Glossary

arc blast
The explosive result of an arcing fault. As current begins passing through ionized air, large volumes of ionized gases, along with metal from the vaporized conductors, are rapidly expelled, creating such hazards as intense heat, thermoacoustic shock wave, molten metal, shrapnel, blinding light, toxic smoke and contact with energized components.

arc flash
The radiant energy and blinding light created by an electric arc passing through air between conductors. Arc temperature can reach 35,000°F, burning skin and igniting clothing at a distance.

arc flash hazard analysis
Part of an electrical hazard analysis, an arc flash hazard analysis is a method of identifying:

• The arc flash protection boundary
• The proper personal protective equipment (PPE) and protective clothing necessary, based on the incident energy present at the working distance for the task to be performed
• Appropriate safety-related work practices

Included in the process are a short circuit study to determine the available "bolted" fault current at each location in the system, the gathering of all relevant overcurrent protection device data to accurately predict the clearing time of the arc fault current, and the calculation of arc flash protection boundaries and incident energies, which are used to determine PPE categories.

bolted fault current
A bolted fault is created when two or more phases are mechanically connected at the same point. This can occur if phases are crossed when power conductors are installed. Bolted faults create high current levels and intense magnetic fields, and can result in deformation and/or breakage of structural components, melting of bus bars or conductors, and weakening of connection points.

current-limiting fuse
A UL Listed, current-limiting fuse must clear a short circuit current in less than one half cycle. By isolating a faulted circuit before the fault current has sufficient time to reach its maximum value, a current-limiting fuse tremendously limits the total electrical energy delivered to the fault, reducing both the magnitude and duration of a fault current.

electrically safe work condition
A state in which the conductor or circuit part to be worked on or near has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if necessary.

flash protection boundary
An approach limit at a distance from exposed live parts within which a person could receive a second-degree burn if an electrical arc flash were to occur.

fuse clearing I²t rating
A value that takes into account a fuse’s peak let-thru current (I_p) and total clearing time. Fuse clearing I²t values are derived from oscillograms of fuses tested within their current-limiting range and can be used to determine the level of protection provided to circuit components under fault current conditions.

fuse let-thru current
The current passed by a fuse while the fuse is interrupting a fault within its current-limiting range.

IEEE 1584, Guide for Performing Arc Flash Calculations
Identifies a systematic, nine-step approach for performing a comprehensive arc flash hazard analysis.

incident energy
The amount of energy impressed on a surface at a distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per square centimeter (cal/cm²).

IP20-rated devices
Finger-safe devices, such as fuse holders, power distribution blocks and disconnect switches, that reduce shock hazards and minimize the chance of an arc flash being initiated by accidental contact.

NFPA 70E, Standard for Electrical Safety in the Workplace
A national consensus standard for electrical safety. It outlines the requirements of employers whose employees are to work on equipment that has not been placed into an electrically safe work condition. One of these requirements is to perform an arc flash hazard analysis, which enables the employer to determine the flash-protection boundary and the category of personal protective equipment (PPE) needed.

OSHA 29 CFR Part 1910, Occupational Safety and Health Standards
The OSHA regulation that promotes the safety of employees working on or near electrical equipment. OSHA 29 CFR Part 1910 clearly defines employers' responsibilities, including:

• Equipment must be deenergized before work is performed
• Lockout/tagout procedures must be used
• If equipment cannot be deenergized prior to work:
  • Employees must be properly protected
  • Employers are responsible for performing a hazard assessment

peak let-thru chart
Peak let-thru current (I_p) data presented in the form of a graph, where the X axis is available fault current in RMS symmetrical amps and the Y axis is instantaneous peak let-thru current in amps.

peak let-thru current
When a fault current begins to rise in the first half cycle and the element in a current-limiting fuse melts, the added impedance created by the resultant arc causes the current to max out and begin to decrease. The highest instantaneous value reached is referred to as peak let-thru current, and is expressed as a peak instantaneous value (I_p). I_p is useful in predicting peak electromagnetic forces created throughout the faulted circuit. The peak forces will be proportional to the square of the instantaneous value of current of the first half-cycle peak, or I_p², for current-limiting fuses.

peak let-thru tables
Based on peak let-thru charts, these easy-to-use tables reflect fuse tests at 15% power factor at rated voltage, with prospective fault currents as high as 200,000 amperes. At each prospective fault current, let-thru data is given in two forms — I_RMS and I_p. Using let-thru tables saves time and reduces the possibility of errors.

PPE categories
Defined by NFPA 70E, the categories of personal protective equipment workers are required to wear for the work to be performed, based on the incident heat energy calculated at the appropriate working distance. Cal/cm² are the units of incident energy the PPE can withstand. Note that appropriate face, hand and foot protection also is required.

working distance
The distance between a potential arc source and a worker's body and face. The default value is normally 18 inches.