I have worked with single phase and 3 phase 4 wire but not that much with 3 phase 3 wire 240 volt. I was working at a warehouse with 3 phase 3 wire 240volt the other day. They called me out because the maintenance man had changed a 1000w HPS ballast in one of the outside fixtures and he could not get it to work. He said he was getting strange voltages from the 240volt time clock that was feeding the fixture.
Now I have seen a 3 phase wild leg where the voltage to ground is 0 volts but on this one I was getting - 10 volt - 185 volt - 234 volts - to ground on each leg but getting 243 volts between each leg. I checked back at the main and got the same voltage. Checked a 3 phase compressor disconnect and got the same voltage. So now I'm scratching my head. I turned off the double pole breaker, in the 3 phase panel, feeding the outside light. The voltage changes through out the building - 85 volts - 156 volts - 122 volts - on each leg to ground.
Now I'm thinking I have to stop listening to this maintenance man about this voltage thing and find the problem with the outside light HE FIXED. I take it apart and he's got the ground from the building connected to the ballast wire that is clearly marked lamp. One hot connected to the 240 volt marked feed for the ballast which it right. The common which you would hook up the other side of the 240 Volt is hooked to the lamp wire. And the other leg of the 240 volt is capped off and not used.
To make a long story even longer, I hooked everything up right on the ballast and the light works. I just don't understand why having this ballast hooked up wrong would change the voltage through the whole building.