A positronic relay was technology on board Starfleet vessels in the 24th century, used to power some parts of a ship. It was also used in Soong-type androids.
Sometime in the 2350s, Kathryn Janeway knocked out power to six decks on the USS Al-Batani by mis-aligning the positronic relays on that ship. She later told the story to her first officer on the USS Voyager, Commander Chakotay. (VOY: "Shattered")
In 2371, Data experienced an overload in his positronic relay while on the Amargosa observatory due to his emotion chip being recently installed. (Star Trek Generations)