Electrical Engineering Terms S

Sealtight. The trade name for liquid-tight flexible metallic conduit.


A name was given to a group of materials or to compounds of these materials that are neither good insulators nor good conductors of electricity. Because of their four valence electrons (free electrons in the outer shell of the atom), their electrical properties are said to stand midway between conductors (which contain one or two valence electrons) and insulators (which contain seven or eight valence electrons).

Static Electricity.

This type of electricity is induced and said to reside as a charge upon the surface of a dielectric or another insulating body. As distinguished from dynamic or current electricity (produced electricity ― dynamos), this type of electricity is an imbalance of electrons created by the friction of two or more bodies, substances, or materials rubbed against each other. The friction strips free electrons from one surface ─ causing a positive charge on that surface, as it deposits the free electrons on the other surface ─ causing a negative charge on that surface.

Static Ground Wire.

The chain of transformers in the electric-utility transmission/ distribution system is connected in a 3-phase AC, 3-wire, delta primary, and a 3-phase AC, 4-wire, wye secondary. The static ground wire is a 4th, non-fused, non-switched, non-circuit ground wire run from the common connection of the three single-phase AC transformer windings with the grounding-electrode conductor(s) in each transformer secondary, and as a static ground along the top of the steel towers to the next transformer location with the three ungrounded fused conductors which interconnect the two transformers.

Stray Currents / Earth Currents.

Electrical current flowing through the ground due to a natural difference of pressure (static electricity).

Scan cycle

The first step in a PLC program execution that reads input status, executes ladder logic and updates the output status of a PLC.

Service Factor (SF).

Normally abbreviated to the acronym SF, this is a qualifying term found on the nameplate of an electric motor. The number of this term is used in two different ways: (1) The motor is so constructed that operation in overload will not overheat or otherwise damage the motor: A number greater than 1 indicates that the motor can be allowed to develop more than its rated or nameplate HP (be operated to the indicated overload percentage) without causing undue deterioration of the insulation. (2) The number by which the horsepower rating of an electric motor is multiplied to determine the maximum safe load the motor may be expected to carry continuously at its rated voltage and frequency.

Short-Time Duty.

If an electric motor is nameplate-rated with this type of duty cycle, it is rated to operate at a substantially constant load for a short and definite, specified time.

Single-Phase Alternating Current (AC) System / Single-Phase AC Power. This term describes an electrical power transmission or distribution system consisting of two or three intentionally interrelated conductors that are generated by or derived from a single 2-wire AC source (as compared to a 3-phase AC electrical power transmission or distribution system in which three or four distribution conductors are purposely interrelated.)


As used with AC electric motors: the difference between the synchronous speed of the rotating electromagnetic field created within the confines of the stator assembly by the branch-circuit power supply and the actual running or turning speed of the rotor. The difference in the two speeds is the power supply (voltage source) for the rotor windings.

Squirrel-Cage Motor.

This term describes an asynchronous AC induction motor in which power from the branch-circuit is applied only to the stator windings. The power in the squirrel-cage rotor circuit is induced by the rotating magnetic field created in the stator circuit.

Squirrel-Cage Rotor.

This term describes an electric-motor rotor constructed of a series of rods or bars uniformly spaced around and extended the length of the shaft of the rotor. The rods are attached to the rotor shaft by short-circuiting ring assemblies on each end. The assembly resembles the wheel found in a squirrel cage. Submersed in a cylindrical mold that is filled with aluminum, the cooled aluminum forms the armature winding of this type rotor.

Synchronous. This term can be described in two different ways: (1) As applied to electrical power distribution systems and utilization equipment: two or more operations, occurrences, systems, or equipment that is simultaneous in action and in time (in phase). (2) As used in electrical/electronic theory and circuit analysis: a waveform that has some timing relationship (in sync with) with another waveform.

Synchronous Motor.

This type of electric motor can be described in two different ways: (1) A type of electric motor that maintains a constant speed as long as the speed of the generator supplying it remains constant. (2) A special type of AC motor that contains a wire-wound armature rotor assembly, which can be separately excited by an external DC supply through slip rings on the rotor shaft. The motor is started as a 3-phase AC induction motor through an amortisseur winding, and once running, DC is applied to the electromagnet mounted on the rotor. The fixed electromagnetic field of the rotor assembly locks in and rotates in unison or in step (synchronized) with the phase of the alternating current which operates it.

Synchronous Speed.

In a 3-phase AC motor, the rotating speed of the electromagnetic field created by the windings in the stator assembly when electrical power is applied. Governed by both the number of poles per phase in the construction of the stator assembly and the frequency of the AC power supply, the synchronous speed of a 2-pole motor operating at 60 Hz is 3600 rpms. In comparison, the synchronous speed of the same 2-pole motor operates at 50 Hz would only be 3000 rpms.

Supervised Location.

As used with transformers, a location where only qualified persons will monitor and service the transformer installation.

3-Phase AC Transformer.

The respective circuit windings of three single-phase AC transformers mounted on a common iron frame (iron core).

3-Phase Delta Connection.

The apparent series-connection of the three single-phase AC sources or loads in a 3-phase AC electrical power distribution system in which the terminal connections (winding-end connections) within an alternator, transformer, motor, or heating bank are triangular like the Greek letter delta (D). When wired in this configuration, the three single-phase AC waveforms are electrically 1200 apart; the 3-phase AC line voltage (phase-to-phase voltage) and the single-phase AC winding voltage are equal, and the 3-phase AC line current is the Ö3 (1.732) times greater than the single-phase winding current.

3-Phase Wye Connection.

The apparent parallel-connection of the three single-phase AC sources or loads in a 3-phase AC electrical power distribution system in which the terminal connections (one end of the winding-end connections) within an alternator, transformer, motor, or heating bank have a common connection that forms the alphabet letter Y. When wired in this configuration: the three single-phase AC waveforms are electrically 1200 apart; the 3-phase AC line current (phase-to-phase current) and the single-phase AC winding current are equal, and the 3-phase AC line voltage is the Ö3 (1.732) times greater than the single-phase AC winding voltage.

3-Phase Alternating Current (AC) System / 3-Phase AC Power.

This term refers to the electrical power generation, transformation, transmission, or utilization of three single-phase AC, 2-wire sources on an interconnected 3-wire or 4-wire system. The three single-phase alternating currents are normally of equal frequency and amplitude (when supplying balanced loads), but differ in phase from each other by one-third of a period (one-third the length of an AC cycle — 1200).

Scientific notation.     

A classification for representing a large or small number as a number between 1 and 10 times a power of ten.


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