Power supplies off their design load can go a long way over nominal voltage.
Years ago I was working on an HF radio in a nice twin-engined aircraft, with the external PSU connected to save running the battery down. The owner was doing something with the Nav kit (all switched on and displaying) when his wife - an energetic and aggressive woman - yelled at us for having the battery master "on" while using external power.
She switched off the battery master. Cue loud sequence of popping sounds as CBs and fuses blew. My test meter showed 70 volts (on a 28 volt system). All the electric instruments in the panel, all the bulbs, and the VHF Nav/Comm radios had to be replaced. He reckoned that switch change cost him over £10k.
They divorced not long after.
Last edited by Keef; 28th Nov 2012 at 08:52.