Incident Energy

What Does Incident Energy Mean?

Incident energy is the measure of thermal energy that would be generated by a piece of electrical equipment, at a specified distance from it, during an electrical arc flash.

Incident energy is an important consideration for workplace safety, as it specifies the amount of thermal radiation (heat) to which a worker would be exposed.

Safeopedia Explains Incident Energy

Arc flashes are one of the primary hazards faced by anyone who works on electrical equipment. These flashes are discharges of light and heat that result from an arc fault. Incident energy can thus be understood as a quantitative expression of the strength or intensity of an arc flash.

The Concept of Incident Energy

Incident energy is described in terms of the amount of energy emitted by an arc fault over a given area. The common units used to express measurement of incident energy are calories per centimeter squared (cal/cm²) and joules per centimeter squared (J/cm²).

In the United States, OSHA standards 1910 and 1926 require employers to conduct an analysis of the potential incident energy that a worker could be exposed to in the even of an arc fault.

The goal of this analysis is to prevent workers from being exposed to levels of incident energy that could cause second-degree burns – assumed to be 1.2 cal/cm² (5 J/cm²). The point at which the worker exposure exceeds this amount is defined in the National Fire Protection Association’s 70E Standard for Electrical Safety in the Workplace.

Calculating Incident Energy

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. The calculations may also be based on the equations in NFPA 70E or NESC. 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 personal protective equipment (PPE) workers must wear to protect themselves under the NFPA 70E standard.

Strategies for Mitigating Arc Flash

Some effective strategies to reduce the frequency and severity of arc flash incidents include:

• Performing an incident energy analysis to calculate the amount of energy an arc flash could release
• Providing employees with appropriate PPE, such as flame-resistant clothing, eye protection, gloves, and Class E electrical hard hats
• Posting warning labels on any piece of equipment that poses an arc flash hazard and on floors to indicate the proper distance from hazardous electrical equipment
• Training employees on the dangers of arc flashes
• Reducing the available fault current by using non-current limiting breakers (NCLBs)
• Shortening arc flash events by decreasing fault clearance times with a bus differential scheme, arc flash relay system, arc quenching switchgear, or other measures