DM Electrical Exam key terms

The relationship stating that voltage equals current multiplied by resistance (E = I × R). Knowing any two of these values lets you calculate the third, making it the basis for nearly all circuit calculations.

The relationship expressing electrical power as voltage times current (P = E × I), with variations using resistance. It is used to size conductors, breakers, and equipment for the load they must carry.

The opposition a material offers to the flow of electric current, measured in ohms. It depends on the material, its length, its cross-sectional area, and temperature.

The loss of voltage along a conductor caused by its resistance as current flows. Excessive drop reduces equipment performance and is limited by code and design practice on feeders and branch circuits.

The maximum current a conductor or device can carry continuously under stated conditions without exceeding its temperature rating. It is determined from tables and adjusted for ambient temperature and conductor count.

Reducing a conductor's allowable ampacity to account for conditions such as high ambient temperature or many current-carrying conductors bundled together. Adjusted ampacity must still protect the load and coordinate with overcurrent devices.

A device or system that interrupts current when it exceeds a safe level, protecting conductors and equipment from damage. Fuses and circuit breakers are the common forms.

A resettable automatic switch that opens a circuit when current exceeds its rating or when a fault occurs. It can be manually reset after clearing the problem.

A one-time protective device containing a metal element that melts and opens the circuit when current exceeds its rating. It must be replaced after operating.

Unintended current flowing from an energized conductor to ground through a path other than the intended circuit. It is dangerous because it can energize metal parts and cause shock or fire.

An unintended electrical discharge across a gap in damaged or loose wiring that produces high heat without necessarily tripping a standard breaker. Special breakers detect the signature and de-energize the circuit.

A device that compares current on the hot and neutral conductors and trips when a small imbalance indicates current leaking to ground. It protects people from shock in wet or damp locations.

A protective device that senses the current signature of arcing faults in branch-circuit wiring and opens the circuit. It is required in many dwelling branch circuits to reduce fire risk.

Connecting electrical equipment and systems to the earth or to a reference point to establish a stable voltage reference and provide a path for fault current. It works together with bonding to clear faults safely.

Intentionally joining metal parts to create a continuous, low-impedance path so they stay at the same potential. Proper bonding prevents dangerous voltage differences between equipment during faults.

The conductor that connects the non-current-carrying metal parts of equipment to the grounding system. It provides the low-impedance fault path needed for overcurrent devices to operate quickly.

A safety procedure that de-energizes equipment, then locks and tags the disconnect so it cannot be re-energized while work is in progress. It protects workers from unexpected energization.

A sudden, explosive release of energy caused by an electrical arc, producing intense heat, light, pressure, and molten metal. It is a major hazard when working on or near energized equipment.

Clothing and gear such as insulated gloves, arc-rated clothing, and face shields worn to reduce injury from shock, arc flash, and burns. Selection depends on the hazard and task.

Acceptable to the authority having jurisdiction, typically because the item is listed by a recognized testing laboratory or meets an equivalent standard. It signals that equipment meets recognized safety criteria.

Equipment or material that appears in a published list by a recognized testing organization and is found suitable for a specific purpose. Listing confirms the product was evaluated to a standard.

Equipment bearing a label, symbol, or other marking from a recognized testing organization indicating it complies with a specific standard. The label identifies the product and its intended use.

The organization or individual responsible for enforcing the code and approving materials, installations, and interpretations. Their rulings govern acceptance of the work.

Selecting a conductor's cross-sectional area so its ampacity and voltage drop meet the load and code requirements. Sizing must also coordinate with the overcurrent device protecting the circuit.

The maximum number and size of conductors, devices, and fittings permitted inside an outlet or junction box. It prevents overcrowding that damages insulation and causes overheating.

An enclosure where conductors are spliced or connected to devices, protecting the connections and providing access. Code limits its fill and requires it to remain accessible.

A channel such as conduit or tubing designed to hold and protect conductors between enclosures. Fill limits and support requirements keep conductors from overheating or being damaged.

The percentage of a raceway's cross-sectional area occupied by conductors. Limits prevent damage during pulling and allow heat to dissipate from the conductors.

A method of totaling connected loads, applying demand factors, and sizing service and feeder equipment to supply them safely. It ensures capacity matches expected demand rather than a simple sum.

A ratio applied to connected loads to account for the fact that not all loads operate at full value at the same time. It allows smaller, more economical service and feeder sizing.

The conductors and equipment that bring utility power from the service point to the building's distribution equipment. It includes the service drop or lateral, meter, and main disconnect.

The device that disconnects all power from the service to the building in one operation. It provides a means to de-energize the system for emergencies and maintenance.

An unintended low-resistance connection between energized conductors or between a conductor and ground, causing very high current. Protective devices must interrupt it quickly to prevent damage.

The presence of a complete, unbroken path for current through a conductor or circuit. It is verified with a meter to confirm connections before energizing equipment.

A test instrument that measures voltage, current, and resistance. Choosing the correct function and range is essential to get accurate readings and avoid damaging the meter.

A meter with a jaw that encircles a conductor to measure current without breaking the circuit. It is useful for checking load current on feeders and branch circuits.

The ability of conductor insulation to resist current leakage, measured with a high-voltage tester. Low readings indicate moisture, damage, or deterioration that can lead to faults.

A test instrument that applies a high DC voltage to measure insulation resistance in ohms. It is used to check motors, cables, and equipment before energizing.