Single-wire earth return

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HVDC SWER power line in Cahora Bassa Cahora Bassa (HVDC) - KNP - 001.jpg
HVDC SWER power line in Cahora Bassa

Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth (or sometimes a body of water) is used as the return path for the current, to avoid the need for a second wire (or neutral wire ) to act as a return path.

A single-wire transmission line is a method of transmitting electrical power or signals using only a single electrical conductor. This is in contrast to the usual use of a pair of wires providing a complete circuit, or an electrical cable likewise containing two conductors for that purpose.

Single-phase electric power

In electrical engineering, single-phase electric power is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 kW in rating.

Electric power the rate per unit of time at which electrical energy is transferred by an electric circuit

Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.


Single-wire earth return is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps. It is also used for high-voltage direct current over submarine power cables. Electric single-phase railway traction, such as light rail, uses a very similar system. It uses resistors to earth to reduce hazards from rail voltages, but the primary return currents are through the rails. [1]

Rural electrification

Rural electrification is the process of bringing electrical power to rural and remote areas. Rural communities are suffering from colossal market failures as the national grids fall short of their demand for electricity. Currently, over 1 billion people worldwide still lack household electric power - a jaw dropping 14% of the global population. Electrification typically begins in cities and towns and gradually extends to rural areas, however, this process often runs into road blocks in developing nations. Expanding the national grid is expensive and countries consistently lack the capital to grow their current infrastructure. Additionally, amortizing capital costs to reduce the unit cost of each hook-up is harder to do in lightly populated areas. If countries are able to overcome these obstacles and reach nationwide electrification, rural communities will be able to reap considerable amounts of economic and social development.

High-voltage direct current

A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be justified, due to other benefits of direct current links. HVDC uses voltages between 100 kV and 1,500 kV.

Submarine power cable

A submarine power cable is a transmission cable for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneath salt water but it is also possible to use submarine power cables beneath fresh water. Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River.


Lloyd Mandeno, OBE (18881973) fully developed SWER in New Zealand around 1925 for rural electrification. Although he termed it "Earth Working Single Wire Line", it was often called "Mandeno’s Clothesline". [2] More than 200,000 kilometres have now been installed in Australia and New Zealand. It is considered safe, reliable and low cost, provided that safety features and earthing are correctly installed. The Australian standards are widely used and cited. It has been applied around the world, such as in the Canadian province of Saskatchewan; Brazil; Africa; and portions of the United States' Upper Midwest and Alaska (Bethel).

Lloyd Mandeno was a New Zealand electrical engineer, inventor and local politician. He was born in Rangiaowhia, Waikato, New Zealand, on 3 October 1888. He is credited with nine hydroelectric installations and numerous inventions. He served on electric power boards, regional councils and as a deputy mayor.

Order of the British Empire order of chivalry of British constitutional monarchy

The Most Excellent Order of the British Empire is a British order of chivalry, rewarding contributions to the arts and sciences, work with charitable and welfare organisations, and public service outside the civil service. It was established on 4 June 1917 by King George V and comprises five classes across both civil and military divisions, the most senior two of which make the recipient either a knight if male or dame if female. There is also the related British Empire Medal, whose recipients are affiliated with, but not members of, the order.

New Zealand Country in Oceania

New Zealand is a sovereign island country in the southwestern Pacific Ocean. The country geographically comprises two main landmasses—the North Island, and the South Island —and around 600 smaller islands. New Zealand is situated some 2,000 kilometres (1,200 mi) east of Australia across the Tasman Sea and roughly 1,000 kilometres (600 mi) south of the Pacific island areas of New Caledonia, Fiji, and Tonga. Because of its remoteness, it was one of the last lands to be settled by humans. During its long period of isolation, New Zealand developed a distinct biodiversity of animal, fungal, and plant life. The country's varied topography and its sharp mountain peaks, such as the Southern Alps, owe much to the tectonic uplift of land and volcanic eruptions. New Zealand's capital city is Wellington, while its most populous city is Auckland.

Operating principle

SWER is a viable choice for a distribution system when conventional return current wiring would cost more than SWER’s isolation transformers and small power losses. Power engineers experienced with both SWER and conventional power lines rate SWER as equally safe, more reliable, less costly, but with slightly lower efficiency than conventional lines. [3] SWER can cause fires when maintenance is poor, and bushfire is a risk. [4]

Schematic of SWER Swer.gif
Schematic of SWER

Power is supplied to the SWER line by an isolating transformer of up to 300 kVA. This transformer isolates the grid from ground or earth, and changes the grid voltage (typically 22 or 33 kV line-to-line) to the SWER voltage (typically 12.7 or 19.1 kV line-to-earth).

