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Circuit Breaker Terms and Definitions

Alternating current (AC): Current that reverses its direction of flow at regular intervals.

Arc: A sustained discharge of electricity across a gap in a circuit or between electrodes, usually accompanied by the electrodes (contacts) being vaporized and/or melted by the extreme heat of the arc.

Arc chute: A structure that contains arc dividers. Also known as an arc extinguisher.

Arc extinguisher: See arc chute.

Arcing contact: The contact of a switching device on which an arc is drawn after the main contacts have parted.

Backboard assembly: The section of a circuit breaker that supports stationary contacts, the primary line, and load disconnects.

Bell alarm and lockout: A device installed on LVPCBs to send an alarm and/or to lock out the circuit breaker when it trips.

Buffer assembly: A mechanical component that absorbs the shock of a mechanism when a circuit breaker opens and prevents the contact assemblies from overdriving when a circuit breaker closes.

Capacitance charging current (IC): Current that flows upon the application of DC voltage to charge the capacitance of the electrical system under test.

Circuit breaker: An electrical device that opens and closes a circuit by non-automatic means and automatically opens a circuit when a predetermined current overload is reached, without damage to itself.

Contact gap: The distance between the two closest points of the moving and stationary contacts in a circuit breaker.

Contact sequence: The difference in contact spacing as arcing (moving) contacts make contact with stationary contacts in a circuit breaker.

Contact wipe: The amount of over-travel a contact assembly has once it initially makes contact.

current sensor: A sensor that reduces load current to a smaller value that is sent to the solid-state logic circuit where, if the value of current and time duration exceed its set points, will send a trip signal to a trip coil.

Dielectric absorption current (IDA): The polarizing current that is drawn by the insulation system (dielectric) to align the dipole molecules within the insulation.

Dipole molecule: A molecule that is affected by the magnetic field created by current flow through its insulation.

Direct current (DC): Current that flows in only one direction.

Driving pawl: A circuit breaker component that advances the drive gear to charge closing springs.

Energized electrical work permit (EEWP): A document issued by an employer to allow specific work to be performed on energized electrical equipment with various limitations.

Fifth wheel: A device with a swiveling, weight-bearing connection used to move heavy loads.

Floor tripper: An interlock that protrudes from the bottom of the circuit breaker and causes the circuit breaker to trip open and the closing springs to discharge when a circuit breaker is racked in or out of its cubicle.

Frame rating: The continuous current rating of all current-carrying parts of a low-voltage circuit breaker, excluding its OCPD.

Fuse: An electrical overcurrent protective device with a fusible portion that is heated and broken by the passage of excessive current.

Guard circuit: A function on a megohmmeter used to eliminate unwanted return current from test measurements.

Hipot tester: A test instrument that measures insulation resistance by measuring leakage current.

Incident energy: The heat from an electrical arc; proportional to time.

Institute of Electrical and Electronics Engineers (IEEE): A professional association that is dedicated to advancing technological innovation and excellence of electrical and electronic equipment.

Insulated-case circuit breaker (ICCB): A circuit breaker that is similar in construction to an MCCB but typically uses an electronic or digital OCPD and has much higher interrupting ratings.

Interphase barrier: An electrical safety device that prevents contact with energized electrical components and protects against spark scatter.

Interrupting rating: See short-circuit current rating.

Latching pawl: A circuit breaker component that locks the drive gear in place when charging the closing springs.

Leakage current (IL): Resistive current that flows through insulation and is the variable that is measured with an insulation resistance test instrument.

Lower band time limit: The time that represents the minimum operating time on a time-current characteristic curve.

Maintenance closing: See slow closing.

Maximum total clearing time: The time required to interrupt an arc and is considered to be from when the arc starts until it is completely extinguished.

Maximum total clearing time line: The line on an INST curve that represents the maximum amount of time for circuit breaker contacts to open and extinguish an arc.

Medium-voltage power circuit breaker (MVPCB): A circuit breaker that operates with voltages ranging from 2300 V to 69,000 V.

