Introduction: Two Eras of Overhead Lines
Overhead power lines were built for decades on bare conductors of types A, AS, M. Aluminium and steel-aluminium conductors, suspended on insulators, still form the basis of backbone networks. At the end of the 20th century, the self-supporting insulated wire — SIP — appeared, which gradually displaced bare wires in 0.4 kV distribution networks. The competition between these two technologies intensifies at the stage of connecting a private house or laying a line inside a settlement.
A bare conductor is cheap, technologically efficient when tensioning long spans and remains non-alternative at voltages of 35 kV and above. SIP is safer, is not afraid of clashing in the wind, reduces leakage losses and allows routing the line in dense vegetation without clearing paths. The difference between them is not simply in a polyethylene layer, but in the philosophy of network construction. A review of types and characteristics of bare conductors is given in the material overhead wires, and the design and types of SIP are described in detail in the guide about SIP cable.
Comparative Table of SIP and Bare Conductors
| Parameter | SIP (SIP-4, SIP-2) | Bare conductor (A, AS, M) |
| Insulation | Light-stabilised cross-linked or thermoplastic polyethylene | Absent |
| Conductor material | Aluminium (SIP-4 without a core, SIP-2 with a load-bearing neutral steel core) | Aluminium (A), steel-aluminium (AS), copper (M) |
| Cross-sections, mm² | 16–120 | A: 16–300; AS: 35–400; M: 4–185 |
| Voltage | 0.4–1 kV (SIP-2), up to 20 kV (SIP-3) | 0.4–1150 kV |
| Self-supporting capability | Yes, load-bearing conductor or steel core | No, suspended on insulators with messenger wire fastening |
| Risk of short circuit during clashing | Absent | High |
| Operation in forested areas | Permitted without clearing paths | Requires route clearing |
| Corona and leakage losses | Minimal | Higher, especially in wet weather |
| Installation complexity | Requires anchor clamps, special tensioning equipment | Simpler, traditional hardware, but insulators and taller poles are needed |
| Operational safety | Possible contact without disconnection (with intact insulation) | Dangerous to life upon contact |
| Service life | Not less than 25 years | 30–50 years depending on atmospheric conditions |
| Line cost | Higher in materials, lower in clearing and maintenance | Lower in wire, higher in insulators, crossarms and regular tree trimming |
Design, Materials and Regulatory Framework
Bare conductors for overhead lines are produced according to GOST 839‑80. Type A — aluminium solid or stranded conductor, AS — steel-aluminium, where the steel core bears the mechanical load and the aluminium shell conducts the current. Copper types M and MG are used on a limited basis due to their high price. The diameter and weight per metre are determined by the cross-section. The steel-aluminium conductor AS‑50/8 is a classic for rural 10 kV overhead lines, where 50 is the nominal cross-section of aluminium, 8 — of steel. The stranded structure imparts flexibility, but not comparable to a cable in insulation.
SIP is insulated aluminium conductors twisted around a neutral load-bearing conductor or, in the case of SIP‑4, a self-supporting design without a core. The insulation of cross-linked polyethylene withstands heating up to 90 °C in normal operation. Domestic standards are GOST 31946‑2012 and GOST R 52373‑2005. Letters in the marking: SIP‑1 — with a bare load-bearing neutral conductor, SIP‑2 — with an insulated load-bearing one, SIP‑4 — without a separate load-bearing element. For 10–20 kV networks, SIP‑3 with a shielded conductor is used. The cross-section range for low-voltage versions is from 16 to 120 mm², which covers any loads of the private sector and small business.
Electrical Parameters, Losses and Heating
The permissible continuous current for aluminium bare conductors is higher than for SIP of the same cross-section, thanks to better cooling in the open air. For example, an A‑50 wire outdoors carries about 215 A, whereas SIP‑4 4×50 — around 140 A per conductor due to mutual heating and the insulating layer. This difference is not critical for a 15 kW domestic service entrance, but is important when calculating backbone lines. The voltage drop per kilometre for AS‑70 is about 5 % at a current of 100 A; for SIP of the same cross-section it is slightly higher, which is compensated by the typically short length of SIP lines.
At voltages of 0.4 kV, the SIP insulation completely eliminates leakage currents through contaminated insulators, dampness and tree branches. In rainy weather, a bare wire on insulators loses up to several percent of energy to leakage, which over the service life translates into appreciable amounts. At high voltages, bare conductors suffer from corona discharge, which creates radio interference and destroys the surface of the conductor. SIP‑3 solves this problem for 10–20 kV networks.
Mechanical Strength, Spans and Sag
The main advantage of steel-aluminium conductors is the ability to cover huge spans. AS‑70/11 is installed with a span of up to 200 metres and more without intermediate poles. This is indispensable when crossing rivers, ravines, railways. SIP is inferior in span length: for SIP‑4 4×50 the maximum span is about 40–50 metres, for SIP‑2 with a load-bearing conductor — up to 90 metres. Therefore, on high-voltage backbone lines, where poles are spaced far apart, bare AS still reigns.
On the other hand, SIP wins in conditions of ice and wind. Insulated conductors do not clash, and the adhesion of wet snow has less effect on the integrity of the line. The mechanical strength of the load-bearing conductor or messenger wire in SIP‑2 is sufficient to bear the weight of the wire and ice, and anchor clamps reliably fix it to the pole. A bare wire, if one conductor breaks, is capable of falling to the ground, creating a lethal danger; SIP continues to hang on the load-bearing conductor even if the others are damaged.
