General Electrical Glossary

This glossary defines fundamental electrical terms including voltage, current, resistance, impedance, frequency, and power relationships used throughout industrial electrical systems, motors, and variable frequency drives.

Part of the VFDS.com glossary. View the full glossary index.

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Ambient Temperature

Ambient temperature is the temperature of the environment surrounding equipment. For a VFD installed inside a cabinet, the relevant ambient temperature is generally the air around the drive inside that cabinet, as specified by the manufacturer. Operating above an equipment’s rated ambient temperature may require derating or additional cooling and can shorten component life.

Amps

Amps, or amperes, are the unit of measurement for electrical current and represent the flow of electric charge through a conductor. Current levels determine heating, conductor sizing, and protection requirements in electrical systems.

Related terms: Current

Arc Flash

An arc flash is the rapid release of thermal energy, pressure, light, and molten material caused by an electrical current traveling through air between energized conductors or from a conductor to ground. An arc flash can cause severe burns, equipment damage, fire, hearing injury, and other hazards. The severity depends on factors such as fault current, system voltage, working distance, and the time required for protective devices to clear the fault.

Related terms: Arc Flash Study

Arc Flash Study

An arc flash study is an engineering analysis used to estimate the incident energy that a worker could be exposed to during an arc-flash event at specific electrical equipment. The study uses system configuration, available fault current, protective-device settings, clearing times, equipment geometry, and working distance to establish incident-energy values and equipment labels. Its results support risk assessment, safe work practices, and selection of appropriate personal protective equipment.

Related terms: Arc Flash

Breaker Coordination Study

A breaker coordination study evaluates the operating characteristics and settings of circuit breakers throughout an electrical distribution system. Its purpose is to have the protective device closest to a fault open first when practical, limiting the outage to the smallest possible part of the system. The study compares time-current curves and considers overload protection, fault clearing, equipment protection, and arc-flash incident energy.

Building Management System (BMS)

A building management system (BMS) is a computerized system that monitors and controls building services, such as heating, ventilation, air conditioning, and lighting. In motor-control applications, a BMS may command fans, pumps, and VFDs and monitor their operating status. In this context, BMS means Building Management System; the same abbreviation can also refer to a Battery Management System.

Canadian Standards Association (CSA)

The Canadian Standards Association, which operates as CSA Group, is a not-for-profit organization that develops standards and provides testing, inspection, and certification services. CSA standards and certification marks appear on many electrical products. The applicable mark and certification record identify the standards and markets covered, which may include Canada, the United States, or both.

Circuit Breaker

A circuit breaker is a resettable protective device that automatically opens an electrical circuit when it detects excessive current caused by an overload or short circuit. Circuit breakers help protect conductors and electrical equipment from damage and must be selected according to voltage, current, trip characteristics, and interrupting rating.

Contactor

A contactor is an electrically operated switching device used to connect or disconnect a power circuit. A control signal energizes or operates its mechanism to change the state of its contacts. Contactors commonly switch motors, heaters, and other loads, but a contactor alone does not provide motor overload protection. In VFD systems, contactor placement and switching must follow the drive manufacturer’s requirements.

Related terms: Two Contactor Bypass, Three Contactor Bypass

Core Saturation

Core saturation occurs when the magnetic core of a transformer cannot support any additional magnetic flux. When saturation occurs, current increases rapidly, leading to overheating, distortion, and reduced efficiency.

Related terms: Inductance, Current, Transformer

Current

Current is the flow of electric charge through a conductor and represents how much electrical charge passes a point in a circuit over time. It is measured in amperes (amps), where one ampere equals one coulomb of charge flowing per second. In electrical systems, current is driven by voltage and limited by the impedance of the circuit.

In motors and variable frequency drive systems, current is closely related to torque production and load demand. As mechanical load increases, the motor typically draws more current to produce the required torque. Excessive current can lead to overheating, insulation damage, or protective trips, which is why current monitoring and protection are critical in electrical equipment.

DIN Rail

A DIN rail is a standardized metal rail used to mount terminal blocks, relays, circuit breakers, power supplies, controllers, and other electrical components inside a panel or enclosure. Components designed for DIN-rail mounting can be installed, arranged, and replaced without requiring separate mounting holes for each device.

Disconnect

A disconnect is a device used to isolate a circuit or piece of equipment from its electrical supply. It provides a defined means of disconnecting power for maintenance, emergency response, or equipment shutdown. A disconnect does not necessarily provide overcurrent protection unless it incorporates properly selected fuses or another protective device.

