1. The problem statement, all variables and given/known data Does anyone know why number 1 VHF communications system powered from 28Vdc Battery Bus or 28Vdc Essential Bus and not from another source such as 28Vdc Main Bus or similar power source. 2. Relevant equations 3. The attempt at a solution I was struggling to find reasons as to why this maybe so, but i was thinking that it may be because if they do operate on the 28Vdc Essential Bus, they can operate on standby power in case of emergency?