Arc flash boundary means the distance from an exposed live part at which an unprotected worker would likely receive a second-degree burn in the event of an arc flash. Both qualified and unqualified workers may cross the arc flash boundary as long as they are wearing proper arc flash PPE, as specified in NFPA 70E. Unqualified persons must be escorted by a qualified person to cross this boundary.

Arc flash hazard means a dangerous condition associated with the possible release of energy caused by an electric arc. An arc flash hazard is classified as such if the incident energy produced by it would expose some part of the employee’s body to the amount of energy that could cause a second-degree burn (1.2 cal/cm2).

Arc flash suit means a complete arc-rated clothing and equipment system that covers the entire body, except for the hands and feet. An arc flash suit may include pants or overalls, a jacket or a coverall, and a beekeeper-type hood fitted with a face shield. These suits usually have a high ATPV rating.

The arc rating on clothing or equipment indicates the level of arc flash energy that it has been tested to protect against. The arc rating is based on the arc thermal performance value (ATPV) of the clothing material. The ATPV is expressed in calories/square centimeter (cal/cm2) and records the incident energy level that could cause a second-degree burn through the particular material.

Note: All arc-rated clothing is also flame-resistant (FR) but not all FR clothing is arc-rated. FR clothing without an arc rating has not been tested for exposure to an electric arc.

Available fault current means the largest amount of current capable of being delivered at a point on the system during a short-circuit condition or a ground fault. If the DC supply is a battery system, the term "available fault current" refers to the prospective short-circuit current.

Balaclava (sock hood) means an arc-rated hood that protects the whole head and neck, except for the facial area of the eyes and nose.

De-energized means a system is deprived of electrical energy or the electrical energy is exhausted out of the system. The NFPA 70E definition is, “Free from any electrical connection to a source of potential difference and from electrical charge; not having a potential different from that of the earth.” Note that de-energized does not describe an electrically safe work condition, e.g. a circuit that has been disconnected but not controlled and verified. Because a de-energized circuit can easily be energized while an employee is working on it, the circuits energizing the parts shall be locked out, tagged, or both. See 29 CFR 1910.333(b)(2).

An electrical safety program is a documented establishment of workplace systems, consisting of electrical safety principles, policies, procedures, and processes that appropriately address all site-specific electrical hazards.

Electrically safe work condition means the state in which an electrical conductor or circuit part has been disconnected from energized parts, locked out/tagged in accordance with established standards, tested to verify the absence of voltage, and, if necessary, temporarily grounded for personnel protection.

Enclosure means the case or housing of an equipment system– or the fence or walls surrounding an installation– intended to prevent personnel from unintentionally contacting energized electrical conductors or circuit parts or to protect the equipment from physical damage.

Energized means something is electrically connected to, or is, a source of voltage.

Equipment grounding conductor (EGC) is the conductive path(s) that provide(s) a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor, or to the grounding electrode conductor, or both.

Fault current means the amount of current delivered at a point on the system during a short-circuit condition.

Ground fault means an unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.

Ground, or earth, means the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

Ground-fault circuit interrupter (GFCI) is a device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device.

Incident energy means the amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is typically expressed in calories per square centimeter (cal/cm2).

Incident Energy Analysis is one component of an arc flash risk assessment used to predict the incident energy of an arc flash for a specified set of conditions. The analysis involves calculating the arc flash energy and the arc flash boundary, so as to select the appropriate PPE when working on or near the electrical equipment. An Incident Energy Analysis is also called an “Arc Flash Study.”

Limited approach boundary is the distance from an exposed live part at which a shock hazard exists. Both qualified and unqualified workers may cross the limited approach boundary as long as they are wearing proper shock hazard PPE, as specified in NFPA 70E. Unqualified persons must be escorted by a qualified person to cross this boundary.

Overcurrent means any current in excess of the rated current of equipment or the ampacity of a conductor. It may result in overload, short circuit, or ground fault. Also see the definition for a "Current-limiting overcurrent protective device (OCPD)".

A qualified person is one who has demonstrated skills and knowledge related to the construction and operation of electrical equipment and installations and has received safety training on how to identify the hazards and reduce the associated risk. Some of the regulations and standards also specify that a qualified person must have the knowledge to work safely on energized circuits and must be completely familiar with precautionary techniques and safety equipment.

Restricted approach boundary is the distance from an exposed live part at which there is an increased risk of shock from arc-over or inadvertent movement. Unqualified workers may not cross this boundary. Qualified workers must wear PPE appropriate to the voltage they are exposed to, as specified in NFPA 70E. At a minimum some form of documentation is required to cross this boundary. Often an approved work permit is required.

Risk assessment means an overall process that identifies hazards, estimates the likelihood of occurrence of injury or damage to health, estimates the potential severity of injury or damage to health, and determines if protective measures are required.