Power relay

Power relays were components located behind the walls in corridors aboard starships and starbases.

Trident scanners are primarily used for repairing power relays aboard Federation starships during the 23rd century.

Power relays were labeled GNDN on the USS Enterprise (NCC-1701). 

Background
In Star Trek: The Original Series, GNDN was an "in-joke" produced by the set designers of the series.

According to an interview with John Jefferies, one of the set designers, stated in the TOS Season 2 DVD special features:
 * "The plumbing and pipes in the Enterprise all had color codes and some form of nomenclature on it. That would be a series of letters and numbers and something that looked like you could tell what this pipe was on this side of the wall, as it came out of the wall. And GNDN with a number behind it and then a color code became very common throughout the Enterprise. GNDN means 'Goes nowhere, does nothing'."

This became a well-known enough prank that when the DS9 crew re-built some of the old Enterprise sets for the 30th anniversary tribute, "Trials and Tribble-ations", they made sure to include a GNDN pipe. (Check the labeling on the white pipe in the lower left corner of the photo.)