Megavolt ampere (MVA) rating: The rated voltage times the rated interrupting current of a circuit breaker.

Megohmmeter: A high-resistance ohmmeter used to measure insulation deterioration on various conductors by measuring high-resistance values during high-voltage test conditions.

Micro-ohmmeter: A device used to measure the resistance of an electrical conductor down to the micro-ohm range.

Molded-case circuit breaker (MCCB): A circuit breaker that uses a molded case to house and support its current-carrying components as well as to be a part of the insulation system.

Nameplate: A metal plate attached to a device, such as a circuit breaker, that lists its technical specifications.

Organic lubricant: A lubricant composed of emulsifiers and oil.

Overcurrent: Electrical current in excess of the equipment limit, total amperage load of a circuit, or conductor or equipment rating.

Overcurrent protective device (OCPD): A circuit breaker, fuse, or switch with an element that disconnects or discontinues current flow when the amount of current exceeds the design load. Also known as a trip device.

Over travel: The distance a circuit breaker mechanism travels once the contacts touch and the mechanism latches.

Persistence: The amount of time vaporized metal remains a vapor between the contact faces in a medium-voltage circuit breaker.

Personal protective equipment (PPE): Clothing, glasses, gloves, hard hats, respirators, or other safety devices designed to protect workers against safety hazards in the work area and from injury.

Potentiometer: A variable-resistance electric device that divides voltage proportionally between two circuits.

Predictive maintenance (PdM): The monitoring of wear conditions and equipment operation characteristics for comparison against a predetermined tolerance to predict potential malfunctions or failures.

Preventive maintenance (PM): Scheduled work required to keep equipment in peak operating condition.

qualified person: A person who has special knowledge, training, and experience in the installation, programming, maintenance, and troubleshooting of electrical circuit breaker equipment and also has the safety skills and knowledge that allow him/her to perform the task safely.

Rated temperature rise: The temperature rise of an electrical device or component above ambient, or surrounding air, temperature.

Restrike: A resumption of current between the circuit breakers contacts if the circuit breaker fails to clear an arc.

Safety interlock device: A device used to ensure that a circuit breaker does not get racked while the circuit breaker is in the closed position.

Schematic diagram: A drawing that shows electrical system circuitry with symbols that depict electrical devices and lines representing conductors.

Secondary injection test: A test used only on solid-state and digital OCPDs in which test current is injected directly into the logic and timing circuits.

Serious violation: A safety violation in which there is a substantial probability that death or serious physical harm could result and the employer knew or should have known of the hazard.

Short circuit: An unintentional connection of two ungrounded conductors that have a potential difference between them.

Short-circuit current rating: The maximum short-circuit current a circuit breaker can safely clear without external damage. Also known as the interrupting rating.

Slow closing: A method of mechanically blocking closing springs from operating when a circuit breaker is manually closed. Also known as maintenance closing.

Spark scatter: The molten metal and debris that may be ejected from an arc chute when a circuit breaker interrupts a short circuit at or very near its rating.

Spreader bar: A rigging device that is suspended from a crane hook to support lifting lines at various locations.

Thermal imager: A device that detects heat patterns in the infrared-wavelength spectrum without making direct contact with the workpiece.

Thermal-magnetic OCPD: An OCPD that reacts to the heat created by the copper loss (I2R) when current passes through a conductor.

Treeing: Irreversible internal degradation caused by the formation of conductive carbonized pathways.

Trip device: See overcurrent protective device (OCPD).

Under voltage OCPD: A device that senses when the voltage of a circuit drops below a predetermined value.

Upper band time limit: The time that represents the maximum operating time on a time-current characteristic curve.

Willful violation: A safety violation committed with an intentional disregard of, or plain indifference to, the requirements of the Occupational Safety and Health Act and regulations.

Wipe: The additional compression of a preloaded spring that is used to apply force to the vacuum interrupter contacts and to provide opening kick-off force.

Zero crossover point: The point at which current alternates and begins to change direction.

About Ahmed Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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