What are the general pros and cons of using high voltage vs. low voltage for 
electrical appliances? Some countries like US use 110 volts, some other 
countries use 230 volts.

Obviously, there is sufficiently little difference in the big picture that both 
standards have survived in different jurisdictions. (In fact, there are more 
than two standards: there are places with 110, 120, 130, 220, 230 240 V nominal 
line voltage [generally +/-6%] plus both 50 and 60 Hz frequency standards. 
There has been a little progress toward increasing standardization, but it has 
been very slow.) The existence of the various standards has been largely the 
result of local politics and historical accident. 

Roughly speaking, to operate a particular appliance requires a particular 
amount of POWER, which (at least for resistive loads) is current times voltage. 
If you double the voltage, you draw half the current to achieve the same power. 
The primary advantage of lower current is that you lose less power in the wires 
feeding current to the appliance (or you can use smaller, cheaper wires for the 
same power loss rating). On the other hand, the higher voltage is somewhat more 
dangerous if accidentally touched or if there is an accidental short circuit. 
Some experienced electricians are relatively casual about touching 110 V 
circuits, but all respect 230 V. (This constitutes a "don't-try-this-at-home 
thing, though--it's quite possible to get a fatal shock or start a fire with 
110 V!) Current trends are toward the use of even lower voltages (24 V, 12 V, 5 
V, 3.3 V...) for any devices which don't draw much total power to increase 
safety. Power is rarely distributed at these lower voltages; rather it is 
converted from 110 V or 230 V by a transformer at the earliest opportunity. 
Even in North America, 220-240 V is commonly used in residential appliances for 
most high-power electrical appliances (ovens, furnaces, dryers, large motors, 
etc.) so that the supply current and supply wire size can be smaller. Higher 
power industrial applications often use 480 V or more. And, of course, 
transmission lines use progressively higher voltages as the distance and total 
power go up (22,000 V for local distribution to 1,000,000 V for long distance 
lines).

For further reading, one good newsgroup discussion on the issue can be found at 
sci.engr.lighting:

http://groups.google.com/groups?hl=en&threadm=6bg74c%24r7%
40cucumber.demon.co.uk&rnum=12&prev=/groups%3Fq%3Dpros%2Bcons%2B120%2B240%
2Bvolts%26hl%3Den%26start%3D10%26sa%3DN

That was very informative. Thanx.

Unless I'm way off, there are some dangerous comments here! Voltage has little 
to do with the danger of electricty in terms of shock, it's all about current. 
You could touch 10,000 volts at low current and live, or die with 10 volts at 
high amperage (current).

I don't know about electricians being more cautious with 220, but in america, 
the difference between 110 and 220 isn't just the voltage, the 220 is also 
three phase (beyond this question, but it has three active connections instead 
of two). The amount of available current for these 220 three phase circuits 
tends to be higher.

rick

The US uses 110V for domestic circuits, but with a centre earth, so the effective potential above 
ground is 55V. The UK uses 240V, with the neutral line strapped to eath, so the potential above 
ground is 240V.

What hurts is the number of Joules you get. (I x V x time). For a given current, more volts are 
worse. In addition, higher voltages are more likely to paralyse the nerves, hence keeping you 
hand stuck on the live wire for longer. 

Fit a residual-current circuit breaker to detect ground leakage and cut power quickly to reduce 
the dangers of electrocution.

Because 220V will carry about twice the power for the same thickness of wire, electric kettles 
take about twice as long to boil in the US as in the UK, hence the popularity in the US of 
stovetop kettles (a utensil I have only seen when camping in the UK).

Actually, both comments are way off. Voltage is the driving force of
the process
we call electric shock. Higher the voltage, other things being the
same, means higher current and more absorbed energy (which is measured
in Joules). More absorbed energy (in comparable time interval) means
higher temperature inside
of a cell, which one cause of the damage. Cell starts dying  at about
75 C, which is actually used as medical procedure for some conditions,
such as BPH.
www.jparisi.com/bph/thermotherapy.htm 
Do not try it at home: If it says "High Voltage" do not speculate
about the Joules! Stay away. Voltage is indicator of danger, of the
extent of the possible damage.
The comment about the kettle is wrong as well. If you import a kettle
from Europe and plug it in the US outlet, it will take over 4 time as
long to boil the water. It would be a silly thing to do. If you buy
one here, made for US voltage, it works as good, as fast, as theirs
does there. Since stores here do not carry them, I gor mine at
http://www.amazon.com/exec/obidos/tg/stores/detail/-/kitchen/B00004S9H7/104-6935778-87735140
 it works fine.

To respond to a couple of the issues raised in the Comments:

1. As Rick notes, it is really current that can injure and kill. You
can indeed work safely around kilovolt circuits if the current is
limited to safe levels (as it is in many [but by no means all!]
products which use such voltages.)

However, the current which flows through the body is often limited
only by the resistance of the body and the connection to it (the
contact resistance at the skin). This is usually the case around
typical 110 V and 240 V wiring. In that case, what determines whether
or not an injury will occur is the voltage difference between two
points of contact (such as a bare wire and ground). The current will
be that voltage difference divided by the net resistance (including
contact resistance and body resistance). Furthermore, there are often
non-linear effects which cause the net resistance to go down with
voltage. For example, the current may cause your hand muscles to grasp
the live wire harder. That's why higher voltages are more dangerous in
accidental contact situations.

Certainly, low voltages at high current can also be dangerous. You can
even get a nasty injury from, say, a 5 V power supply if you happen to
put your hand near an arc from an accidental short circuit of several
amps.

2. 220 V circuits in the USA are generated more than one way. In
industrial applications, they are usually 3-phase (two versions).
Higher-powered residential appliances such as electric ovens, dryers
and furnaces use a 2-phase system. The ac power delivered to the house
comes in on two wires, both 110 V above ground, but 180 degrees out of
phase with each other. Most household circuits provide a connection
between one of these lines and a "neutral" or ground wire.

The high power devices connect between the two live lines which are
220 V apart. In both cases, there should be (but may not be in some
older wiring) a third "ground" wire which should be connected to any
metal chassis that may be present in a given appliance. This ensures
that the chassis cannot "float" to some voltage that might give you a
shock, and that any accidental short circuit to the chassis does not
pose a shock hazard. The safety ground wire is physically connected to
the neutral wire back at the fuse or circuit breaker box for the
house, but only the neutral wire should carry any current under normal
conditions.

You may also have encountered an additional safety feature which is
now required by most electric codes for wet locations (near sinks,
outdoors, etc.). This is "GFI" or ground fault interruption.
Essentially, there is a local circuit breaker near the point of use
which interrupts the circuit if a current leakage to the safety ground
lead is detected.

The higher voltage can deliver twice the energy with the same wire
thickness. Insulation is usually the same for both and is rated at 600
volts. I think safety would be greatly enhanced if the circuit was
isolated from ground by the utility at the point of entry. If all (US)
home appliances used a 230 volt 2-wire (supply and return) from a
ground-isolated source, used smaller fuses, and had a third wire to
ground any metal surfaces, shock and fire hazards would be
substantially reduced. There would be no additional need for isolation
transformers and ground-fault interrupt devices. I have a small
business (micrometer.com) that deals extensively with electrical
utility topics.