Transformer electrical artefact that transfers energy through electromagnetic induction

A transformer is a static electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described the induced voltage effect in any coil due to changing magnetic flux encircled by the coil.

Volt-ampere unit used for the apparent power in an electrical circuit

A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current. In direct current (DC) circuits, this product is equal to the real power in watts. Volt-amperes are useful only in the context of alternating current (AC) circuits.

Voltage difference in the electric potential between two points in space

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres, with a number of distribution transformers along its length. At each transformer, such as a customer's premises, current flows from the line, through the primary coil of a step-down isolation transformer, to earth through an earth stake. From the earth stake, the current eventually finds its way back to the main step-up transformer at the head of the line, completing the circuit. [3] SWER is therefore a practical example of a phantom loop.

Ground (electricity) reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth

In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.

Electrical network interconnection of electrical components or a model of such an interconnection, consisting of electrical elements

An electrical network is an interconnection of electrical components or a model of such an interconnection, consisting of electrical elements. An electrical circuit is a network consisting of a closed loop, giving a return path for the current. Linear electrical networks, a special type consisting only of sources, linear lumped elements, and linear distributed elements, have the property that signals are linearly superimposable. They are thus more easily analyzed, using powerful frequency domain methods such as Laplace transforms, to determine DC response, AC response, and transient response.

In areas with high-resistance soil, the resistance of the soil wastes energy. Another issue is that the resistance may be high enough that insufficient current flows into the earth neutral, causing the grounding rod to float to higher voltages. Self-resetting circuit breakers usually reset because of a difference in voltage between line and neutral. Therefore, with dry, high-resistance soils, the reduced difference in voltage between line and neutral may prevent breakers from resetting. In Australia, locations with very dry soils need the grounding rods to be extra deep. [5] Experience in Alaska shows that SWER needs to be grounded below permafrost, which is high-resistance. [6]

The secondary winding of the local transformer will supply the customer with either single ended single phase (N-0) or split phase (N-0-N) power in the region’s standard appliance voltages, with the 0 volt line connected to a safety earth that does not normally carry an operating current.

A large SWER line may feed as many as 80 distribution transformers. The transformers are usually rated at 5 kVA, 10 kVA and 25 kVA. The load densities are usually below 0.5 kVA per kilometer (0.8 kVA per mile) of line. Any single customer’s maximum demand will typically be less than 3.5 kVA, but larger loads up to the capacity of the distribution transformer can also be supplied.

Some SWER systems in the USA are conventional distribution feeders that were built without a continuous neutral (some of which were obsoleted transmission lines that were refitted for rural distribution service). The substation feeding such lines has a grounding rod on each pole within the substation; then on each branch from the line, the span between the pole next to and the pole carrying the transformer would have a grounded conductor (giving each transformer two grounding points for safety reasons).

Mechanical design

Proper mechanical design of a SWER line can lower its lifetime cost and increase its safety.

Since the line is high voltage, with small currents, the conductor used in historic SWER lines was Number-8 galvanized steel fence wire. More modern installations use specially-designed AS1222.1 [7] [8] high-carbon steel, aluminum-clad wires. Aluminum clad wires corrode in coastal areas, but are otherwise more suitable. [9] Because of the long spans and high mechanical tensions, vibration from wind can cause damage to the wires. Modern systems install spiral vibration dampers on the wires. [9]

Insulators are often porcelain because polymers are prone to ultraviolet damage. Some utilities install higher-voltage insulators so the line can be easily upgraded to carry more power. For example, 12 kV lines may be insulated to 22 kV, or 19 kV lines to 33 kV. [9]

Reinforced concrete poles have been traditionally used in SWER lines because of their low cost, low maintenance, and resistance to water damage, termites and fungi. Local labor can produce them in most areas, further lowering costs. In New Zealand, metal poles are common (often being former rails from a railway line). Wooden poles are acceptable. In Mozambique, poles had to be at least 12 m (39 ft) high to permit safe passage of giraffes beneath the lines. [9]

If an area is prone to lightning, modern designs place lightning ground straps in the poles when they are constructed, before erection. The straps and wiring can be arranged to be a low-cost lightning arrestor with rounded edges to avoid attracting a lightning strike. [9]



SWER is promoted as safe due to isolation of the ground from both the generator and user. Most other electrical systems use a metallic neutral connected directly to the generator or a shared ground. [3]

Grounding is critical. Significant currents on the order of 8  amperes flow through the ground near the earth points. A good-quality earth connection is needed to prevent risk of electric shock due to earth potential rise near this point. Separate grounds for power and safety are also used. Duplication of the ground points assures that the system is still safe if either of the grounds is damaged.

