<P> For three - phase at utilization voltages a four - wire system is often used . When stepping down three - phase, a transformer with a Delta (3 - wire) primary and a Star (4 - wire, center - earthed) secondary is often used so there is no need for a neutral on the supply side . For smaller customers (just how small varies by country and age of the installation) only a single phase and neutral, or two phases and neutral, are taken to the property . For larger installations all three phases and neutral are taken to the main distribution panel . From the three - phase main panel, both single and three - phase circuits may lead off . Three - wire single - phase systems, with a single center - tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America . This arrangement is sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the UK . Small power tools and lighting are supposed to be supplied by a local center - tapped transformer with a voltage of 55 V between each power conductor and earth . This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage of 110 V between the two conductors for running the tools . </P> <P> A third wire, called the bond (or earth) wire, is often connected between non-current - carrying metal enclosures and earth ground . This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools . Bonding all non-current - carrying metal parts into one complete system ensures there is always a low electrical impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault . This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device (breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state . All bond wires are bonded to ground at the main service panel, as is the neutral / identified conductor if present . </P> <P> The frequency of the electrical system varies by country and sometimes within a country; most electric power is generated at either 50 or 60 hertz . Some countries have a mixture of 50 Hz and 60 Hz supplies, notably electricity power transmission in Japan . A low frequency eases the design of electric motors, particularly for hoisting, crushing and rolling applications, and commutator - type traction motors for applications such as railways . However, low frequency also causes noticeable flicker in arc lamps and incandescent light bulbs . The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency . The original Niagara Falls generators were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker). Most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some 25 Hz industrial customers still existed as of the start of the 21st century . 16.7 Hz power (formerly 16 2 / 3 Hz) is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland . Off - shore, military, textile industry, marine, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds . Computer mainframe systems were often powered by 400 Hz or 415 Hz for benefits of ripple reduction while using smaller internal AC to DC conversion units . In any case, the input to the M-G set is the local customary voltage and frequency, variously 200 V (Japan), 208 V, 240 V (North America), 380 V, 400 V or 415 V (Europe), and variously 50 Hz or 60 Hz . </P> <P> A direct current flows uniformly throughout the cross-section of a uniform wire . An alternating current of any frequency is forced away from the wire's center, toward its outer surface . This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation that cancel the propagation of electricity toward the center of materials with high conductivity . This phenomenon is called skin effect . At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths . The skin depth is the thickness at which the current density is reduced by 63% . Even at relatively low frequencies used for power transmission (50 Hz--60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors . For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost . Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced . This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area . The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I R loss). </P>

What is the voltage range in which alternating current equipment can be operated
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