| OK thanks for this ... but why wasn't the same effect visible on 240V ... is | it an effect that is proportional to the voltage or square of the voltage?
UK circuits are typically higher amperage, such as 30 amps, and have a correspondingly higher amperage wire, less you plug in too many things on several outlets and overload it. US circuits are typically lower amperage, such as 15 or 20 amps, and have a smaller wire, thus more resistance.
For a given amount of power, a heavy duty appliance at 120 volts will draw twice the current as the same appliance at 240 volts.
So lets say some appliance needs 1200 watts, but draws 12000 watts just briefly to start up. In the UK that would be 5 amps and 50 amps on a
30 amp circuit. In the US that would be 10 amps and 100 amps on a 15 amp (typically) circuit.
Also the "other circuit" where it is not seen could be on the other side of the split phase wiring used in the US (and not in the UK). You get
240 volts between opposite ends of the transformer winding, but only 120 volts between either and the neutral (grounded) wire from the center. And that 120 volts is what is typically used, but the full 240 volts is available on circuits wired for it.
Heavier appliances tend to be wired up to their own circuit and also tend to be wired up to 240 volts instead of 120 volts (so they use only half the current, same as in UK).