Understage Isolators
The understage isolators, more commonly called "Stage Power", are mains power isolators located in the plant room under the stage. These device take in mains power from the National Grid and outputs two feeds used in the venue.
There are two isolators, both providing 415V, 100A Three Phase (3P+N+E) power through 125A Ceeform protruding from the floor in the stage left wing. One provides power to the dimmer system, the other to the distro box, which splits into the sound phase and the rubber box phase.
Turning Understage Isolators on or off are the first and last things you'll do in the venue each time you're in it. Unsurprisingly this is one of the first things you'll learn when joining Ents.