High voltage
The difference between high voltage and low voltage depends on the situation and on the field of science or industry involved. In general, 'high voltage' is capable of producing sparks and delivering shocks to humans, while 'low voltage' is not.
Safety and insurance industry
Various safety and insurance organizations place the threshold between high and low voltage in the region of 40 to 60 volts. Voltages above this range are sometimes capable of producing heart fibrillation if they produce electric currents in body tissues which happen to pass through the chest area. The electrocution danger is mostly determined by the low conductivity of dry human skin, and if skin is wet (especially with electrolytes,) or if there are wounds, or if the voltage is applied to electrodes which penetrate through the skin, then even voltages far below 40V can be lethally 'high.' On the other hand, extremely high voltages (above approx. 500V) have a naturally defibrillating effect, so sometimes a high voltage can be safer than a low voltage.
Sparks in air
High voltages make violet corona discharge and obvious sparks. Voltages below about 500-700 volts cannot produce easily visible sparks or glows, so by this rule these voltages are 'low.' However, under conditions of low atmospheric pressure, or in an environment of noble gas such as argon, neon, etc., sparks appear at much lower voltages. 500 to 700 volts is not a fixed minimum for producing sparks, but it is a rule of thumb.
Science classroom devices
A high voltage is not necessarily dangerous. Physics demonstration devices such as Van de Graaff generators, Wimshurst machines, and Tesla coils can produce voltages approaching one million volts, yet at worst they deliver a brief sting. During the discharge, these machines apply high voltage to the body for only a millionth of a second or less. Since electrical power equals the voltage multiplied by the current, or P = V · A, in order to be dangerous, an electric power supply must also produce a significant current in the heart muscle continuing for many milliseconds, or it must deliver enough energy to damage tissue through heating. Since the current is only a few micro amperes, the power is only a few milliwatts
Electrostatic attraction/repulsion
The terminals of DC high voltage machines can attract dust, lint, and bits of paper. On an everyday scale, voltages higher than a few thousand volts are required in order to create an electric field with a gradient large enough to produce obvious forces. On the other hand, the forces depend on the distance from the electrodes and the electrode shapes, and at the microscopic scale of MEMS machines, even a few tens of volts acts like a 'high voltage.'
Power lines
In power grids, the optimum voltage usually falls well into the 'high voltage' range. Electric energy flow (i.e. power) is the product of voltage and current. High voltage at low current can give the same energy flow as low voltage at high current. However, because of Ohm's law, the heating of the wires and the waste of energy is proportional only to current. Utility companies avoid wasting energy by transmitting it at low currents, but at very high voltages. In the past it was easy to trade voltage for current using a transformer, but transformers required that alternating current be used. Today even DC voltages can be stepped up and down easily by using a switching power supply.
See also
