Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

out-of-step
an abnormal condition when generators in a power system cannot operate in synchronism.
out-of-step relay
a protective relay that senses that a synchronous generator has pulled out of step, and is operating at a frequency different than the system frequency.
outage
(1) the percentage of time or area for which a communication system does not provide acceptable quality.

(2) loss of power from all or part of a power system.
outage inferencing
the act of identifying the probable location of an outage based on information received from customer trouble calls and power monitoring units.
output impedance
the ratio of the drop in voltage to the current drawn is known as the output impedance of the electric source and is measured in units of ohms.
output power
(1) the difference in the power available under perfectly matched conditions and the reflected power taking the output return loss into account, expressed in watts.

(2) in lasers, the useful output from a laser oscillator.
over-compounded
a compound DC generator in which the terminal voltage increases as load current increases. Extra turns are added in the series winding to generate the additional voltage after compensating for the armature voltage drop and the armature resistance drop.
overcurrent
(1) current in a circuit that exceeds a preset limit.

(2) motor current magnitude in the normal circuit path exceeding the full-load current.
overcurrent protection
(1) the act of protecting electrical and electronic devices or circuits from a dangerous amount of input current.

(2) the effect of a device operative on excessive current.
overcurrent relay
a protective relay that operates when fed a current larger than its minimum pick-up value.
overexcited
the condition where the field winding current is greater than a rated value that produces the rated MMF in the armature. With a synchronous or a DC machine, the excitation current is a direct current in the field windings.

If a machine is overexcited, the excess MMF must be counterbalanced in the armature. In the case of a DC motor, the overexcitation is counterbalanced by the increase of armature current, which is translated by the increase of both torque and speed. In the case of synchronous motor, a leading component of the armature current is present and the machine operates at a leading power factor.
overlap angle

See commutation angle
overload heater
a term used to describe the thermal sensors that detect motor overload currents. Usually located on the motor starter, the heaters cause the overload relay to operate.
overload protection
a protective device which opens the circuit to a piece of electrical equipment or power line in the event of current exceeding the upper design limit.
overload relay
a device designed to detect and interrupt motor overload conditions.

Motor overload relays may be actuated by thermal (temperature), magnetic (current), or electronic (voltage and current) sensors.
overvoltage
a voltage having a value larger than the nominal voltage for a duration greater than 1 minute.
overvoltage relay
a protective relay that operates on overvoltage.
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