Why These Terms Matter

Electrical engineering carries its own dense vocabulary of acronyms and abbreviations that span equipment nameplates, protective relay settings, one-line diagrams, and test reports. Unlike the NEC's Article 100 definitions — which define legal terms used in the code itself — this glossary focuses on the shorthand power systems engineers, electricians, and technicians use every day on drawings, in relay setting sheets, and in conversation: device function numbers, power quality terms, protection concepts, and testing terminology.

Getting these terms wrong has real consequences. Confusing an ANSI device 50 (instantaneous overcurrent) with a device 51 (time-overcurrent) changes how a relay is expected to behave during a fault. Mixing up THD with TDD changes whether a harmonic study passes IEEE 519 limits. This glossary defines 55 of the terms most likely to appear on a one-line diagram, a relay coordination study, or a PE exam, organized alphabetically with plain-language explanations and the standards where each concept originates.

A

AFCI — Arc-Fault Circuit Interrupter
A device that detects the electrical signature of arcing — from damaged insulation, loose terminations, or cord damage — and interrupts the circuit before the arc can ignite adjacent combustibles. Distinct from a GFCI, which detects current leaking to ground rather than arcing. Required by NEC 210.12 for most dwelling unit branch circuits.
ampacity — Ampere Capacity
The maximum current a conductor can carry continuously without exceeding its insulation temperature rating, under stated conditions of use. Ampacity tables (NEC 310.16) account for conductor size, insulation type, and installation method; correction factors apply for ambient temperature and conductor bundling.
ANSI device number — American National Standards Institute Device Function Number
A standardized numbering system (from IEEE C37.2) used to label protective relay functions on one-line diagrams regardless of manufacturer. For example, device 50 is instantaneous overcurrent, 51 is time-delay overcurrent, 27 is undervoltage, 59 is overvoltage, and 87 is differential protection. Learning the common device numbers lets an engineer read any one-line diagram without a legend.
ARC (arc flash) — Arcing Fault
A short circuit through air between two energized conductors, or between an energized conductor and ground, that produces intense heat, light, and pressure. Arc flash incident energy calculations (per IEEE 1584 and NFPA 70E) determine the required PPE category for work on energized equipment.
ASD — Adjustable Speed Drive
A general term for any electronic device that controls motor speed by varying the frequency or voltage supplied to the motor. VFD (variable frequency drive) is the most common type of ASD used with AC induction motors.
ATS — Automatic Transfer Switch
A switch that automatically transfers a load between two power sources — typically utility power and a backup generator — when the primary source fails, and transfers back once it is restored. ATS ratings, transfer time, and open/closed transition type are critical for standby power system design per NFPA 110.

B

BIL — Basic Impulse Level (Insulation Level)
The voltage that equipment insulation can withstand from a standardized lightning impulse test wave without failure, expressed in kV. BIL ratings determine how much surge and lightning protection margin exists in transformers, switchgear, and cable insulation; higher system voltages require higher BIL ratings.
BESS — Battery Energy Storage System
A system of battery modules, inverters, and controls that stores electrical energy and discharges it on demand — used for peak shaving, backup power, renewable firming, and grid services. NFPA 855 governs stationary BESS installation requirements including spacing, fire suppression, and thermal runaway detection.
BPT — Bushing Potential Transformer
A voltage-sensing device built into a high-voltage bushing (commonly on power transformers) that provides a scaled-down voltage signal for metering and protection without a separate standalone PT. Used where space or cost make a dedicated potential transformer impractical.

C

CT — Current Transformer
An instrument transformer that steps down a large primary current to a standardized secondary current (commonly 5A or 1A) for metering and protective relaying. CT ratio, accuracy class, and burden must be selected together — an undersized or saturated CT gives inaccurate relay inputs during high-current faults.
CB — Circuit Breaker
A device that opens and closes a circuit both manually and automatically, interrupting current on a predetermined overcurrent condition without damage to itself when applied within its rating. Distinguished from a fuse, which must be replaced after operating.
CPT — Control Power Transformer
A small transformer that steps down line voltage to a lower control voltage (commonly 120V) used to power relays, contactor coils, indicator lights, and control circuits within a motor control center or switchgear lineup, isolated from the main power circuit.
CVT — Capacitor Voltage Transformer
A voltage-sensing device used on high-voltage transmission systems that uses a capacitive voltage divider ahead of a conventional transformer, reducing cost compared to a magnetic potential transformer at very high voltages (typically above 69 kV).

D — E

DEMAND FACTOR — Demand Factor
The ratio of the maximum demand of a system to its total connected load. A demand factor below 1.0 reflects that not every connected load operates simultaneously; NEC Article 220 uses tabulated demand factors to reduce calculated service and feeder loads below the sum of nameplate ratings.
DTOC — Definite-Time Overcurrent (relay)
An overcurrent relay function that operates after a fixed time delay once current exceeds its pickup setting, regardless of how far above pickup the current is. Contrasted with inverse-time overcurrent (IDMT), where the operating time shortens as fault current increases.
EGC — Equipment Grounding Conductor
The conductive path — typically the bare or green wire — that connects the metal frames of equipment to the grounded system, providing a low-impedance path for ground fault current to return to the source and trip the overcurrent device. Defined in NEC Article 100 and sized per Table 250.122.
EMF — Electromotive Force
The voltage generated by a source (battery, generator, or transformer) that drives current through a circuit. Distinguished from voltage drop, which is the loss across impedance as current flows; EMF is the driving potential before any drop is subtracted.

