| 340 / | HD | COMMUNICATIONS: ELECTRICAL |
| 286.01 | DF | SYSTEMS {24} |
| 287 | DF | .~ Signal box type (e.g., to call messenger, plural fire alarm boxes) {11} |
| 293 | DF | .~.~ Variable signal (e.g., police and fire, first and third alarm) {1} |
| 294 | | .~.~.~ Dial selector for variable signal |