Addressing several items:
Modern devices separate the two-wire
power circuit from user-accessible parts of the device either with a third
wire - the
ground, or by being double-insulated with rubber or plastic,
etc. Prior to this, a variety of schemes were used - from devices where the chassis was directly bonded to the
neutral wire, capacitively bonded to both the hot and
neutral wires, or in a few cases, had large internal parts directly bonded to the hot
wire. In the olden days, absolute
polarity made a big difference, as plugging in the device backwards (or to a mis-wired
receptacle where hot and
neutral were reversed) could present full
line voltage to the large metal exterior of the device. This hazard still continues tody with the use of vintage guitar amps, but for the most part,
polarity doesn't matter.
In a simple two-wire
circuit, a balanced
system such as the two hot legs of a residential North American 240/120 service and a European 240v
system with 240v on one hot and the other
wire at
ground potential are totally indistinguishable at the device
level. If you put an (isolated) oscilloscope across these
power conductors, you would see exactly the same waveform. Similarly, two legs of a 208v 3-phase
system are indistinguishable from two legs of either a North American or a European 240v
system, other than the
voltage difference.
The difference between a center-tapped 240v
system like we use in NA and the European 240v
system does start to matter when you consider fault potential. In the North American
system, the
voltage potential between a hot
leg and
ground is 120v, so unless you have managed to be extra unlucky by touching two phases at the same time, you're only going to be shocked to 120v, which in most situations is survivable. In the European
system, every
shock between
line and
ground is a 240v
shock, which is significantly more dangerous. In cases where
cord damage is likely such as
power tools on construction sites, code mandates that a step-down
transformer is used so that the
power tools are run from 120v, reducing the
shock potential.
Someone asked if it is possible to have
power with 60v between each hot
leg and
ground, and 120v between two hot legs. Yes, this exists and is sometimes used in technical
power applications and fancy recording studios. It is even safer than the regular North American
power system as a
shock now is only 60v, and regular 120v devices can run on this
power without issue as the device only cares about the
voltage difference between it's
line wires, which is 120v.
