Not necessarily. You could design the plug like "copper tip, plastic ring, copper sleeve". With the right dimensions the contacts aren't shorted together during

Not necessarily. You could design the plug like "copper tip, plastic ring, copper sleeve". With the right dimensions the contacts aren't shorted together during insertion. Alternatively, use a ring which is segmented in 120 degree pieces, and place two contacts 180 degree apart in the socket: any two ring segments might be shorted, but never all three - and there will always be at least two segments with a connection. Use a bridge rectifier to get a fixed polarity out of it.