At the main supply find the main switch that controls the supply to that DB. Place a padlock through the switch where possible, to lock it in the off. A distribution board, also known as a DB box, is like the central hub of an electrical system. It contains multiple circuit breakers and connects various electrical circuits to ensure the safe flow of electricity throughout the building. ac power lines for power supplies and I/O circuits. high-power digital dc I/O lines — to connect dc I/O modules rated for high power or with input circuits with long time-constant filters for high noise rejection. This is necessary because it is not practical to run dozens, or even hundreds, of different electrical lines directly into the.