S

S - Abbreviation for second(s)

Sag - A momentary reduction in the voltage for a duration from a half-cycle to a few seconds. Also referred to as a dip.

Screen - To surround or encase a circuit with metal in order to reduce the effect of electric or magnetic fields. A screened cable is one which incorporates an outer layer of an earthed metal skin. To be effective for transient overvoltage protection purposes, the screen must be a good conductor and earthed at both ends.

Self-inductance - See inductance

Semiconductor - Name applied to a group of materials whose ability to conduct electric charge is greater than that of an insulator, but less than that of a conductor. Semiconductors are used in a wide variety of solid state components (such as transistors, integrated circuits, and diodes) and hence in most electronic systems. Where a semiconductor is connected to metal or to another semiconductor of different conductivity, a junction is formed. A junction is forward biased when current is flowing through it and reverse biased when it is blocking the flow of current. Bias therefore refers to the direction of the applied voltage.

Shield - See screen

Short circuit - A deliberate or (more usually) accidental low resistance connection, on an electrical circuit. Its effect is to equal voltages at two points and allow current to flow. If the short circuit is the result of a fault, it will usually cause problems.

Shunt - Alternative term for parallel

Sine wave - The natural waveform of voltage generated by a coil of wire revolving in a uniform magnetic field. It is therefore the waveform an AC main power supply approximate to.

Single phase - Power supply incorporating one live conductor. See phase

Spike - Alternative term for transient overvoltage

Star supply - Three phase power supply incorporating a neutral conductor

Suppression level - Voltage at which transient overvoltages are controlled or suppressed

Surge - Common, but ambiguous alternative for transient overvoltages

Switchfuse - Fuse incorporating a switch or isolator, thereby enabling the power supply to devices fed from it to be turned off.

Switching transient - Transient overvoltage caused by the operation of large inductive or capacitive loads.

Symmetrical mode - See differential mode