A graphic of the secondary plasma system of a Galaxy-class starship
The electro-plasma system (EPS for short, or more specifically the electro-plasma distribution network, the term preferred by engineers (citation needed • edit)) was the primary form of energy distribution on starships.
The technology was based on storing energy in a highly energetic plasma, "electro-plasma," and distributing it throughout the ship via plasma conduits, called EPS conduits. The system of conduits was also referred to as the plasma grid. EPS conduits usually started at a matter-antimatter reaction assembly, also called the warp core, where matter was converted to energy, and extended to all areas of a ship. Plasma conduit power levels were regulated by plasma coolant ducts, and monitored by plasma conversion sensors.
EPS manifold circa 2151 technology
Very large conduits extended from the warp core, through the nacelle struts, and into the nacelles to facilitate the massive power transfer to the warp coils needed to create a warp field. A plasma stream was directed by plasma injectors at the warp coils.
In addition to distributing power to the warp nacelles, various EPS taps were placed on the conduits throughout the ship to enable other systems to access electro-plasma wherever it was needed. From the EPS taps, the energy was distributed through conventional electricity; however, this conversion often occurred deep inside the components of a subsystem. Some systems used plasma distribution manifolds (or plasma manifolds, for short) to manage the power conversion level.
Electro-plasma could also be bled off into warp plasma canisters or a plasma infuser for mobile use as an energy source.
The USS Enterprise-D had a primary plasma system and a backup secondary plasma system. Additional systems were the life support power flow and the auxiliary replicator system. (TNG: "The Mind's Eye")
A massive EPS explosion in cargo bay 4 was detected by internal sensors in 2368; it later turned out that it was just a malfunction caused by tampering with the conduit by solanogen-based lifeforms. (TNG: "Schisms")
Engineer in command Lieutenant Paul Porter assumed that a problem within the EPS conduits caused the environmental control's problems when the temperature in main engineering increased. (Star Trek: First Contact)
During the Second Battle of Chin'toka, after the USS Defiant was crippled by a previously unencountered energy weapon deployed by the Dominion's Breen allies, repeated torpedo strikes from Breen interceptors inflicted multiple hull breaches and caused eight separate plasma coolant leaks from the EPS system, toxifying the air aboard the Defiant and ultimately forcing Captain Benjamin Sisko to order the Defiant's crew to abandon ship. (DS9: "The Changing Face of Evil")