A plasma relay is used to distribute plasma throughout starships. They are used on Vulcan, Earth Starfleet, Federation and Dominion ships.
In 2161, Commander Charles Tucker connected two live plasma relays aboard the NX-01 Enterprise in an attempt to prevent a group of alien trespassers from killing Captain Jonathan Archer and possibly other crewmembers. The resulting explosion incapacitated the aliens, but the ignited plasma from the relay thermalized Tucker's lungs and he died soon after. (TNG: "These Are the Voyages...")
When a plasma relay was damaged it produced radiation which could be detected by another starship. If the replicator power converter was crossed with the plasma manifold there was a great chance the plasma relay would explode, although this did not happen always. (DS9: "Defiant")
- When this action was done on the USS Defiant, and the plasma relay did not exploded, it would render it's Cloaking device useless for at least thirty minutes.
In 2375, Bronok and four other members of the Cardassian Rebellion were ordered by Kira Nerys to plant a bomb in a Jem'Hadar attack ship's plasma relay, but the Cardassians ignored orders and placed it in the intercooler matrix. (DS9: "Tacking Into the Wind")