F — G

FLA — Full-Load Amperes
The current a motor or piece of equipment draws when operating at its rated output under rated conditions. Motor overload protection (NEC 430.32) and conductor sizing are based on FLA from the equipment nameplate or NEC Table 430.250, not on the motor's calculated horsepower alone.
GFCI — Ground-Fault Circuit Interrupter
A device that continuously compares current on the hot and neutral conductors and trips when it detects an imbalance as small as 4-6 mA, indicating current is leaking to ground through an unintended path — commonly a person. Required by NEC 210.8 in bathrooms, kitchens, outdoor locations, and other wet or damp areas.
GFP — Ground-Fault Protection (of equipment)
A protective scheme, required by NEC 230.95 on services rated 1000A or more at 150V to ground or less (solidly grounded wye systems), that detects and clears ground faults at higher current levels than a personnel GFCI, primarily to limit equipment damage and arcing fault energy rather than to protect people.
GSU — Generator Step-Up (transformer)
A transformer that raises a generator's output voltage (commonly 13.8-25 kV) up to transmission voltage for delivery to the grid. GSU transformers are among the largest and most critical transformers at a power plant or substation.

H — I

HRC — High Rupturing Capacity (fuse)
A fuse class designed to safely interrupt very high fault currents without violent case rupture, by using sand-filled or similar current-limiting elements. Common in industrial and utility applications where available fault current is high.
IDMT — Inverse Definite Minimum Time (overcurrent relay)
An overcurrent relay characteristic where operating time decreases as fault current increases, following a defined curve (per IEEE C37.112 or IEC 60255), while approaching a minimum time at very high currents. IDMT curves let engineers coordinate multiple protective devices so the one closest to a fault trips first.
IEEE — Institute of Electrical and Electronics Engineers
A professional organization that publishes widely adopted electrical standards, including IEEE 519 (harmonic limits), IEEE 1584 (arc flash calculations), IEEE C37 series (protective relaying and switchgear), and IEEE 1547 (distributed energy resource interconnection).
ICCB — Insulated Case Circuit Breaker
A molded-case-style breaker built to a more robust, field-serviceable standard than a standard MCCB, often with adjustable trip settings and interchangeable trip units, used in applications between typical MCCBs and full low-voltage power circuit breakers (LVPCBs).
ISC — Short-Circuit Current (Interrupting/Available Fault Current)
The maximum current that would flow at a point in a system during a bolted fault, before any protective device operates. Available fault current calculations determine the required interrupting rating of every breaker and fuse in the system (NEC 110.9) and drive arc flash incident energy.

K — L

kAIC — Kiloamperes Interrupting Capacity
The maximum fault current, in thousands of amperes, that a circuit breaker or fuse is rated to safely interrupt. A breaker's AIC rating must equal or exceed the available fault current at its point of installation, or it risks catastrophic failure during a fault.
LVPCB — Low-Voltage Power Circuit Breaker
A drawout-style breaker (typically ANSI/IEEE C37.13 rated) used in switchgear at 600V and below for main and feeder protection in larger commercial and industrial systems, offering higher interrupting capacity and more sophisticated trip units than a molded-case breaker.
LOTO — Lockout/Tagout
The OSHA-mandated procedure (29 CFR 1910.147) for de-energizing equipment, physically locking disconnecting means in the open position, and tagging them before performing maintenance, preventing unexpected re-energization while workers are exposed.

M — N

MCC — Motor Control Center
A factory-assembled enclosure housing multiple motor starter units — combination starters, VFDs, or soft starters — fed from a common bus, used to centralize motor control and protection in industrial and commercial facilities.
MCCB — Molded-Case Circuit Breaker
The most common type of circuit breaker used in commercial and residential panelboards, built with an insulating plastic (molded) case, fixed or adjustable thermal-magnetic or electronic trip settings, and typically not drawout or field-serviceable.
NEMA — National Electrical Manufacturers Association
A trade association that publishes equipment standards, including NEMA enclosure ratings (NEMA 1, 3R, 4, 4X, 12, etc.) describing the degree of protection an enclosure provides against dust, water, and corrosion, and NEMA motor frame and design letter standards (Design A/B/C/D).
NETA — InterNational Electrical Testing Association
An organization that publishes the ANSI/NETA acceptance and maintenance testing standards used to commission and periodically verify electrical distribution equipment — insulation resistance, contact resistance, and protective relay testing among them.

