Electrical Safety Glossary

These are the core terms tested across ArcReady's Safety and Theory question banks, defined in one place for quick reference. For the full explanation behind each concept, see the complete Study Guide.

Ammeter

A meter used to measure current, connected in series with the circuit so the current to be measured flows through it. A clamp-on ammeter measures current by sensing the magnetic field around a single conductor without breaking the circuit.

Anode

The terminal of a diode that current flows into during forward bias. Represented on a schematic as the back (flat side) of the triangle symbol.

Approach Boundary

One of the defined distances from exposed energized conductors that dictate what protection and qualification a worker needs — see Limited Approach Boundary, Restricted Approach Boundary, and Arc Flash Boundary.

Arc Blast

The pressure and shock wave produced by an arc flash event, distinct from the thermal (burn) hazard. Both are dangerous, but PPE arc ratings address burn severity specifically.

Arc Flash

A plasma explosion caused by current jumping through ionized air between conductors, or from a conductor to ground. Temperatures can reach 35,000°F — about four times the surface temperature of the sun — in microseconds.

Arc Flash Boundary

The distance from an arc source at which an unprotected worker would receive a second-degree burn (the NFPA 70E threshold is 1.2 cal/cm²). Anyone working inside this boundary must wear arc-rated PPE.

Arc Flash Label

A label required by NFPA 70E on equipment that has undergone a hazard analysis, stating the equipment's incident energy, arc flash boundary, minimum PPE arc rating, and working distance.

Arc Rating (ATPV / EBT)

The maximum incident energy a piece of PPE can absorb before there is a 50% probability the wearer receives a second-degree burn. PPE must be rated at or above the calculated incident energy for the task.

Bonding

The practice of interconnecting metallic parts of an electrical system so no voltage difference can exist between them, preventing sparking and shock from potential differences. Distinct from grounding, which connects to earth.

Cathode

The terminal of a diode marked with a painted band, representing the direction current is blocked in reverse bias. Shown on a schematic as the vertical bar at the tip of the diode triangle.

Clamp Meter

A current-measuring tool that clamps around a single conductor and reads current from its magnetic field, without needing to break the circuit. Clamping around all conductors of a balanced circuit reads zero because the fields cancel.

Coil

The schematic symbol (a circle) for an electromagnetic coil inside a relay, contactor, or solenoid. When energized, it changes the state of its associated contacts.

Contactor / Motor Starter (MS)

An electrically controlled switch that uses a coil to close heavy-duty power contacts, commonly used to start and stop motors from a low-voltage control circuit.

Diode Test Mode

A digital multimeter setting that measures the actual forward voltage drop across a diode, generally the most accurate way to test a diode compared to standard resistance mode.

Electrically Safe Work Condition

A state in which a circuit has been de-energized, locked, tagged, tested for absence of voltage, and grounded where appropriate — the required condition before most electrical work under NFPA 70E.

Energized Work Permit

A required, dual-signed document (qualified worker and management representative) that justifies and authorizes performing work on equipment that cannot be put into an electrically safe work condition.

Fuse

A protective device, shown on a schematic as a small rectangle or S-curve, that interrupts current flow when it exceeds a rated value, protecting the circuit and equipment downstream.

GFCI (Ground Fault Circuit Interrupter)

A device that monitors for a current imbalance between hot and neutral conductors and trips in about 25 milliseconds once leakage current reaches 4–6 mA — well below the threshold for cardiac fibrillation.

Ground

The reference point at 0 volts, or a safety connection to earth. Shown on schematics as progressively shorter horizontal lines or a downward triangle.

Grounding

The connection of electrical system components to the earth, providing a return path for fault current and establishing a stable voltage reference. Complementary to, but distinct from, bonding.

Hazard/Risk Category (HRC) / PPE Category

NFPA 70E's four-tier system (Category 0–4) that specifies the minimum arc rating and PPE items required for a given incident energy range.

Incident Energy

The amount of thermal energy, measured in calories per square centimeter (cal/cm²), that a worker's body could absorb at a defined working distance during an arc flash. Drives all PPE category decisions.

Insulating Gloves

Rubber gloves rated for a specific voltage class, used as primary shock protection. Must be electrically tested every 6 months (ASTM D120) and are typically worn with leather protector gloves (ASTM F696).

Let-Go Threshold

The current level (roughly 10–20 mA) at which muscle contraction can prevent a shock victim from voluntarily releasing an energized object.

Limited Approach Boundary

The outermost approach boundary; the closest distance an unqualified person may approach exposed energized conductors, and only when escorted by a qualified person.

Line Voltage (V-line)

In a three-phase system, the voltage measured between any two phase conductors.

Lockout/Tagout/Tryout (LOTOTO)

The eight-step procedure for achieving an electrically safe work condition: identify energy sources, notify affected employees, identify disconnects, don PPE, de-energize, lock and tag, release stored energy, then verify absence of voltage ("tryout").

Node (schematic)

A solid dot on a schematic marking a point where two or more wires are electrically connected. Crossing lines without a dot are not connected.

Normally Closed (NC) Contact

A switch contact that is closed (conducting) at rest and opens when its associated coil or actuator is energized.

Normally Open (NO) Contact

A switch contact that is open (non-conducting) at rest and closes when its associated coil or actuator is energized.

Ohm's Law

The relationship E = I × R between voltage (E, volts), current (I, amperes), and resistance (R, ohms) — the foundation of every circuit calculation.

Ohmmeter

A meter that supplies its own small current to measure resistance. Must only be used on de-energized circuits — using one on a live circuit gives an incorrect reading and can damage the meter or injure the user.

OL (Overload / Overflow Reading)

On a digital multimeter in resistance mode, an OL reading means the measured resistance exceeds the meter's range — on a circuit path, this indicates an open circuit.

Overload Relay (OL)

A thermal or electronic protective device wired in series with a motor that opens a control-circuit contact when current exceeds a trip setting, protecting the motor from overheating.

Phase Voltage (V-phase)

In a three-phase system, the voltage measured between one phase conductor and the neutral point.

Qualified Person

Under NFPA 70E and OSHA, someone who has demonstrated the skills and knowledge related to the construction and operation of specific electrical equipment and has been trained to recognize and avoid its hazards. Qualification is competency-based and task-specific, not a job title.

Restricted Approach Boundary

An approach boundary closer than the Limited boundary, where shock risk becomes significant. Only qualified persons wearing appropriate shock PPE and working under a written plan may cross it.

Step Potential

A voltage difference between two points on the ground near a fault, which can drive current foot-to-foot through a person standing nearby — a hazard distinct from direct contact shock.

Three-Phase Power

An AC power system using three sinusoidal voltages spaced 120° apart, producing constant (non-pulsating) power delivery — the standard for industrial and commercial motor loads.

Turns Ratio

The ratio of primary to secondary winding turns in a transformer (N1/N2), which determines the voltage step-up or step-down ratio (V1/V2 = N1/N2).

Unqualified Person

Anyone who has not been trained and demonstrated competency to recognize and avoid the electrical hazards of a specific task or equipment. Unqualified persons may only approach as far as the Limited Approach Boundary, and only when escorted.

Ventricular Fibrillation

A disruption of the heart's electrical rhythm, typically occurring at roughly 100–200 mA of current through the body, in which the heart quivers rather than pumps blood — the primary lethal mechanism in electrical shock.

Voltmeter

A meter connected in parallel across a component or between two nodes to measure voltage. High internal impedance means it draws negligible current and does not disturb the circuit being measured.

Working Distance

The distance between a worker's face/chest and a prospective arc source, used as the reference point for an incident energy calculation. Common values are 18, 24, and 36 inches.