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Ampere - The unit by which the flow of current through a conductor is measured.

Arc - A discharge of electricity through a gas, such as air.

Attachment Cap - See Plug.

Circuit - The path along which electric current flows from start to finish is called a circuit. The circuit includes the generator or battery which starts the current, the wires, and any electrical device that the current operates. If any part of the circuit is removed, the current cannot flow. The circuit is then broken or open. Because electric current seeks to complete its circuit, it will travel along any path that is presented (path of least resistance), which is why humans are at risk of electrocution when they handle damaged tools or cords, or contact un-insulated wires. In effect, they become part of the circuit.

Circuit breaker - A protective device which automatically opens, or trips, a circuit, without damage to itself, when the current exceeds a predetermined level.

Conductor - A substance or body that allows a current of electricity to pass continuously along it. Metals, such as copper or aluminum, are good conductors. In a circuit, current-carrying wires are termed "conductors", as in a flexible cord.

Current - The flow of electrons through a conductor, measured in amperes (amps). If the current flows back and forth through a conductor, it is called alternating current (AC). If the current flows in one direction only, as in a car battery, it is called direct current (DC). AC is most widely used because it is possible to increase ("step up") or decrease ("step down") the current through a transformer. For example, when current from an overhead power line is run through a pole-mounted transformer, it can be stepped down to normal household current. Also, alternating current can travel enormous distances with little loss of voltage, or power.

Cycle - When alternating current flows back and forth through a conductor, it is said to cycle. In each cycle, the electrons flow first in one direction, then the other. In the United States, the normal rate for power transmission is 60 cycles per second, or 60 Hertz (Hz).

De-energize - To free from any electric connection and/or electric charge.

Electricity - The flow of an atom's electrons through a conductor.

Electrode - A conductor used to establish electrical contact with a nonmetallic part of a circuit.

Energize - To direct electric current through a conductor. Power lines and wires can be intentionally energized (or de-energized) to carry current to an electrical device. But conductive surfaces which are unintentionally energized, like the metal case of a tool, the metal housing of a circuit box, or a metal object such as an aluminum ladder, present a danger of electrocution.

Fault - An insulation failure that exposes electrified conductors, causing current to leak and possibly resulting in electric shock.

Fuse - A protective device which allows a piece of metal to become part of a circuit. The metal melts under heat created by excessive current, thereby interrupting the circuit and preventing the flow of electricity from exceeding the circuit's current-carrying capacity.

GFCI (Ground-fault circuit interrupter) - A device that detects an insulation failure by comparing the amount of current flowing to electrical equipment with the amount of current returning from the equipment. Whenever the difference is greater than 5 milliamps, the GFCI trips and thereby interrupts the flow of electricity.

Ground - A conducting connection, intentional or unintentional, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

Ground-fault - A fault, or insulation failure, in the wire used to create a path to ground.

Grounding - To prevent the buildup of hazardous voltages in a circuit by creating a low-resistance path to earth or some other ground plane.

Guarding - Placement of live parts of electrical equipment where they cannot accidentally be contacted, such as in a vault, behind a shield, or on a raised platform, to which only qualified persons have access.

Impedance - Opposition to the flow of alternating (AC) electric current. See Resistance.

Insulation - Non-conductive materials used to cover or surround a conductor, permitting it to be handled without danger of electric shock.

Insulator - Any material, such as glass or rubber, that prevents the flow of electric current.

Kilowatt - One thousand watts.

Lockout - To lock a switch in the "off" position by means of a padlock, or to lock electrified equipment behind a locked door, to which only qualified persons have the key.

Low-Impedance - Low resistance to A/C current.

Milliampere - A unit of measurement equaling one thousandth (1/1000) of an ampere.

Ohm - The unit by which resistance to electrical current is measured. From Ohm's Law (Current=Voltage/Resistance, or in other words, Current=Voltage/Ohms), a mathematical expression of the relationship between these three elements.

Overcurrent - Any current in excess of the rated capacity of equipment or of a conductor.

Phase - In AC power systems, load current is drawn from a voltage source which typically takes the form of a sine wave. Ideally, the current drawn by the loads in the system is also a sine wave. With a simple, resistive load such as a light bulb, the current sine wave is always aligned with the voltage sine wave. This is called single-phase. A single-phase power system normally uses three wires, called hot, neutral, and ground, and the voltage is typically 120/240. Most home and office outlets operate in this manner. With some loads, such as motors, and in high voltage systems, the current sine wave is purposely delayed and lags behind the voltage sine wave. The amount of this lag is expressed in degrees and is called a phase difference. A common example is three-phase power, where the system has three "hot" wires, each 120 degrees out of phase with each other.

Plug - A device to which the conductors of a cord are attached, which is used to connect to the conductors permanently attached to a receptacle.