Arc Flash Study: What It Involves and What It Asks of Your Plant

An arc flash study is how a plant learns what could happen at each switchboard, panel and motor control center if a fault turned into an arc. The study estimates the energy a worker standing at that equipment could be exposed to, sets the distance inside which protection is needed, and produces the labels that tell people what to wear before they open a door. It is engineering work. This guide explains what the engineer does, what your plant is asked to provide, and where an electrical contractor like Wattsmith fits, which is installing the labels and keeping the equipment the way the study modeled it.

What is an arc flash study?

An arc flash study is an engineering analysis of a facility's electrical system that calculates, for each piece of equipment, the energy an arcing fault could release and the boundary inside which a worker needs protection. A qualified engineer performs it, and its results become the labels on the equipment.

The study sits inside a wider electrical safety program. NFPA 70E, the standard most plants build that program around, calls for an arc flash risk assessment before anyone works on or near exposed energized parts. The assessment decides whether a hazard exists, how severe it could be, and what protective equipment and practices the work requires. A study gives that assessment its numbers. Without one, a plant is limited to the general tables in NFPA 70E, which only apply within the conditions the tables state.

We do not perform arc flash studies. We install the labels a study produces, which is part of the industrial electrical work we do for San Antonio plants.

What does the engineer do during an arc flash study?

The engineer collects data on the equipment, conductors and protective devices, builds a model of the system from the utility service down, runs a short circuit study and a protective device coordination review, then calculates incident energy and arc flash boundaries at each location. The results go into a report and a set of labels.

Most studies follow the same order. The engineer starts with a one line diagram of the system, often updating an old one to match what is actually installed. Data collection follows: the rating and type of each piece of equipment, the size and length of the conductors between them, the transformers, and the type and settings of every breaker, fuse and relay that would clear a fault. The serving utility provides the available fault current at the service, which across most of San Antonio means a request to CPS Energy.

With the model built, the short circuit study works out how much current could flow in a fault at each point. The coordination review looks at whether the protective devices would open in the right order and how fast. Then the incident energy calculation, usually done to the method in IEEE 1584, combines the fault current, the clearing time, the equipment configuration and the working distance into the energy a worker could face. The arc flash boundary is the distance at which that energy falls to the level NFPA 70E treats as the onset of a second degree burn.

What does a plant need to provide for an arc flash study?

A plant provides access to its drawings and equipment, the protective device settings on record, a contact who knows the system, and planned time to open equipment safely for data collection. The more accurate the data, the more the results match the equipment people actually work on.

• Existing one line diagrams, panel schedules and equipment drawings, even if they are out of date.
• Records of breaker trip settings, fuse types and relay settings, if the plant keeps them.
• A list of equipment added, moved or replaced since the last drawings were made.
• A plant contact who can open areas and answer questions about how the system runs.
• Scheduled access to equipment, coordinated with production and the plant's safe work procedures.

Gaps in the data are the most common reason a study takes longer than planned. When a breaker setting or a conductor length cannot be confirmed, the engineer has to assume a value, and an assumption changes the result. Good records save time on the study and make its labels more trustworthy.

What does an arc flash label show after the study?

Under NFPA 70E, a label from the study shows the nominal voltage, the arc flash boundary, and at least one way to choose protection, such as the incident energy at a stated working distance or a PPE category. NEC 110.16 also calls for warning labels on equipment likely to be worked on while energized.

NEC 110.16 sets the field marking that warns of the hazard on switchboards, panelboards, industrial control panels, motor control centers and similar equipment. For service equipment rated 1200 amps or more, the code asks for a more detailed label that includes the available fault current and the clearing time of the service overcurrent device, or a label that meets NFPA 70E. The engineer's study is what supplies those figures.

Installing those labels correctly is its own job, covered in our guide to arc flash labels on industrial equipment. Each label goes on the equipment it names, made to last in the conditions where it is mounted.

How often does an arc flash study need to be updated?

NFPA 70E calls for the arc flash risk assessment to be reviewed at intervals of no more than five years, and updated whenever a major modification or renovation takes place. A new transformer, a changed breaker setting or a utility service change can each move the results for equipment downstream.

Small changes add up. A replaced breaker with a different trip curve, a longer feeder after a move, or a larger motor on a line can change the fault current or the clearing time somewhere in the system. Any of those can make a label wrong in either direction. That is why changes to modeled equipment are best recorded as they happen, rather than reconstructed at the next review.

When our electrical repair or maintenance work changes equipment the study modeled, we note it for your engineer instead of guessing at a new label.

Where does the electrician fit in an arc flash study?

The electrician installs the labels the study produces, keeps the equipment and protective device settings as the study modeled them, and flags every change that could affect the results. The electrician does not calculate incident energy or choose protective equipment levels; that belongs to the engineer and the plant's safety program.

In practice, that means three habits. Labels go on exactly as the study lists them, matched to each piece of equipment by its name in the report. Replacement breakers and fuses match the type and settings the study used, unless the engineer approves a change. And every addition, removal or replacement of modeled equipment is written down and sent to the engineer, so the next review starts from what is really installed.

A motor control center is often where those changes happen, as buckets are added and starters swapped. Our guide to motor control center installation covers that equipment.

What goes wrong with arc flash studies, and how is it prevented?

Studies go wrong when the field data is assumed rather than confirmed, when labels end up on the wrong equipment, or when the plant changes and nobody tells the engineer. Accurate records, labels matched to the report and a habit of flagging every change keep the study useful.

• Confirm breaker settings and conductor lengths in the field instead of reading them off old drawings.
• Match each label to the equipment name in the report before it goes on.
• Keep replacement protective devices to the type and setting the study used, or ask the engineer first.
• Send every change to modeled equipment to the engineer as it happens.

For the rest of our industrial scope, see commercial electrical services. If you have a finished study and need the labels installed, or equipment has changed since the study was done, reach us through our contact page.