Incident Energy

Definition - What does Incident Energy mean?

Incident energy is a term of measurement that describes the amount of thermal radiation (heat) that a worker would encounter at a given distance from an arc fault, a high power discharge of electricity from a piece of electrical equipment.

One of the primary hazards faced by employees working on electrical hazards is the potential to be exposed to an arc flash, which refers to the discharge of light and heat (thermal energy) that occurs as part of an arc fault. Incident energy can thus be understood as expressing the strength of an arc flash in quantitative terms.

Safeopedia explains Incident Energy

The occupational health and safety requirements that workers operating on or in proximity to electrical hazards must comply with are prescribed based on the amount of incident energy that could be emitted by the hazard at a given working distance. The concept of incident energy is described in terms of the amount of energy that is emitted by an arc fault over a given area; thus, common units that are used to expressed measures of incident energy include calories per centimeter squared (cal/cm2) and joules per centimeter squared (J/cm2).

An understanding of the potential incident energy that could be generated by a system is important for knowing the amount of protection needed against arc faults. In the United States, an analysis of the potential incident energy that a worker could be exposed to in the case of an arc fault is a requirement under OSHA standards 1910 and 1926.

The goal of incident energy protection can be concisely expressed as preventing the worker from being exposed to sufficient incident energy to cause second-degree burns, which is assumed to be 1.2 cal/cm2 (5 J/cm2) for general worker-protection purposes. The point at which worker exposure exceeds this amount is defined as the arc-flash boundary by the National Fire Protection Association's 70E standard.

There are multiple ways to calculate the incident energy of a system, only some of which are recommended under OSHA’s standard. The calculation method that OSHA accepts for most types of equipment (15 kilovolts (kV) or less) is detailed under the Institute of Electrical and Electronics Engineers’ 1584b-2011 standard. For higher voltages, OSHA recommends the use of ARCPRO, a software package. Calculating the incident energy requires knowing how long an arc lasts, the worker's distance from the source of the arc, and the amount of current associated with the arc.

The results of these studies are used to determine the types of PPE that workers must wear to protect themselves under the NFPA 70E standard. OSHA recognizes the use of NFPA 70E as a method through which an employer can meet OSHA’s electrical safety standards, and conversely, may view a failure to implement 70E as evidence that an employer failed to meet his or her general duty to ensure employee safety.

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