Electrical Control Panel

An electrical control panel is an assembled system containing electrical and electronic components used to control, monitor, and protect a machine or industrial process. It may include circuit breakers, contactors, relays, controllers, power supplies, terminal blocks, VFDs, and operator controls housed within an enclosure.

Enclosure

An enclosure is a housing that surrounds electrical equipment or components to help protect people from contact with hazardous parts and protect the equipment from its surroundings. Depending on its design and rating, it may provide protection against dust, water, corrosion, or other environmental conditions. VFDs, motors, and control panels use different enclosure designs suited to their installation requirements.

Related terms: VFD Cabinet

Equipment Submittal

An equipment submittal is a collection of technical documents provided for review to show how proposed equipment meets a project’s requirements. It may include product data, electrical ratings, dimensional drawings, wiring diagrams, installation information, and certification details. The required contents and review process depend on the project specifications.

Frequency

Frequency is the number of complete AC voltage or current cycles that occur each second, measured in Hertz (Hz). In power systems, standard supply frequency is typically 50 Hz or 60 Hz, depending on the region. In AC motors, frequency directly influences rotational speed because the speed of the stator’s rotating magnetic field is proportional to the supply frequency. Variable frequency drives control motor speed by adjusting the output frequency supplied to the motor.

Fuse

A fuse is an overcurrent protective device containing an element that melts and opens the circuit when current exceeds its designed operating characteristics. Once a fuse operates, it must be replaced. Fuses are selected according to factors such as voltage, current, interrupting rating, time-current characteristics, and the equipment or conductors being protected.

Hand-Off-Auto (HOA) Switch

A Hand-Off-Auto (HOA) switch is a three-position selector switch used to choose an equipment’s control mode. Hand typically selects a local or manual operating command, Off disables the normal run command, and Auto allows operation from an external controller or automatic control sequence. Exact behavior depends on the wiring and programming. The Off position does not provide electrical isolation. On many VFDs, the same function is provided by keypad buttons.

Heat Sink

A heat sink is a thermally conductive component that transfers heat away from electrical or electronic devices and releases it into the surrounding air or another cooling medium. Heat sinks commonly use aluminum or copper with fins or other surface-area-enhancing features to help prevent component overheating and maintain reliable operation.

Related terms: Thermal Management

Hertz

Hertz is the unit of frequency representing cycles per second in an alternating current waveform. Electrical power systems commonly operate at 50 Hz or 60 Hz.

Related terms: Frequency

Impedance

Impedance is the total opposition to current flow in an AC circuit, combining both resistance and reactance. Resistance opposes current directly, while reactance results from inductance and capacitance that oppose changes in current and voltage. Impedance influences current levels, voltage drop, fault behavior, and overall system stability in AC power systems. It is measured in ohms (Ω), the same unit used for electrical resistance.

Inductance

Inductance is the property of an electrical conductor or coil that resists changes in current by storing energy in a magnetic field. When current flowing through an inductive component changes, the magnetic field around the conductor expands or collapses, generating a voltage that opposes the change in current according to electromagnetic principles. Inductance is measured in Henries (H).

In AC power systems and motor drives, inductance influences current flow, voltage behavior, and harmonic performance. Inductive components such as reactors, transformers, and motor windings help smooth current, limit inrush current, reduce harmonic distortion, and affect power factor and transient response.

Ingress Protection (IP) Rating

An Ingress Protection (IP) rating is a code defined by IEC 60529 that describes an enclosure’s protection against access to hazardous parts, the entry of solid foreign objects, and harmful water ingress. In common codes such as IP54, the first digit identifies protection against access and solid objects, while the second identifies protection against water. An IP rating does not by itself establish corrosion resistance or compliance with a NEMA enclosure type.

International Electrotechnical Commission (IEC)

The International Electrotechnical Commission (IEC) is an international organization that develops and publishes standards for electrical, electronic, and related technologies. IEC standards address equipment such as motors, drives, switchgear, and electrical enclosures. An IEC reference should identify the applicable standard because different standards cover different product characteristics, test methods, and safety requirements.

Intrinsic Safety

Intrinsic safety is a hazardous-location protection method that limits the electrical and thermal energy available in a circuit so it cannot ignite the specified explosive atmosphere under defined operating and fault conditions. Equipment described as intrinsically safe must be suitable for the application and installed as part of a compatible system. This method commonly applies to instrumentation and control circuits, rather than the main power circuits of motors or VFDs.