A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the ground, and bonded to the transformer earth and tank. A good ground resistance is 5–10 ohms which can be measured using specialist earth test equipment. SWER systems are designed to limit the voltage in the earth to 20 volts per meter to avoid shocking people and animals that might be in the area.

Other standard features include automatic reclosing circuit breakers (reclosers). Most faults (overcurrent) are transient. Since the network is rural, most of these faults will be cleared by the recloser. Each service site needs a rewirable drop out fuse for protection and switching of the transformer. The transformer secondary should also be protected by a standard high-rupture capacity (HRC) fuse or low voltage circuit breaker. A surge arrestor (spark gap) on the high voltage side is common, especially in lightning-prone areas.

Most fire safety hazards in electrical distribution are from aging equipment: corroded lines, broken insulators, etc. The lower cost of SWER maintenance can reduce the cost of safe operation in these cases. [4]

SWER avoid lines clashing in wind, a substantial fire-safety feature, [4] but a problem surfaced in the official investigation into the Black Saturday bushfires in Victoria, Australia. These demonstrated that a broken SWER conductor can short to ground across a resistance similar to the circuit's normal load; in that particular case, a tree. This can cause large currents without a ground-fault indication. [4] This can present a danger in fire-prone areas where a conductor may snap and current may arc through trees or dry grass.

Bare-wire or ground-return telecommunications can be compromised by the ground-return current if the grounding area is closer than 100 m or sinks more than 10 A of current. Modern radio, optic fibre channels, and cell phone systems are unaffected.

Many national electrical regulations (notably the U.S.) require a metallic return line from the load to the generator. [10] In these jurisdictions, each SWER line must be approved by exception.

Cost advantages

SWER’s main advantage is its low cost. It is often used in sparsely populated areas where the cost of building an isolated distribution line cannot be justified. Capital costs are roughly 50% of an equivalent two-wire single-phase line. They can cost 30% of 3-wire three-phase systems. Maintenance costs are roughly 50% of an equivalent line.

SWER also reduces the largest cost of a distribution network: the number of poles. Conventional 2-wire or 3-wire distribution lines have a higher power transfer capacity, but can require 7 poles per kilometre, with spans of 100 to 150 metres. SWER's high line voltage and low current also permits the use of low-cost galvanized steel wire (historically, No. 8 fence wire). [9] Steel's greater strength permits spans of 400 metres or more, reducing the number of poles to 2.5 per kilometre.

If the poles also carry optical fiber cable for telecommunications (metal conductors may not be used), capital expenditures by the power company may be further reduced.


SWER can be used in a grid or loop, but is usually arranged in a linear or radial layout to save costs. In the customary linear form, a single-point failure in a SWER line causes all customers further down the line to lose power. However, since it has fewer components in the field, SWER has less to fail. For example, since there is only one line, winds can’t cause lines to clash, removing a source of damage, as well as a source of rural bush fires.

Since the bulk of the transmission line has low resistance attachments to earth, excessive ground currents from shorts and geomagnetic storms are more rare than in conventional metallic-return systems. So, SWER has fewer ground-fault circuit-breaker openings to interrupt service. [3]


A well-designed SWER line can be substantially upgraded as demand grows without new poles. [11] The first step may be to replace the steel wire with more expensive copper-clad or aluminum-clad steel wire.

It may be possible to increase the voltage. Some distant SWER lines now operate at voltages as high as 35 kV. Normally this requires changing the insulators and transformers, but no new poles are needed. [12]

If more capacity is needed, a second SWER line can be run on the same poles to provide two SWER lines 180 degrees out of phase. This requires more insulators and wire, but doubles the power without doubling the poles. Many standard SWER poles have several bolt holes to support this upgrade. This configuration causes most ground currents to cancel, reducing shock hazards and interference with communication lines.

Two-phase service is also possible with a two-wire upgrade:[ citation needed ][ discuss ] Though less reliable, it is more efficient. As more power is needed, the lines can be upgraded to match the load, from single wire SWER to two wire, single phase and finally to three wire, three phase. This ensures a more efficient use of capital and makes the initial installation more affordable.

