Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

instantaneous-trip circuit breaker
essentially an inverse-time circuit breaker with the thermal element removed. It will only trip magnetically in response to short-circuit currents. Thus, it is often referred to by other names, such as magnetic circuit breaker and magnetic-only circuit breaker.
inverse-time circuit breaker
a circuit breaker in which the allowed current and time are inversely proportional. It contains a thermal element and a magnetic element in series. The thermal element is designed to trip as a result of heating over time in response to overload currents, while the magnetic element is designed to trip magnetically, with no intentional time delay, in response to short-circuit currents. Also called a thermal-magnetic circuit breaker.
magnetic circuit
the possible flux paths within a system consisting of a source of flux (electromagnets, permanent magnets), permeable flux carrying materials (steel, nickel) and non-flux carrying materials (aluminum, air).
molded case circuit breaker
a low voltage air circuit breaker that includes thermal and/or magnetic overcurrent sensing which directly trips the breaker. The molded case circuit breaker is nearly always manually closed, opened, and reset.
motor circuit
the three components of an electrical circuit are source, load, and interconnecting circuit conductors. A motor circuit is an electrical circuit designed to deliver power to a motor. It includes the over-current protective devices, controller, disconnect switch, circuit conductors, and the motor itself.
motor circuit protector (MCP)
a listed combination motor controller containing an adjustable instantaneous-trip circuit breaker and coordinated motor overload protection. MCPs can provide short-circuit and bolted ground-fault protection via the circuit breaker magnetic element, overload protection via the overload device, motor control, and disconnecting means all in one assembly.
motor control circuit
a circuit containing devices such as the start/stop switches, mainline coil, main-line sealing contacts, overload contacts, timers and timer contacts, limit switches, antiplugging devices, and anything else used to control devices in the motor circuit.
oil circuit breaker
a power circuit breaker that uses oil as an insulating and arc-clearing medium.
open-circuit test
a transformer test conducted by applying nominal voltage on the low voltage side while keeping the high voltage side open. By measuring the power in, current, and voltage, the magnetizing reactance of the transformer equivalent circuit can be determined.
residual current circuit breaker
European term for ground fault interrupter.
SF6 circuit breaker
a power circuit breaker where sulfur hexaflouride (SF6)gas is used as an insulating and arc clearing agent.
short circuit
a condition on the power system where energized conductors come in contact (or generate an arc by coming in close proximity) with each other or with ground, allowing (typically large) fault currents to flow.
short circuit admittance
the admittance into an N-port device when the remaining ports are terminated in short circuits. For port 1 of a 2-port device, it is the input admittance into port 1 when port 2 is shorted.
short circuit gain-bandwidth product
a measure of the frequency response capability of an electronic circuit. When applied to bipolar circuits, it is nominally the signal frequency at which the magnitude of the current
gain degrades to one.
short-circuit protection
the beneficial effect provided by an overcurrent device when it acts to interrupt short-circuit current.
short-circuit test
a transformer test conducted by placing a few percent of rated voltage on the voltage side while the low voltage winding is shorted. By measuring the voltage, current, and input power, it is possible to calculate the equivalent winding impedance for the transformer equivalent circuit.
subtransient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis subtransient open-circuit time constant.
transformed circuit
an original circuit with the currents, voltages, sources, and passive elements replaced by transformed equivalents.
transient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis transient open-circuit time constant.
transient short-circuit
time constant pass and stop bands are critical. A longer filter length generally implies that the filter can have a steeper transition band to a similarly shaped shorter length filter.
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