At the airfields I have worked at, essential items like airfield lighting, ILS, radar and (of course) R/T were on a "no-break" supply. As London Mil has said, a huge battery of, er batteries supplies those essential services while the standby jenny coughs itself into life. We had a tropical fish tank in the Approach room and even that managed to get wired in to the "no-break" supply.
I seem to remember that the only times the jenny failed to kick in was whenever the system was tested by deliberately chopping the mains. Then it was "cough, cough, f@rt, silence".