(covers information from several alternate timelines)
The primary EPS relays were integral components to Constitution-class ships.
In 2154 of the mirror universe, Dr. Phlox disengaged the primary EPS relays of the USS Defiant, which had been transported from the "prime" universe and back in time from the year 2268. Disengaging the relays had the effect of disabling the Defiant's warp drive and shutting down main power. When Commander Tucker discovered Phlox's act of sabotage, he disabled Phlox and re-engaged the relays, restoring the ship to full power. (ENT: "In a Mirror, Darkly, Part II")