A system is said to be transient or in a transient state when a process variable or variables have been changed and the system has not yet reached a steady state.The time taken for the circuit to change from one steady state to another steady state is called the transient time.

A transient response is the response of a system to a change from an equilibrium or a steady state.

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

A transient event is a short-lived burst of energy in a system caused by a sudden change of state.

Passive mechanical systems designed to depressurize the transformer a few milliseconds after the occurrence of an electrical fault. Those system prevent any fire by avoiding transformer tank rupture and subsequent fire damages.

See lifecycle

An overhead electrical line running from a utility pole, to a customer's building or other premises.

Series circuits are sometimes referred to as current-coupled or daisy chain-coupled.

The amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units which are so calculated as to be proportional to the total input and to the mass processed.

In nuclear physics, separation energy is the energy needed to remove one nucleon (or other specified particle or particles) from an atomic nucleus.

IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".

A sensor is a device, module, machine, or subsystem whose purpose is to detect events or changes in its environment and send the information to other electronics.

Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic variables of the body or system, such as volume or pressure.

Semiheavy water is the result of replacing one of the protium in light water with deuterium.

Semiconductors are materials which have a conductivity between conductors and nonconductors or insulators. Used to control current in circuits.

A semi-submersible platform is a specialised marine vessel used in offshore roles including as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes.

A semi-arid climate, semi-desert climate, or steppe climate is the climate of a region that receives precipitation below potential evapotranspiration, but not as low as a desert climate.

Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes or any external circuit. Self-discharge decreases the shelf life of batteries and causes them to have less than a full charge when actually put to use.

In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency.

An emissions cleaning process whereby NOx is reduced by a catalyst, combined with a reagent that is either an aqueous solution of urea or ammonia.