O — P

OCPD — Overcurrent Protective Device
A general term covering any circuit breaker or fuse that opens a circuit when current exceeds a safe level, protecting conductors and equipment from overload and short-circuit damage. NEC conductor ampacity and OCPD sizing rules are closely linked (NEC 240.4).
OL — Overload (relay/protection)
A protective device or relay function that detects sustained current above a motor's or conductor's rated capacity — but below short-circuit levels — and trips before thermal damage occurs. Motor overload relays (NEC 430 Part III) are sized based on motor FLA and service factor, separately from the branch-circuit short-circuit protection.
PF — Power Factor
The ratio of real power (kW) to apparent power (kVA) in an AC circuit, reflecting the phase difference between voltage and current caused by inductive or capacitive loads. A low power factor means the system delivers more current than the real work performed requires, increasing losses and often triggering utility power factor penalties; capacitor banks are commonly installed to correct it.
PT — Potential Transformer
An instrument transformer that steps down a high system voltage to a standardized low secondary voltage (commonly 120V) for metering and protective relay voltage inputs, paired with CTs to give relays a scaled view of system conditions.
PPE — Personal Protective Equipment
In an electrical context, the arc-rated clothing, face shields, gloves, and other protective gear required when working on or near energized equipment, with the required category determined by an arc flash incident energy calculation per NFPA 70E.

Q — R

RCD — Residual Current Device
The IEC/international equivalent term for what North American practice calls a GFCI — a device that detects an imbalance between line and neutral current and disconnects the circuit, used broadly across Europe and other IEC jurisdictions for shock protection.
RTD — Resistance Temperature Detector
A temperature sensor whose electrical resistance changes predictably with temperature, commonly embedded in motor windings and transformer windings to monitor for overheating and feed protective relay thermal trip functions.
RVSS — Reduced-Voltage Solid-State (starter)
Another common name for a solid-state soft starter — a motor starting device that limits inrush current and mechanical stress by ramping voltage up gradually at start, then bypassing to full-voltage operation, as distinct from a VFD that continuously varies frequency during running.

S

SCADA — Supervisory Control and Data Acquisition
A system architecture of sensors, remote terminal units (RTUs) or PLCs, and a central control room that monitors and controls distributed industrial or utility equipment in real time, widely used for substation automation and distribution feeder control.
SCCR — Short-Circuit Current Rating
The maximum available fault current an entire assembly — such as an industrial control panel — can withstand without unsafe failure, as distinct from the interrupting rating of a single breaker within it. UL 508A governs SCCR calculation methods for control panels.
SPD — Surge Protective Device
A device installed at service equipment, panelboards, or point-of-use that limits transient overvoltages (from lightning or switching surges) by diverting excess energy to ground. NEC 230.67 requires SPDs on most new dwelling unit services.
SWGR — Switchgear
An assembly of switching, protective, and metering devices — breakers, fuses, buses, and instrumentation — arranged to control and protect electrical distribution at a given voltage level. Switchgear is broadly classified by voltage class (low, medium, high) and construction (metal-enclosed, metal-clad, arc-resistant).

T

TDD — Total Demand Distortion
A harmonic distortion measure, defined in IEEE 519, that expresses harmonic current as a percentage of the maximum demand load current (rather than the instantaneous fundamental current used by THD), providing a more consistent basis for comparing harmonic performance against utility limits.
THD — Total Harmonic Distortion
A measure of how much a voltage or current waveform deviates from a pure sine wave due to harmonic components, expressed as a percentage of the fundamental. High current THD from nonlinear loads (VFDs, switch-mode power supplies) can cause transformer overheating, neutral conductor overloading, and nuisance tripping if not mitigated with filters or derating.
TOV — Temporary Overvoltage
A power-frequency overvoltage of relatively short duration (seconds to minutes) caused by events like load rejection, single-line-to-ground faults on ungrounded systems, or ferroresonance — distinct from a lightning-caused transient, which lasts microseconds.

U — V

UPS — Uninterruptible Power Supply
A battery-backed power system that provides instantaneous, uninterrupted power to critical loads during a utility outage, bridging the gap until a generator starts or providing ride-through for loads (like data center servers) that cannot tolerate any interruption at all.
VFD — Variable Frequency Drive
A power electronic device that controls AC motor speed and torque by varying the frequency and voltage of the power supplied to the motor, enabling significant energy savings on variable-load applications like pumps and fans compared to constant-speed operation with mechanical throttling.
VT — Voltage Transformer
An alternate term for a potential transformer (PT) — an instrument transformer that steps down system voltage for metering and relay inputs; the terms VT and PT are used interchangeably depending on regional convention.

W — Z

WPT — Wireless Power Transfer
Technology that transfers electrical energy across an air gap using magnetic or electromagnetic coupling rather than a direct conductive connection, used in applications from EV charging pads to implantable medical devices.
XFMR — Transformer
Common engineering-drawing shorthand for "transformer" — a static device that transfers electrical energy between circuits at different voltage levels through electromagnetic induction, with no moving parts.
Z (impedance) — Transformer Impedance
A transformer nameplate value, expressed as a percentage, representing the voltage drop across the transformer's internal impedance at rated current, expressed as a percentage of rated voltage. Impedance directly limits available fault current on the secondary side and is a key input to short-circuit and protective coordination studies.