final results of the study are used to choose arc-rated personal protective equipment (PPE) and clothing with a high enough arc rating and to set an arc flash boundary. This information is also needed for arc flash (equipment) labels and electrical work permits when the equipment is turned on. The picture above shows what can happen when things don’t go as planned.

What law or rule says that the arc flash study must be done? Arc Flash Risk Assessment, NFPA 70E 130.5, says that arc flash personal protective equipment (PPE) must be chosen using either the arc flash PPE category method or an incident-energy analysis.

The PPE category method is based on tables in NFPA 70E that put PPE into categories based on the type of equipment and other factors. For the incident-energy analysis method, you have to figure out how much incident energy an arc flash could cause. This value is used to choose the arc rating of PPE. The NFPA 70E doesn’t require a certain way to figure out the incident-energy analysis. But most people around the world use the IEEE 1584 standard.

NFPA 70E and IEEE 1584 define incident energy as “the amount of thermal energy impressed on a surface during an electric arc event at a certain distance from the source.” In other words, it tells how bad an arc flash is and is measured in calories per centimeter squared (cal/cm2).

Even though the calculations in IEEE 1584 are called a “incidence-energy analysis,” this is only one part of what most people think of as a “arc flash study.” A risk control method for the arc flash, arc flash (equipment) labels, and a shock risk assessment may be added as well.

A short-circuit and coordination study is also needed to figure out how much fault current is available at the equipment location and how long an arc will last based on how long it takes a certain protective device upstream to work and clear the fault during an arc flash. This information can be found in studies that have already been done, as long as the results have been checked to make sure they are correct. If there aren’t any studies already done or if they can’t be checked for accuracy, new or updated studies may need to be done. For these extra studies, standards like the IEEE standards should be used.

Legal
Before we go any further, we need to talk about two things. First, the person doing the study should have the skills and knowledge to do it. In this age of “there’s an app for that,” mistakes can happen that could be very bad. IEEE 1584 says that a qualified person is “a person who performs arc-flash hazard calculations by using skills and knowledge related to the construction and operation of the electrical equipment and installation and has experience in power system studies and arc-flash hazard analysis.”

Note that the IEEE 1584 definition of “qualified person” refers to the person doing the study, while the NFPA definition refers to the electrical worker.

Second, even though I’m very involved with many of the standards used for an arc flash study, the views I share here are my own and may or may not reflect those of any particular standards organization.

Arc flash investigation: 480V panelboard

A 480V three-phase panelboard was chosen as an example of how to do an arc flash study so that the steps could be shown. There needs to be a lot of information, which will be talked about step by step. The reader is also encouraged to get a copy of IEEE 1584, version 2018 for all the equations and details.

The arc flash study is like the old saying, “You can’t see the forest for the trees.” It just means don’t get so focused on the little things that you can’t step back and see the big picture. It is easy to get lost in the study, just like it is easy to get lost in the forest. I want you to remember one simple idea: the main goal of an arc flash study is to choose arc-rated personal protective equipment (PPE) that is enough for the calculated incident energy.

This means that if one assumption leads to a calculated incident energy of 4.2 cal/cm2, another leads to a calculated incident energy of 4.6 cal/cm2, and the third leads to a calculated incident energy of 5.2 cal/cm2, PPE with a minimum arc rating of 8 cal/cm2 is considered enough no matter which of the three calculated incident energy values is used. We will talk about the effect on the arc flash boundary at a later time.

I like to compare it to a study that doesn’t work right. When doing one, let’s say a certain panel has a rating of 22 kiloamperes for stopping a short circuit (kA). If different assumptions were used in the study, the calculated short-circuit current could be different, such as 12.5, 13.5, or 14.1 kA. But the 22-kA panel is good enough for all three values in the end. Accuracy is very important, and this statement is not meant to make using exact data less important. Just make sure you keep your eyes on the bigger picture.

Step one: Getting information

An arc flash study needs three types of data: impedance data for a short circuit study, protective device data to figure out how long the arc will last, and equipment data to figure out the size of the enclosure, the gap between the bus and the electrode configuration. Depending on how old the system is and how much documentation is already there, collecting data could be a big part of the whole study. Often, not all of the data is available, so it may be necessary to make safe assumptions and write them down.

Creating a computer model of the power system being studied is the first step in an arc flash study. With their large libraries, commercially available software can make the process easier by letting you make a one-line diagram and enter the data you need.

Don’t let yourself get stuck in what I call “data paralysis.” This is when not all of the data is available right away, which stops the study in its tracks. When data isn’t yet ready, one way to fill in the blanks is to use very unusual data that stands out. So, an initial model can be finished. For example, if the lengths of the conductors are unknown, use lengths of 3 feet for now. This lets a working model be made before the study gets too complicated. Once all the information is in, the odd numbers can be changed.

Start by making a one-line diagram of the power system you want to study.

Second step: Available fault current

After the model has been made, the first step in calculating is to find out how much bolted three-phase fault current is available at each location. This is one of the main things that are used to figure out how much energy hit the object. I often say that the fault current shows how strong or explosive the arc flash is. When there is more current, the arc flash is stronger. But, as we’ll see in the next part of this article, that doesn’t always mean the most incident energy.

A separate fault current study found that the three-phase bolted fault current at the panel in the figure below, where our example will start, is 28,500A.

When the author set up an arc flash test, the fault current flowed across the gap, ionizing the air and making a plasma.

Step 3: Short-circuit the current by arcing

In step two, a traditional fault current study is used to find the “bolted” fault current, which is usually used to test the ability of protective devices to stop the flow of electricity. When something is “bolted,” it means that the short is a solid connection and there are no other problems at the fault.

During an arc flash, however, the conductors or conducting object that caused the fault either melt back or are blown back, leaving a gap in the path of the current. The picture above shows what one of the author’s arc flash tests looked like. The fault current flows across the gap, making a plasma by ionizing the air. For this calculation, IEEE 1584 equations need a lot of information, such as bolted three-phase short-circuit current, bus gap, electrode configuration, and voltage. This is called a “arcing fault current.”

In IEEE 1584, there is a table that shows “typical” bus gaps. In low-voltage distribution equipment, such as panelboards, switchboards, and motor control centers, the gaps are usually 25 mm to 32 mm. The 25 mm was chosen because this arc flash study example is a panelboard. The user can put their own bus gaps in place. But you should know that the gaps can be different on the same piece of equipment and on different pieces of equipment.