This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
In 2161, Commander Tucker connected two live plasma relays aboard Enterprise NX-01 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. (ENT: "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")
Lieutenant Farrell asked Lieutenant Commander Geordi La Forge in 2371 if she could begin a diagnostic on the USS Enterprise-D's port plasma relays, as she believed the generators were malfunctioning. La Forge agreed, but she was unable to begin the diagnostic as the Enterprise was attacked shortly thereafter by Lursa and B'Etor. (Star Trek Generations)
Around the same time, B'Elanna Torres was working on a plasma relay on the USS Voyager, however, she was taken over by the consciousness of Chakotay in which she was forced to eject the warp core. (VOY: "Cathexis")
While attempting to remove Borg technology from Voyager, Torres told Seven of Nine to begin removal of the technology from the plasma relays in Jefferies tube 13-Alpha, section 12. Torres herself had problems removing the technology, which disrupted the matter-antimatter reaction and kept the ship stuck at sublight speeds. (VOY: "The Gift")
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")
In 3189, several EPS relays of the USS Discovery were damaged when a plasma manifold ruptured as the ship crash landed onto a glacier on a planet. These relays were repaired by the time the ship left the planet. (DIS: "Far From Home")