Ray,
As Tim points out, typical for U.S. homes is for the house to be fed from a medium voltage power distribution line via a step down transformer. This transformer has a center tapped secondary with the center tap forming a neutral between the two secondary winding halves. 240 volts is available across the entire secondary winding for large demand loads (air conditioning, electric clothes dryer, etc.) while 120 volts is available between either side and neutral for lighting and other appliances.
In this case the transformer is functioning as a phase splitter so that as the AC waveform is approaching a positive peak on one side of neutral, it is going towards a negative peak on the other side. The two phases are a creation of the transformer and stay perfectly 180 degrees out of sync due to the split transformer winding. Lower cost LED lighting can use a half wave rectifier so that it only is supplied voltage on half of the waveform and if two of these lights are on different halves of the 120 circuit then you will have them emitting peak brightness 180 degrees out of phase with each other creating some odd effects. Note that the full 240 volt available is a single phase and the split phase is purely a construct of the transformer reversing one side with respect to neutral.
Although you won't typically find it for a home; businesses, farms, and large power consumers will use tri-phase power with three "hot" lines all out of phase with one another. This is true multiple phase power unlike that "pseudo-split" created by the typical home transformer.
Quality LED lighting is fed regulated DC and this isn't a problem, the worst are stuff like cheap Christmas lights where the LED itself acts as a rectifier diode and flickering is very obvious to the human eye. Incandescent lighting has significant thermal lag so even though the power through it is changing throughout the AC cycle the human eye (and camera) don't perceive a difference in illumination unlike sources that respond quickly in response to applied voltage changes.
It is a little more complex than the UK but the U.S. system has some advantages and some difficulties. For best power utilization the house load should be as evenly distributed across each side of neutral because any current imbalance flows back through the neutral line. AND if there is a fault with the neutral line, bad things can occur because the voltage will no longer split evenly across the two 120 volt sides which can cause one side to rise to a damagingly high voltage while the other side drops to voltage low enough to damage motors. In terms of damage to electrical gear, the loss of neutral is about the worst in terms of carnage.
Rodger
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