Split-phase electric power

A single-phase three-wire system (English split- phase electric power ) is a power supply system, which is commonly (up to about 100 kW) used in North America for single-family homes and small businesses. It is the AC version of Edison now historical three-wire direct current system. There is voltage on two conductors, it is often mistakenly referred to as " two-phase flow " system. In fact, however, the same single-phase alternating current is applied to the two conductors.


A transformer that feeds a single-phase three-wire system has high voltage side, a primary winding of the line voltage from the medium-voltage network, which is operated for example with 10 kV. With star-shaped medium voltage networks with earthed neutral point, as are common in the North and Central American area as the United States, the primary winding connected between one phase and the grounded star point is connected. In triangular medium-voltage networks without operational earthed star point as they are common in the UK, the primary between two phase conductors is connected.

In rural extended coverage areas, particularly in Australia, New Zealand and Canada, single-phase three -wire systems are also supplied with only one pole of the medium-voltage network on the high side for cost reasons. As an operative return conductor is used on the high voltage side of the ground. These systems are referred to as Single-Wire Earth Return ( Swer ).

On the secondary side of the transformer is so designed that in each case between the neutral conductor (N) and any of the two other conductors, L1 or L2, for example, 120 V alternating voltage concern. This voltage is common in the U.S. and Canada. Between the two conductors L1 and L2 double the voltage, that is 240 V is applied. The center tap on the secondary side is usually grounded. This is also necessary to enable the operation of residual current devices in single-phase three -wire systems.

In rural areas of the UK are 230 V / 460 V, single-phase three -wire systems used to connect farms and a small number of remote, of related houses. However, is the users of such service connections, in which only two outer conductor of the medium-voltage network are guided to, then no three-phase alternating current ( "AC " ) are available - the operation of, for example, three-phase asynchronous motors for agricultural machinery is only through additional circuits such as the Steinmetz circuit or with a frequency possible.

In Germany and in many other regions in Europe, where since the 1950s all households are supplied as standard with a three-phase AC power, the obsolete single-phase three-wire system variant is uncommon and largely unknown.

Three-phase expansion

A common, especially in North America extending the single-phase three -wire system for the three- phase alternating current is the Delta 4 -Wire, High Leg Delta or Red -Leg Delta system. In Europe, this type of low voltage supply is completely unusual.

This extension provides the advantage that it requires no change to the usual American space single-phase three wire connections in the area of ​​120 V/240 V. To supply the local transformers a Dreiphasenanspeisung from the medium-voltage network is necessary.

On the secondary side of the transformer and connected as a three-phase triangle in local transformer, usually connected in rural areas in the form of three single pole transformers, as shown in the adjacent sketch. The terminals L1 and L2 and the grounded neutral conductor N, the corresponding to the single-phase three-wire system and connections is provided by an outer conductor. The additional fourth terminal L3 is referred to as "High Leg" and supplied by the remaining two phase conductors. Thus arises between L1, L2 and L3 with a three-phase 240 V can be used for example for three-phase induction motors.

Since the grounded neutral conductor is not unlike the common European systems such as the TT system in the star point, but in a Mittelpunktanzapfung a winding in the triangle to different voltages between the three phases L1, L2 and L3 yield to the grounded neutral conductor N:

From the higher voltage of L3 with respect to ground or N, the terms High Leg derived. To avoid connection errors and in accordance with Article 110.15 of NFPA 70, this port must be marked in orange or red. The U.S. NFPA 70, as " National Electrical Code " (NEC ), is comparable to the usual especially in Germany DIN VDE standard (VDE 0100) approximately.

A disadvantage of the high -leg delta system, as well as in the single-phase three-wire system, is that the outer conductors are loaded very unevenly in the medium-voltage network and thereby can increase the appearance of unbalanced loads. A distribution of the single-phase consumers on all three phases is not possible in this system and an unbalanced load can be compensated only for several local transformer stations away. Because of the unbalanced load the transformers in North America are also mostly as three separate single-phase transformers, with possibly different power, run and not combined into a single three-phase transformer with the same output with a smaller core cross section.