Common Terms and Definitions

Arc Flash Study

Arcing Fault

An arc flash is the result of the rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or ground.

Arc Flash Study Analysis

An arc flash hazard analysis shall determine the arc flash boundary, the incident energy at the working distance, and the personal protective equipment that people within the arc flash boundary shall use. The arc flash hazard analysis shall be updated when a major modification or renovation takes place. It shall be reviewed periodically, not to exceed 5years, to account for changes in the electrical distribution system that could affect the results of the arc flash hazard analysis. The arc flash hazard analysis shall take into consideration the design of the overcurrent protective device and its opening time, including its condition of maintenance. The opening and clearing times of overcurrent protective devices can result in increased incident energy. Both larger and smaller available short-circuit currents could result in higher available arc flash energies as well.

Arc Flash Hazard Analysis

An Arc Flash Hazard is a dangerous condition associated with the possible release of energy caused by an electric arc. A study investigating a worker’s potential exposure to arc flash energy, conducted for the purpose of injury prevention and the determination of safe work practices, arc flash boundary, and the appropriate levels of personal protective equipment (PPE). Key changes throughout the Standard replace the phrase “hazard analysis” with “risk assessment” to enable a shift in awareness about the potential for failure. Change in naming from “Hazard Risk Category” to “Arc Flash PPE Category.”

Arc Flash Labels

Electrical equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures, motor control centers that are in other than dwelling units, and are likely to require examination, adjustment, servicing, or maintenance while energized, shall be field marked with a label containing all the following information: (1)At least one of the following: Available incident energy and the corresponding working distance, Minimum arc rating of clothing, Required level of PPE, Highest Hazard/Risk Category (HRC)for the equipment, (2) Nominal system voltage, (3) Arc flash boundary

Arc Flash PPE Category

The PPE table defines the Personal Protective Equipment Categories and clothing descriptions used in the reports and labels. Different label colors may be assigned for each PPE Category, the Bus Detail and Arc Flash Label will apply the colors based on the PPE Categories calculated. The NFPA 70E provides Standard Guidelines for Arc-Rated Clothing and other Personal Protective Equipment (PPE) for use with an Arc Flash Risk Assessment. Table H.3 (a) provides a summary of specific sections within the 70E standard describing personal protective equipment for electrical hazards. Guidance on the selection of arc-rated clothing and other personal protective equipment for users who conduct a risk assessments to determine the incident energy exposure (incal/cm2) to the worker is provided in NFPA 70E Table H.3(b).

Boundary, Arc Flash

When an arc flash hazard exists, an approach limit at a distance from a prospective arc source within which a person could receive a second-degree burn if an electrical arc flash were to occur.

Boundary, Limited Approach

An approach limit at a distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists

Boundary, Restricted Approach

An approach limit at a distance from an exposed energized electrical conductor or circuit part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the energized electrical conductor or circuit part.

Boundary, Prohibited Approach

An approach limit at a distance from an exposed energized electrical conductor or circuit part within which work is considered the same as making contact with the electrical conductor or circuit part.

Incident Energy

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

NFPA70E Standard

NFPA 70E requirements for safe work practices protect personnel by reducing exposure to major electrical hazards. Originally developed at OSHA’s request, NFPA 70E helps companies and employees avoid workplace injuries and fatalities due to shock, electrocution, arc flash, and arc blast, and assists in complying with OSHA 1910 Subpart S and OSHA 1926 Subpart K. The Standard includes guidance for making hazard identification and risk assessments, selecting appropriate PPE, establishing an electrically safe work condition, and employee training.

Electrically Safe Work Condition

A state in which an electrical conductor or circuit part has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary.

Qualified Person

One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved. [70, 2011]

Short Circuit and Coordination Study

ANSI Fault

The A_FAULT Short Circuit Study calculates the balanced and unbalanced fault current duties at all buses and provides a detailed Report for branch fault current contributions for a fault at a single bus. The A_FAULT Short Circuit Study algorithm uses the methodology outlined in the ANSI (American National Standards Institute) C37 Standards. ANSI Standard C37.13 is the critical standard used for calculating the fault duty at low voltage buses, whereas ANSI Standard C37.5 and C37.010 are the critical standards used for calculating the fault duty on medium and high voltage buses. For medium and high voltage buses, both closing and latching (withstand and momentary, respectively) duties, and an interrupting (contact separating or breaking) duty are calculated.

Comprehensive Fault

There are two major differences between the Comprehensive Short Circuit Study and the A_FAULT Short Circuit Study. First, the Comprehensive Short Circuit Study reports the fault location X/R ratio based on the complex algebra formulation of the Thevenin equivalent impedance, whereas the A_FAULT Short Circuit Study uses the ANSI method of separately derived resistance and reactance at the fault location. Second, the A_FAULT Short Circuit Study includes specific multipliers to model the ac decrement of motors, and generators that are local to the fault point location. In essence, the A_FAULT Short Circuit Study calculates the fault duties which are compared to the manufacturer’s published ratings.

