“What if I plug a 220V to a 240V?”
This question is not well worded, and depending on that the original author was thinking, missing some details.
Electrical power outlets look like this (or similar)
The two rightmost are the Euro-style “Schuko” grounding pin side contact variety.
The Far Left is the North American two blade with longer pin for 110 Volts.
The Left of Centre outlet is the UK style three pin variety.
Depending on the current, there are larger devices , for things such as electric dryer , and oven machines, but they look similar.
But your question does not make mention of what is being plugged that has the rating of 220V? or the item and its tolerance for voltage fluctuation.
If the tolerance is 10%, then anything +22 volts or -22volts from the ‘standard’ will be fine.
So, the device is ‘rated’ for 230 Volts.
This means that it can tolerate a drop of 23 volts , and an increase of 23 volts.
Suppose the tolerance is -5%/ +15%.
This would mean that the supply can be 220 to 260 Volts.
But that is for a device/ appliance etc that is designed for 230 Volts.
What if it is grandma’s Microwave Oven , and it says 220–240 V, 50 Hz on the label?
Well then you are fine, because the outlet will supply 240 Volts.
But this is for a Generator.
Ah, well, that is a voltage source, and must not be plugged into the home outlets.
What will happen?
You will get a current surge as all devices that have 220, will instantly be energized to 240, and depending what is on the other end of the 220 will now absorb the current, and want to be running at 240.
You have not specified if this is a LOAD at 240, or a SOURCE at 240 that you are plugging into the 220 socket (A reverse plug, or cheater cord).
A plug to plug arrangement is highly dangerous, since if you had another source of power supplying 240 volts AC, and if you got the timing wrong, you would burn out your MCB and damage your generator.
You could have a spark of 460 volts flash between the posts, and the current in the generator would go up to 1,000 amperes, and possibly ‘weld’ the contacts together with such a high voltage, making the generator Circuit breaker inoperable, then melt your home wiring, and cause fire in house
