Arc flash basics: what an arc flash is, what decides how bad it is, and how to read a label

An arc flash is the release of energy that happens when electric current leaves its intended path and jumps through the air between conductors, or from a conductor to ground. That jump is called an arc fault. The arc is a conducting channel of superheated, ionised air and vaporised metal, and it can release heat, bright light, a pressure wave, loud noise and flying molten metal in a fraction of a second.

People often confuse arc flash with electric shock because both come from electrical equipment. They are different injuries. Shock is current passing through the body. Arc flash is mainly a thermal injury caused by energy released around the equipment, and a person can be badly burned without ever touching a conductor. This is why the protective measures for the two hazards are related but not the same.

Arc flash is not only a high-voltage problem. Ordinary low-voltage industrial equipment, such as the 480 V motor control centers and drive panels found in many plants, can sustain an arc. How much energy is released depends on the fault current available and how long it flows, not on voltage alone.

Arc faults start from a small number of recurring causes. Knowing them explains most of the rules you are given.

• Contact by a tool, a part or a dropped item that bridges conductors inside an enclosure.
• Dust, moisture or contamination that lets current track across insulation, including condensation inside a cold panel.
• Insulation that has aged, been damaged or been overheated.
• Loose or corroded connections that overheat and then fail.
• Operating, racking or closing a device that is already faulty or has been damaged, which is why some tasks are done remotely.
• Animals or debris inside equipment, and human error when working on or near exposed parts.

Many of these are maintenance conditions, which is one reason a controls technician who opens panels needs to understand the hazard even when their own work is on 24 V control circuits.

You do not need the calculations to understand the factors. A qualified engineer uses them in an arc flash study, and the factors explain why the same panel can be more or less dangerous after a change to the system.

This is the reason a change to a system can quietly change the hazard. Replacing a breaker with a slower one, adding a transformer or changing the supply can alter the fault current or the clearing time. That is why studies are repeated after changes and why labels carry a date.

A handful of terms appear on labels and in standards. They describe the same hazard from different angles.

• Incident energy: the thermal energy that reaches a surface at a stated working distance during an arc. It is usually given in calories per square centimetre, written cal/cm2.
• Working distance: the distance between a person's face and chest and the possible arc, which the incident energy figure is calculated for.
• Arc flash boundary: the distance from the possible arc at which the incident energy falls to a level that would only cause a curable burn. Inside the boundary, arc-rated protection is required and the work belongs to qualified persons. Standards commonly use 1.2 cal/cm2 for this level.
• Arc rating: a number, also in cal/cm2, that describes how much heat energy a piece of clothing or equipment can withstand. Arc-rated clothing is tested for this, which ordinary clothing is not.
• Arc flash study: an engineering analysis by a qualified person, using methods such as those in IEEE 1584, that calculates incident energy and boundaries for each piece of equipment.

An arc flash warning label is fixed to equipment after a study. Layouts differ, but labels typically show the equipment name, the nominal voltage, the arc flash boundary, the incident energy and the working distance it applies to, and information about the protection required. Many also show shock approach boundaries and the date of the study.

What a label gives you is information, not permission. It tells a qualified person how much thermal energy to plan for if an arc occurred in the condition that was studied. It does not say the equipment is safe to work on live, and it does not replace your site procedures, a work permit or the instructions of the person who owns the safety program.

Electrical safety programs use a hierarchy of controls, the same idea used for any hazard. Working from the most to the least effective: eliminate the hazard, reduce it by engineering, warn, set procedures and training, and only then rely on protective equipment.

For electrical work the strongest control is to remove the energy. Equipment that is properly de-energised, locked out and proven dead has no arc flash hazard to protect against, which is why lockout/tagout and establishing an electrically safe work condition are the first line. Engineering controls come next, such as protective device settings that clear faults faster, arc-resistant equipment, remote racking and remote operation. Warnings, such as arc flash labels, come after those, then training and procedures. Arc-rated clothing and equipment is the last layer: it reduces injury if everything else has failed, and does not prevent the arc.

This is why a label never makes live work routine. The aim of the program is to avoid being exposed in the first place.

A PLC panel runs on low-voltage control power, but the enclosure often shares space with, or sits beside, the power that feeds drives, starters and heaters. Opening a panel to trace a fault can put you near parts that carry hundreds of volts and a lot of available fault current. Motor control centers and drive cabinets are the clearest examples.

Practical awareness for a technician means three habits. Know which parts of the cabinet are power and which are control before you open it. Check for the label and read it, so you know the equipment has been studied. And never let a stopped machine or a tripped E-stop persuade you the equipment is dead, because those are control actions and not isolation.

• Only high voltage is dangerous. Low-voltage industrial systems can and do produce arc flash.
• Arc-rated clothing makes an arc flash safe. It reduces burn injury and does nothing about the blast, pressure, noise or shock.
• A label means the equipment is safe to work on. A label states a hazard and the distance it applies to.
• If I am not touching anything I cannot be hurt. Arc flash injures people at a distance.
• A study done once is good forever. Changes to the supply or protective devices can change the result.

This page is awareness training. It will not qualify you to work on energised equipment, select protective gear or perform an arc flash study. Those need a qualified person, your employer's electrical safety program and the standards that apply where you work, such as NFPA 70E in the United States.

To go further, the Industrial Safety track covers why machines are guarded, E-stop circuits, safety relays, lockout/tagout and, in a later unit, electrical hazards including shock, arc flash and approach boundaries. The free lockout/tagout lesson is a good place to start, and the lockout/tagout explained page on this site covers the procedure in plain language.