Since you ask (post19) from memory, and it's a very long time ago, Comet was 115v frequency wild generation - this used for de-ice only, rectified and regulated to 28vDC. Instrument supplies from three? DC-AC 115v 400Hz motor driven inverters. Engine start was 120v DC. I Think the Brit was similar but might have been 28v starters. I seem to remember that the Caravelle had the same ground power requirements as the Comet which would imply the same starter. The GPU cables used to go rigid with the current on engine start !