Utility Source Contribution

A utility component represents an electric power utility company or any primary power supply. Think of this component as a lumped equivalent of all the utility’s generators. You specify the utility’s voltage in per unit and relative voltage angle, and also define its capacity to produce short-circuit current. In PTW, utilities are automatically defined as swing buses.

Equipment Evaluation

The Equipment Evaluation Study module compares the following items: Voltage Rating of cables and protection components versus nominal bus voltage, the Calculated Interrupting Fault Duty versus Interrupting Rating of protection components, and the Calculated Asymmetrical Duty versus Asymmetrical Rating of protection components.

There are two sets of limits: Marginal and Fail. If the evaluation result is less than the Marginal limit, the program will report the evaluation result for the device as Pass. If the evaluation result reaches the Marginal limit but still smaller than the limit of Fail, the program will report the evaluation result for the device as Marginal. Otherwise, the device will be reported as Fail.

Protective Device Coordination

The purpose of the coordination study is to determine the characteristics, ratings and settings of overcurrent protective devices to provide adequate protection for the equipment against short circuits and overloads and optimize the selective operation between devices. The priority is to provide overcurrent protection for the cables, transformers, motors, generators, and relative equipment appurtenances in accordance with applicable guidelines and standards and the equipment fault withstand capabilities. The protective device is set below the equipment damage curve with a degree of flexibility, such that the pickup and short-time settings can be selectively coordinated with a device in series so that the one nearest to the fault will open first. This ensures that a minimum portion of the power system remains energized while the fault is isolated and safely interrupted. Although selectivity between adjacent devices is not always possible, the study assures the optimum coordination and system protection.

Power System Components

Automatic Transfer Switch (ATS)

An auto-transfer switch (ATS) component is a 3-way switch. The “N” normal side could be connected to a utility, the “E” emergency side could be connected to an emergency or standby generator, and the third node connected to the load. The ATS component can be used in both AC and DC systems.


A cable component is a single or stranded conductor with or without insulation and other coverings (single-conductor cable), or a combination of conductors insulated from one another (multiple-conductor cable). A cable’s Library-reference consists of four parts: the cable type (Al (Aluminum) or Cu (Copper)), the duct type (Magnetic, Non-Magnetic, Plastic, or Bus), the Insulation type (THWN, XLP, THHN, EPR, (non-insulated)), and the cable voltage. Cables are modeled in PTW as a series resistance-and-reactance value in ohms/1000 ft.


A synchronous generator component is a synchronous alternating-current machine which transforms mechanical work into electric power. (A synchronous machine is one in which the average speed of normal operation is exactly proportional to the frequency of the system to which it is connected).


An impedance device is any device which has either resistance or reactance, or both. This typically consists of utility and generator source, cables, transformers, induction or synchronous motors, and non-motor loads.


Electrical loads can be most generally divided into non-motor loads and motor loads. The primary difference between the load types is that motor loads are rotating loads that store energy for fault current calculations. Non-rotating loads such as lighting panels, which do not affect faults, should be modeled as non-motor loads, not as motor loads. An induction motor component is an ac motor in which a primary winding on one member (usually the stator) is connected to the power source, and a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current.

Protective Device

A protective device component is a bypass gap or other device which limits the voltage on the capacitor segment to a predetermined level when overcurrent flows through the series capacitor (that is, during system faults, system swings, or other abnormal events), and which is capable of carrying capacitor discharge, system fault, and load current for specified durations. PD Components consist of LV and MV Breakers, Fuses, Motor Overloads, Motor Circuit Protectors, Specialty Devices, Recloser, Relays, and High Voltage Breakers.


The Scenario Manager is a power tool to document and compare different design alternatives and fault conditions under normal and alternative power sources. This tool helps to keep track of historical and future facility expansions. It is also used to identify the worst case arc flash PPE category levels and to develop optimal solutions to harmonic problems.


A transformer component is a device which, when used, will raise or lower the voltage of alternating current of the original source. The two-winding transformer component has two nodes, or connection points. All transformers are modeled as three-phase devices–either three single-phase transformers or a single three-phase type. Note that kVA values are total three-phase values and the transformer rated-voltages are line-to-line voltages.

Uninterruptable Power Supply (UPS)

A UPS is a device which can provide power to the load terminals under normal conditions and upon loss of power to the line side terminals. The capability to continue to supply power upon loss in input power is accomplished by drawing power from an energy storage device such as a battery or flywheel.

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