You don't.
If you are connecting 120 volts, you connect the black wire to the breaker, white wire to the neutral bar, and ground wire to the ground bar. If you are connecting 240 volts connect the black & white wires to the breaker, & ground wire to the ground bar.
No, a GFCI outlet is designed for use with standard 120-volt circuits. Attempting to use a GFCI outlet with a 240-volt split circuit could cause damage to the outlet, the circuit, and could present a safety hazard. It is not recommended to use a GFCI outlet in this manner.
You can't do that. An L14-30 is a 240/120 outlet and you cannot "make" 240 volts from two separate plugs; you have to have a hard-wired 240-volt source (either another type of outlet or wired directly to 240 volts).
No, you cannot simply cap one leg of a 240-volt circuit and change the breaker to convert it into a 120-volt outlet. This requires rewiring the circuit to provide 120 volts, including changing the outlet and possibly other components to be compatible with the lower voltage. It's recommended to consult with a licensed electrician to properly convert the outlet.
Yes <<>> In North America, a three wire 120/240 volt system uses a neutral wire. For 240 volts two "hot" wires are used with no neutral.
A 120-volt outlet is typically used for lower power devices like lamps and small appliances, while a 240-volt outlet can handle higher power devices like ovens and dryers. The main difference is the amount of voltage each outlet can provide, with 240 volts being able to deliver more power for larger appliances.
You can't. The 120 volt GFCI is probably just a 2-wire (hot, neutral and ground) You would have to run a new 3-wire (2 hots, neutral and ground). The two hots are how you get the 240 volts (120+120=240). Also you must make sure the wire is gauged properly. #10 wire for 30 amps, #12 wire for 20 amps, etc.
No, it is not possible to use 240 volts with a 120 volt supply directly. You would need a transformer to step up the voltage from 120 volts to 240 volts. Attempting to use 240 volts with a 120 volt supply without a transformer can damage equipment and pose a safety hazard.
Someone has wired 240 volts into your 120 volt outlet. If you have 240 volts you need a specially configured outlet so that a standard 120 volt plug cannot be inserted. If you have this situation you would see 120 volts to ground and not neutral. Sometimes if you don't look carefully an outlet will look like the standard 120 volt variety but it isn't. One of the slots is horizontal and not vertical although there may be a small vertical split. 240 volts doesn't just magically appear. What you are describing is on purpose. If it really is 120 volt receptacle you need an electrician to put in the proper receptacle or re-wire the circuit.
A four wire plug suggests to me that it is for split voltages. 120/240, hot - neutral - hot - ground will be the four blade connections for shore power in the US. With UK shore power three wire there will be no neutral as you know it. 120 volt equipment will not work. If there is a special berth for 120/240 shore power that is what you will need to tie up to.
It seems like you are describing the Red, Black, White and Ground in your electric panel. There is 240 VAC between Black and Red and 120 VAC between Black and White and 120 VAC between Red and White. The electric panel has two busses that supply 120 VAC on alternating breakers in your panel. Essentially, the Red turns into "black" in the panel for all practical purposes. If you have a 240 VAC circuit it essentially takes up to two vertical positions in your electric panel.
Depends on the PSU, the box on the cord. If it says 120-240 volts, then you can, but you also need an adapter as the outlet itself is different.