Installation, Hardware and Work Safety
Working with a bare conductor requires disconnecting the line. The installer on the pole must use dielectric gloves and insulated tools. Insulators, crossarms, hooks — all this increases the mass of the pole and the installation time. SIP can be laid even without removing the voltage, provided special safety measures are observed, and special piercing connectors make it possible to make branches without stripping the insulation. This speeds up the connection of new subscribers by many times. The technology of laying SIP is shown in detail in the guide SIP cable laying.
Hardware for SIP — anchor brackets, tension clamps, branch piercing connectors — costs more than traditional insulators and hooks, but saves labour costs and increases safety. For bare lines, the qualification of the personnel and compliance with distances are critically important: the PUE sets the minimum clearances to the ground, buildings and trees. If these standards are not observed, a bare conductor becomes a source of increased danger.
Economics: Cost of Materials and Operation
A linear metre of bare aluminium wire is about twice as cheap as a metre of SIP of the same cross-section. However, the total cost of the line consists of the price of the wire, poles, hardware, route clearing work and subsequent maintenance. SIP does not require heavy crossarms and insulators; poles can be lower and lighter; a path does not need to be cut. As a result, the mere elimination of annual branch trimming pays back the difference in cost within a few years.
In rural areas, where lines run through gardens and forests, SIP economically wins unambiguously. In the bare steppe, where spans are large and there is no vegetation, a bare conductor retains the advantage in capital costs. At the same time, the service life of SIP is guaranteed to be 25 years, after which the insulation may degrade under ultraviolet radiation. Steel-aluminium conductors serve 40–50 years if not destroyed by corrosion, but require constant monitoring of the condition of insulators and fasteners.
When to Choose SIP
- Service drops from overhead insulated lines to private houses, dachas, small businesses — safety, speed of installation.
- 0.4 kV distribution networks in populated areas with dense construction and a large number of trees.
- Reconstruction of old overhead lines without replacing poles: SIP is mounted on existing posts.
- Zones with intense ice formation and wind load — clashing is excluded.
- Facilities where uninterrupted operation is important: even if a branch falls, the line is not disconnected.
When to Choose a Bare Conductor
- Backbone lines of 35 kV and above — SIP is not intended for such voltages, except for the limited use of SIP‑3 at 10–20 kV.
- Long spans of more than 100 metres, where high tensile strength is required.
- Remote rural networks with a minimal budget, if a security zone and route clearing can be ensured.
- Temporary lines that do not require insulation durability.
- Operation in desert and steppe terrain with low humidity and without vegetation.
Typical Mistakes in Selection and Laying
- Attempting to tension SIP with the same span as AS. Without calculating the sag and anchor forces, breakage or critical bending occurs.
- Using a bare conductor in a forested area without clearing a path. Falling branches cause frequent failures and fires.
- Saving on the SIP cross-section for a long service entrance. With a load of 15 kW and a distance of 50 metres, a cross-section of 16 mm² can give a voltage drop of more than 5 %, which goes beyond the PUE standards.
- Using simple twists instead of piercing connectors when connecting SIP to a bare conductor. The contact point oxidises, heats up and fails.
- Installing SIP without anchor brackets and tensioning devices — after a year, the insulation wears through at the fixing point.
- Ordering a bare conductor without checking the type: A and AS are confused, which leads to mechanical failure during tensioning.
Frequently Asked Questions
How are SIP‑2 and SIP‑4 deciphered?
SIP‑2 — self-supporting insulated wire with an insulated load-bearing neutral conductor and a steel core. SIP‑4 — without a separate load-bearing element, all four conductors are equally strong aluminium.
Can SIP be used for 380 V voltage?
Yes, SIP‑2 and SIP‑4 are rated for a nominal voltage of 0.6/1 kV, which fully covers a 380 V three-phase network. The cross-section is selected according to the load.
Which wire is better for an overhead service entrance to a house?
For an overhead service entrance, SIP‑4 4×16 or 2×16 is preferable. It is safe, does not require an additional messenger wire, and is installed in one day.
What is the difference between steel-aluminium conductor AS and aluminium A?
AS contains a steel core that bears the mechanical load, which makes it possible to cover large spans. Conductor A is purely aluminium, lighter, but with lower strength.
How to connect SIP to a bare conductor?
Through a special piercing connector or a bimetallic connector. The joint is insulated and protected from moisture. Direct twisting is not permitted due to galvanic corrosion.
Does the SIP insulation affect the cooling of the conductors?
Yes, insulated conductors heat up more at the same current. Therefore, the permissible current for SIP is lower than for a bare wire of a similar cross-section. This is taken into account during design.
Conclusion
The choice between SIP and a bare conductor is a choice between a modern safe distribution network and a backbone technology proven over centuries. Where the voltage is up to 1 kV, where people live nearby and trees grow, SIP has long become the standard. On high-voltage backbones and in uninhabited spaces, bare wire retains its position thanks to mechanical strength and a lower price. A competent project always takes into account the compromise between these two solutions, based on the specific conditions of the route and the requirements for the reliability of the power supply.
