About A.C.: Alternating Current differs from Direct Current in that the two source terminals alternate, or take turns, being the source of excess electrons.

Either A.C. or D.C. can be of high-enough voltage to kill, and either can be grounded to there is a potential (voltage) difference between a 'hot' wire and the earth (ground).

We use A.C. because it allows the use of transformers to step voltages up or down for most efficient use and the least waste of energy.

Since power (the work done) is the product of voltage and current [Watt's Law: P(watts) = E(volts) x I(amps)], you can raise one while lowering the other, and still maintain the power.

Long-distance power transmission is best done at high voltages, because the voltage drop along the conductors is dependent on the current [Ohm's Law: E(volts) = I(amps) x R(ohms)], not the voltage.

However, 'primary' distribution is easier at lower voltages (in the thousands of volts), as insulation is easier, and local utilization voltages are much safer yet, at 240/120(1 ph.), 208/120(3 ph.), or 480/277(3 ph.).

High-voltage/low-current circuitry requires a lot of insulation, whereas high-current/low-voltage circuitry requires a lot of conductor; usually, insulation is cheaper than conductor.

For example, look at the wiring under the hood of your car. The two largest wires are usually the battery cables and the spark-plug wires. The battery cables are mostly copper; the plug wires are msotly insulation.

The voltage-to-ground is the usual concern when it comes to personal safety. Rarely do non-electrically-qualified people come into contact with two (or more) hot wires; most accidental contact is with a single hot wire while grounded.

Larry
www.fineelectricco.com