NEMA

NEMA stands for National Electrical Manufacturers Association, a U.S. industry association that develops standards and technical guidance for electrical equipment. NEMA designations appear in specifications for products such as motors and electrical enclosures. The applicable standard or designation determines what characteristics are being described; the name NEMA alone does not establish a product’s rating or certification.

NEMA Enclosure Type

A NEMA enclosure type identifies the environmental protection an electrical enclosure is designed to provide under specified conditions. Types such as 1, 3R, 4X, and 12 address different combinations of indoor or outdoor use and exposure to conditions such as dust, rain, washdown, or corrosion. Each type has specific requirements, and a higher type number does not necessarily indicate greater protection in every application. NEMA types and IP codes are not directly equivalent, and there is no official IP-to-NEMA conversion.

Ohms

Ohms (Ω) are the unit of electrical resistance, representing how strongly a material opposes current flow. Resistance influences heating, voltage drop, and power losses in circuits.

Related terms: Resistance

Phase

Phase describes the position of a voltage or current waveform relative to another waveform in an alternating current (AC) system. It indicates the timing relationship between waveforms and is typically measured in degrees of electrical angle. Two waveforms are said to be in phase when their peaks and zero crossings occur at the same time, and out of phase when one waveform leads or lags another.

In power systems, phase is especially important in three-phase electrical systems, where three AC waveforms are separated by 120 degrees. This arrangement allows power to be delivered more smoothly and efficiently than single-phase systems. Correct phase relationships are necessary for proper motor rotation, balanced loading, and reliable operation of electrical equipment.

Power Factor

Power factor is the ratio of real power, measured in watts or kilowatts, to apparent power, measured in volt-amperes or kilovolt-amperes, in an AC electrical system. It indicates how effectively electrical power is being converted into useful work.

A power factor of 1.0 represents ideal utilization, where all supplied power performs useful work. Lower power factor values indicate the presence of reactive power, harmonic distortion, or both, which increase current draw without increasing useful output.

In systems with inductive loads such as motors and transformers, power factor is reduced due to phase displacement between voltage and current. Power electronic equipment, including VFDs, can also reduce power factor through current distortion caused by rectification and switching.

Power factor is often described as either displacement power factor, which accounts only for phase shift, or true power factor, which includes both phase displacement and harmonic distortion. Utilities may impose penalties or limits based on true power factor because low power factor increases losses, reduces system capacity, and stresses electrical infrastructure.

Related Article: Power Factor

Pressure Transducer

A pressure transducer is a device that converts measured pressure into an electrical signal for monitoring or control, typically a millivolt or voltage output. Devices with a 4-20 mA current output are often called pressure transmitters, though many suppliers use the terms interchangeably; the datasheet's output specification is the reliable way to confirm compatibility. The output type and range must match the analog input configuration of the receiving drive or controller.

Pressure Transmitter

A pressure transmitter is a pressure sensor that produces a standardized output signal, most commonly 4-20 mA, for use by controllers, drives, or monitoring systems. A current signal resists electrical noise and voltage drop over long wire runs. Because a live-zero signal reads 4 mA rather than 0 mA at the bottom of the range, a reading of 0 mA can indicate a broken wire or failed sensor. In VFD-controlled pumping systems, a pressure transmitter commonly provides the feedback signal that lets the drive adjust motor speed to maintain a pressure setpoint. The terms pressure transmitter and pressure transducer are often used interchangeably.

Primary Winding

The primary winding is the coil of a transformer that is connected to the input power source. It receives electrical energy and creates a magnetic field in the core that enables energy transfer to the secondary winding.

Related terms: Transformer, Secondary Winding

Programmable Logic Controller (PLC)

A programmable logic controller (PLC) is an industrial controller that reads input signals, executes a programmed sequence or control logic, and operates outputs. PLCs coordinate equipment such as motors, VFDs, valves, and sensors. A PLC can send commands to a VFD and monitor its status through wired signals or an industrial communication network.

Purging and Pressurization

Purging and pressurization is a hazardous-location protection method that uses a protective gas, such as clean air or an inert gas, to establish and maintain a protected environment inside an enclosure. Purging removes or reduces a potentially flammable atmosphere before equipment is energized, while pressurization helps prevent the surrounding atmosphere from entering during operation. Required monitoring, alarms, and shutdown functions depend on the protection system and applicable requirements.

Resistance

Resistance is the opposition to electrical current flow within a conductor and is measured in ohms. Electrical resistance converts energy into heat and contributes to power losses.