Customer equipment installed before these upgrades will all be single phase, and can be reused after the upgrade. If small amounts of three-phase power are needed, it can be economically synthesized from two-phase power with on-site equipment.

Power-quality weakness

SWER lines tend to be long, with high impedance, so the voltage drop along the line is often a problem, causing poor regulation. Variations in demand cause variation in the delivered voltage. To combat this, some installations have automatic variable transformers at the customer site to keep the received voltage within legal specifications. [13]

After some years of experience, the inventor advocated a capacitor in series with the ground of the main isolation transformer to counteract the inductive reactance of the transformers, wire and earth return path. The plan was to improve the power factor, reduce losses and improve voltage performance due to reactive power flow. [3] Though theoretically sound, this is not standard practice. It does also allow the use of a DC test loop, to distinguish a legitimate variable load from (for example) a fallen tree, which would be a DC path to ground.


In addition to New Zealand and Australia, single-wire earth return is used throughout the globe.


In 1981 a high-power 8.5 mile prototype SWER line was successfully installed from a diesel plant in Bethel to Napakiak in Alaska, United States. It operates at 80 kV, and was originally installed on special lightweight fiberglass poles that formed an A-frame. Since then, the A frames have been removed and standard wooden power poles were installed. The A-framed poles could be carried on lightweight snow machines, and could be installed with hand tools on permafrost without extensive digging. Erection of "anchoring" poles still required heavy machinery, but the cost savings were dramatic.

Researchers at the University of Alaska Fairbanks, United States estimate that a network of such lines, combined with coastal wind turbines, could substantially reduce rural Alaska’s dependence on increasingly expensive diesel fuel for power generation. [14] Alaska’s state economic energy screening survey advocated further study of this option to use more of the state’s underutilized power sources. [15]

In developing nations

At present, certain developing nations have adopted SWER systems as their mains electricity systems, notably Laos, South Africa and Mozambique. [9] SWER is also used extensively in Brazil. [16]

In HVDC systems

Many high-voltage direct current systems (HVDC) using submarine power cables are single wire earth return systems. Bipolar systems with both positive and negative cables may also retain a seawater grounding electrode, used when one pole has failed. To avoid electrochemical corrosion, the ground electrodes of such systems are situated apart from the converter stations and not near the transmission cable.

The electrodes can be situated in the sea or on land. Bare copper wires can be used for cathodes, and graphite rods buried in the ground, or titanium grids in the sea are used for anodes. To avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at the surface of the electrodes must be small, and therefore large electrodes are required.

Examples of HVDC systems with single wire earth return include the Baltic Cable and Kontek.

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Alternating current electric voltage which periodically reverses direction; form in which electric power is delivered to businesses and residences; form of electrical energy that consumers typically use when they plug electric appliances into a wall socket

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Electrical substation part of an electrical generation, transmission, and/or distribution system

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As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

Distribution transformer transformer that provides the final voltage transformation in an electric power distribution system

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In an electrical installation, an earthing system or grounding system connects specific parts of that installation with the Earth's conductive surface for safety and functional purposes. The point of reference is the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.

Service drop

In electric power distribution, a service drop is an overhead electrical line running from a utility pole, to a customer's building or other premises. It is the point where electric utilities provide power to their customers. The customer connection to an underground distribution system is usually called a "service lateral". Conductors of a service drop or lateral are usually owned and maintained by the utility company, but some industrial drops are installed and owned by the customer.

Delta-wye transformer

A delta-wye transformer is a type of three-phase electric power transformer design that employs delta-connected windings on its primary and wye/star connected windings on its secondary. A neutral wire can be provided on wye output side. It can be a single three-phase transformer, or built from three independent single-phase units. An equivalent term is delta-star transformer.

In an electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short-circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.

Transformer types

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Grounding transformer type of auxiliary transformer, part of earthing systems

A grounding transformer or earthing transformer is a type of auxiliary transformer used in three-phase electric power systems to provide a ground path to either an ungrounded wye or a delta-connected system. Grounding transformers are part of an earthing system of the network. They let three-phase systems accommodate phase-to-neutral loads by providing a return path for current to a neutral.


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  8. IEC 60888 Ed. 1.0 Zinc-coated steel wires for stranded conductors Archived 30 March 2012 at the Wayback Machine
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  11. Stone Power AB discusses low cost networks
  12. "FAQ2".
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