science

Single-wire earth return

Single-wire earth return is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Single-wire earth return rather than just read about it. In short: 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 lowest cost. 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.

Key takeaways

Single-wire earth return belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Single-wire earth return to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Single-wire earth return from memory before moving on to harder problems.

Reference excerpt

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 lowest cost. 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. 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.

History Telegraph circuits from the 1840s used a single wire on poles per circuit and the ground to form a closed circuit. Early telephone circuits were also unbalanced lines, but later were converted to 2 wires per circuit, to form a balanced line system, which was less affected by electromagnetic interference. In 1897, Nikola Tesla patented a high voltage AC transmission and distribution system using a single wire and ground electrodes. From the 1890s and early 1900s, several power companies used the ground instead of the neutral wire to save copper and aluminium. Lloyd Mandeno, OBE (1888–1973) 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". More than 200,000 kilometres (100,000 miles) 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).

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. SWER can cause fires when maintenance is poor, and bushfire is a risk. 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. The voltage changes due to the transition from line-to-line to line-to-earth, typically reducing a 22 kV grid to 12.7 kV SWER or a 33 kV grid to 19.1 kV SWER. The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres (miles), 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. SWER is therefore a practical example of a phantom loop. In areas with higher-resistance soil, the grounding rod can float to higher voltages, wasting energy. The resistance may be high enough to affect self-resetting circuit breakers, which usually reset due to a difference in voltage between line and neutral. 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. Experience in Alaska shows that SWER needs to be grounded below permafrost, which is high-resistance. 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 obsolete 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).

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Single-wire earth return

Start with the simplest possible case. Write down what Single-wire earth return claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Single-wire earth return before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Single-wire earth return ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Single-wire earth return

In research: Single-wire earth return appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Single-wire earth return in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Single-wire earth return is common in secondary-school and first-year university syllabi. It links to neighbouring topics Appropriate technology, Electric power distribution, so understanding it makes those chapters shorter.
In everyday life: Look for Single-wire earth return outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Single-wire earth return in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Single-wire earth return means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Single-wire earth return out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Single-wire earth return in simple terms?

Why does Single-wire earth return matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Single-wire earth return?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Single-wire earth return.