SCCR

SCCR, or Short Circuit Current Rating, is the maximum prospective fault current that an electrical device, assembly, or industrial control panel can safely withstand without causing a fire, explosion, or loss of enclosure integrity.

SCCR is expressed in kiloamperes (kA) and represents the available short-circuit current at the point where the equipment is installed. If the available fault current exceeds the SCCR, protective devices may fail to clear the fault safely, creating a serious safety hazard.

In VFD systems, SCCR is determined by the weakest component in the power path, such as contactors, fuses, circuit breakers, reactors, or the drive itself. Panel-level SCCR is not the same as the SCCR of individual components and must be evaluated as a complete system.

SCCR is a critical compliance requirement under standards such as UL 508A. Proper SCCR coordination ensures that protective devices limit let-through current during a fault and that the system can safely withstand worst-case short-circuit conditions.

Related terms: Amps Interrupting Current, Overcurrent Fault, Overvoltage Fault, Under Voltage Fault

Secondary Winding

The secondary winding is the coil of a transformer that delivers electrical energy to the load. Voltage in the secondary winding is determined by the transformer turns ratio relative to the primary winding.

Related terms: Transformer, Primary Winding

Solar Heat Gain

Solar heat gain is the additional heat absorbed by an object from solar radiation. For outdoor electrical equipment, sunlight can heat an enclosure and increase the temperature of the components inside it. The amount of heating depends on factors such as solar exposure, surface color and finish, enclosure orientation, and shading.

Related terms: Sunshade

Supervisory Control and Data Acquisition (SCADA)

Supervisory Control and Data Acquisition (SCADA) is a system used to monitor industrial equipment, collect operating data, display alarms, and provide supervisory control. It typically communicates with PLCs, remote terminal units, or other controllers that perform local control functions. SCADA allows operators to view and manage processes across a facility or multiple locations.

Surge Protective Device

A surge protective device (SPD) protects electrical equipment from voltage spikes caused by lightning, switching events, or other transients.

Switchgear

Switchgear is an assembly of switching, protective, control, and isolation devices used to manage electrical power distribution. It commonly includes circuit breakers, disconnects, protective relays, control equipment, instrumentation, and bus conductors installed within a metal enclosure. Switchgear allows circuits and equipment to be energized, de-energized, isolated, and protected against electrical faults.

Terminal

A terminal is a conductive connection point where a wire or other conductor connects to an electrical device or circuit. Terminals may use screws, clamps, studs, or other connection methods. On a VFD, different terminals serve functions such as incoming power, motor output, grounding, and control signals.

Terminal Block

A terminal block is an insulated assembly that supports and organizes terminals for connecting electrical conductors. Terminal blocks are commonly used in control panels to join field wiring to internal wiring and provide accessible connection points. Depending on the design, they may include feed-through connections, grounding connections, disconnects, or fuses.

Transformer

A transformer is an electrical device that transfers energy between circuits using electromagnetic induction to increase or decrease voltage levels. It consists of primary and secondary windings wound around a magnetic core and is used for power distribution, isolation, and voltage conversion in electrical systems. Transformers are commonly used in industrial systems to support motor drives, power distribution, and system reliability.

Related terms: Turns Ratio, Primary Winding, Secondary Winding, Core Saturation

Turns Ratio

Turns ratio is the ratio of the number of windings in the primary coil to the number of windings in the secondary coil of a transformer. It determines how voltage is increased or decreased between the input and output.

Related terms: Transformer, Voltage, Primary Winding, Secondary Winding

Underwriters Laboratories (UL)

Underwriters Laboratories is the historical name associated with UL safety research, standards, testing, and certification. Today, these activities are carried out by distinct organizations, including UL Research Institutes, UL Standards & Engagement, and UL Solutions. For electrical equipment, UL marks identify a particular certification status, such as UL Listed or UL Recognized Component. Component recognition does not establish that the complete assembled product is UL Listed.

Voltage

Voltage is the electrical potential difference that drives current through a circuit. Proper voltage levels are essential for equipment performance and longevity.

Volts

Volts are the unit of electrical potential difference that drives current through a circuit. Voltage level determines insulation requirements, power capacity, and system design.

Related terms: Voltage

Watts

Watts (W) are the unit of real power and measure the rate at which electrical energy is converted into useful work or heat. In AC systems, watts represent the portion of power that actually performs work, unlike reactive power which does not produce usable output. Watts are commonly calculated as voltage times current times power factor in AC systems, and they are used to estimate energy consumption, heat load, and operating cost.