A split-phase or single-phase three-wire system is a type of single-phase
electric power distribution. It is the AC equivalent of the original Edison
three-wire direct current system. Its primary advantage is that it saves
conductor material over a single-ended single-phase system while only requiring
single phase on the supply side of the distribution transformer.
Connections A transformer supplying a three-wire
distribution system has a single-phase input winding. The output winding is
center-tapped and the center tap connected to a grounded neutral. As
shown in Fig. 1. either end to center has half the voltage of end-to-end. Fig.
2 illustrates the phasor diagram of the output voltages for a split-phase
transformer. Since the two phasors do not define a unique direction of
rotation for a revolving magnetic field, a split single-phase is not a two-phase
system. In the United States, the practice
originated with the DC distribution system developed by Thomas Edison. By
dividing a lighting load into two equal groups of lamps connected in series, the
total supply voltage can be doubled and the size of conductors reduced
substantially. The line to ground voltage is half the
line-to-line voltage. Lighting and small appliances are connected between a line
wire and the neutral. Large appliances, such as cooking equipment, space
heating, water pumps, clothes dryers, and air conditioners are connected
across the two line conductors, requiring less current and smaller
conductors than would be needed if the appliances were designed for the lower
voltage. If the load were guaranteed to be
balanced, then the neutral conductor would not carry any current and the
system would be equivalent to a single-ended system of twice the voltage
with the line wires taking half the current. This would not need a neutral
conductor at all, but would be wildly impractical for varying loads; just
connecting the groups in series would result in excessive voltage and
brightness variation as lamps are switched on and off.
By connecting the two lamp groups to a neutral, intermediate in potential
between the two live legs, any imbalance of the load will be supplied by a
current in the neutral, giving substantially constant voltage across
both groups. The total current carried in all three wires will always be twice
the supply current of the most heavily loaded half.
For short wiring runs limited by conductor ampacity, this allows three
half-sized conductors to be substituted for two full-sized ones, using 75% of
the copper of an equivalent single-phase system.
Longer wiring runs are more limited by voltage drop in the conductors. Because
the supply voltage is doubled, a balanced load can tolerate double the
voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the
copper of an equivalent single-phase system.
In practice, some intermediate value is chosen. For example, if the imbalance is
limited to 25% of the total load rather than the absolute worst-case 50%, then
conductors 3/8 of the single-phase size will guarantee the same maximum voltage
drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two
single-phase conductors. Technical power
In a so-called technical power system, an isolation transformer with a center
tap is used to create a separate supply with conductors at a balanced 60 V with
respect to ground. The purpose of a balanced power system is to minimize the
noise coupled into sensitive equipment from the power supply.
Unlike a three-wire distribution system, the grounded neutral is not distributed
to the loads; only line-to-line connections at 120 V are used. A
balanced power system is only used for specialized distribution in audio and
video production studios, sound and television broadcasting, and
installations of sensitive scientific instruments.
The U.S. National Electrical Code provides rules for technical power
installations. The systems are not to be used for general-purpose lighting or
other equipment, and may use special sockets to ensure only approved
equipment is connected to the system. Additionally, technical power systems
pay special attention to the way the distribution system is grounded.
Applications = Europe=
In Europe, three-phase 230/400 V is most commonly used. However, 230/460 V,
three-wire, single-phase systems are used to run farms and small groups of
houses when only one of the three-phase high-voltage conductors are available.
In the UK, electric tools and portable lighting at construction sites are
required to be fed from a centre-tapped system with only 55 V between live
conductors and the earth. This system is used with 110 V equipment and therefore
no neutral conductor is needed. The intention is to reduce the electrocution
hazard that may exist when using electrical equipment at a wet or outdoor
construction site. Portable isolating transformers that transform single-phase
230 V to this 110 V system are a common piece of construction equipment.
Generator sets used for construction sites are equipped to supply it
directly. An incidental benefit was that the
filaments of 110 V incandescent lamps used on such systems are thicker and
therefore mechanically more rugged than 230 V lamps.
= Oceania= In Australia and New Zealand, remote
loads are connected to the grid using SWER transmission lines, since it is
cheapest to run only one wire. The primary of the transformer is connected
between the high voltage line and earth, the secondary is a three-wire
single-phase system as described here, the secondary voltage being 230/460 V.
Single phase loads are split between the two circuits. Hot water services use
both circuits. = North America=
This three-wire system is common in countries with a standard phase-neutral
voltage of 120 V. It is commonly used in North America for single-family
residential and light commercial applications.
In North American electrical codes, the split-phase distribution may be carried
to the outlet receptacles. Two 120 volt devices may be plugged into a duplex
receptacle that connects one neutral wire to both outlets. This saves the
cost of one wire back to the panelboard. Such multiwire branch circuits have
special rules in the electrical codes to ensure they are safely applied.
A variation is the 240 V delta 4-wire system, also known as a high-leg or
red-leg delta. This is a three-phase 240 V delta connected system, in which one
winding of the transformer has a center tap which is connected to ground and
used as the system neutral. This allows a single service to supply 120 V for
lighting, 240 V single-phase for heating appliances, and 240 V three-phase for
motor loads. Two of the phases are 120 V to neutral, the third phase or "high
leg" is 208 V to neutral. Multiwire systems split more than two
ways are possible with both AC and DC but have the significant disadvantage
that no matter which point is tied to ground some of the wires will have a
higher earth relative voltage than the utilisation voltage; therefore, such
systems are not used in normal power distribution.
Railways In Sweden split-phase electric power is
also used on some railways. The center tap is grounded, one pole is fed with an
overhead wire section, while the other wire is used for another section.
Amtrak's 60 Hz traction power system in the Northeast Corridor between New York
and Boston also uses split-phase power distribution. Two separate wires are run
along the track, the contact wire for the locomotive and an electrically
separate feeder wire. Each wire is fed with 25 kV with respect to ground, with
50 kV between them. Autotransformers along the track balance the loads
between the contact and feeder wires, reducing resistive losses.
In the UK Network Rail are using autotransformers on all new 50 Hz
electrification, and are converting many old booster transformer [1]
installations to autotransformers, to reduce energy losses [2] and exported
electromagnetic interference, both of which increase when longer, heavier, or
faster trains are introduced, drawing higher peak current from the supply.
Note that booster transformers only "boost" the return of traction current
through its intended path, the "return conductor", rather than randomly through
the earth, and do not boost, but rather reduce, the available voltage at the
train, and introduce additional losses. The autotransformer system enforces the
traction return current taking its intended path, while reducing the
transmission losses, and therefore achieves both required objectives, of
controlling return current leakage to earth and ensuring low energy loss,
simultaneously. There is an initial cost penalty, because the previous return
conductor, insulated to a fairly modest voltage, must be replaced by an
anti-phase feeder, insulated to 25 kV, and the autotransformers transformers
themselves are larger and more expensive than the previous booster transformers.
See also Shared neutral
References ^ Terrell Croft and Wilford Summers,
American Electricians' Handbook, Eleventh Edition, McGraw Hill, New York
ISBN 0-07-013932-6, chapter 3, pages 3-10, 3-14 to 3-22.
^ a b Gonen, Turan. Electric Power Distribution System Engineering, 2nd ed.
CRC Press, 2007, p. 284. ^ NFPA 70, National Electrical Code
2005, National Fire Protection Association, Inc., Quincy, Massachusetts
USA,. no ISBN , articles 640 and 647