Telephone cable: types, marking, device and installation

Telephone Cable: Types, Marking, Design and Installation

The telephone cable has accompanied the development of communications from the first overhead lines to fibre‑optic backbones. Even in the era of mobile communication and the internet, copper distribution networks reliably deliver the signal from the exchange to apartments, offices and industrial facilities. In this article we will examine how a telephone wire is constructed, what types are available on the market, how the types TPP, ShTLP, P‑274M and UTP differ, and how to correctly perform installation and connection. The full range of urban and trunk models is presented in the telephone cable section.

1. What Is a Telephone Cable and What Types Exist

A telephone cable is a symmetrical transmission line for low‑frequency signals, rated for voltages up to 200–300 V DC or AC at voice frequency. Unlike a power cable, a telephone conductor transmits an information signal, not energy. The current in the circuit is measured in milliamperes, and the conductor cross‑section rarely exceeds 1.2 mm². By the number of twisted pairs and sheath construction, the following are distinguished:

• single‑pair distribution wire for subscriber wiring – often called “noodles” because of its flat cross‑section;
• multi‑pair trunk and urban cable, combining from 5 to hundreds of pairs in a single sheath;
• field military wire, mechanically strong and frost‑resistant;
• twisted pair for computer networks, which historically originated from telephone lines.

The key difference between a telephone cable and a power cable lies in the standardised characteristic impedance and low capacitance, which allow the voice spectrum of 300–3400 Hz to be transmitted without critical distortion.

2. History: From Soviet Lines to Modern Networks

The old Soviet telephone cable was manufactured in accordance with GOST as early as the 1960s. The most mass‑produced type was TPP – telephone, with polyethylene insulation of the conductors and a polyethylene sheath. It was laid in cable ducts, on poles and in the ground. The second landmark type remains P‑274M – a field long‑distance communication wire widely used in the army and geological survey parties. It is distinguished by high mechanical strength, copper‑clad steel current‑carrying conductors and frost‑resistant insulation.

With the spread of the internet, UTP twisted pair came into apartments, inheriting many design solutions of telephone cables. However, the classic two‑core distribution wire is still actively used by Rostelecom and other fixed‑line operators to connect subscribers to multi‑service nodes.

3. Classification by Design and Application

The diversity of telephone cables can be reduced to several groups.

3.1. By Number of Cores and Pairs

• two‑core – 1 pair, usually for subscriber wiring from the distribution box to the socket;
• four‑core – 2 pairs, for connecting two telephone numbers or system phones;
• multi‑pair – 5 pairs, 10 pairs, 30, 50 and more, for trunk lines.

3.2. By Type of Insulation and Protective Coverings

• with polyethylene insulation – types TPP, TPPep, TPPepZ, possessing low dielectric permittivity;
• with PVC insulation – TPVng, non‑flame propagating;
• armoured – with tape armour for underground laying, e.g. TPPepB;
• shielded – with an overall screen of aluminium foil or copper braid, types TPPep, FTP.

3.3. By Area of Application

• urban telephone cable – for laying in cable ducts, collectors and along building walls;
• trunk – multi‑pair, connecting exchanges to each other;
• distribution – from the distribution cabinet to the subscriber;
• indoor – flat wire ShTLP 4 white, run along the skirting board to the telephone socket;
• field military – mechanically strong, operating at low temperatures;
• connecting – short cords with RJ11 or RJ45 connectors for connecting devices.

Some models contain a supporting messenger wire for aerial suspension – this is how an outdoor wire running from a pole to a house is arranged.

4. Marking and Decoding of Common Types

The alphanumeric designation of a telephone cable carries information about the materials, construction and performance properties. Let us consider several examples.

• TPpep 10×2×0.5: T – telephone, P – polyethylene core insulation, P – polyethylene sheath, ep – screen of aluminium‑polyethylene tape. 10×2×0.5 means 10 pairs of conductors with a diameter of 0.5 mm;
• TPpepZ: the index “Z” indicates filling of the core with a hydrophobic compound, thanks to which the cable can be laid in damp manholes and the ground without additional sealing;
• TPpep ndg: with a sheath that does not propagate flame;
• TPVng: insulation and sheath made of PVC compound that does not support combustion during group laying;
• ShTLP 4 white: telephone line flat cord, 4 cores, white colour. ShTLP 4 cores cu – version with copper cores;
• P‑274M: field long‑distance communication wire, modernised;
• UTP: unshielded twisted pair used for voice and data transmission. The utp communication cable is a variant of the telephone cable adapted for high‑speed protocols.

A detailed review of the TPP types of all modifications and their compliance with GOST 31943‑2012 is presented in the article in detail about TPP cable.

5. Construction and Materials Used

The current‑carrying conductors are made of soft copper wire. In distribution and trunk cables, the conductor diameter is 0.32, 0.4, 0.5, 0.64 or 0.7 mm. In field models such as P‑274M, the conductor is steel, clad with copper, which gives high tensile strength. The insulation of each conductor is made of solid or cellular polyethylene; the colour coding complies with IEC 60304. The conductors are twisted into pairs with a pitch that differs between adjacent pairs to reduce cross‑talk. Then the pairs are twisted into elementary bundles of 5 or 10 pairs, forming the core.

Over the core, a binding tape, a screen of aluminium‑polymer tape with a drain wire, and then a sheath of polyethylene or PVC are applied. Armoured versions receive a layer of steel tapes and an outer protective jacket. Cables with a messenger wire have an integrated steel wire in the common sheath for aerial installation.

6. Technical Characteristics of Typical Representatives

Below are summarised the parameters of several popular types. The data are based on the technical specifications and factory certificates.

The loop resistance of conductors with a diameter of 0.5 mm is about 200 Ω/km. To stabilise parameters in trunk cables, the pairs may have thicker insulation or a different twist pitch. The internet speed using ADSL technology reaches 24 Mbit/s on short lines, but falls rapidly as the length increases.

7. Pair Marking and Pinout

In a multi‑pair cable, each pair has its own count colour. The tip conductor is coloured white, red, blue or another colour, the ring conductor – blue, green, brown and others. In ShTLP 4 white cords, red, green, yellow and black cores are encountered. To connect to an RJ11 socket or RJ11 plug, the middle contacts 2 and 3 are used for one line; two lines use contacts 2–5 and 3–4. The device can be connected to the line using an RJ11 plug or the old RTShK‑4 plug, which is still found in Soviet‑built houses. The wire pinout does not tolerate errors – reversing a pair leads to a loss of signal.

8. How to Crimp a Telephone Cable and Install a Joint

Crimping of an RJ11 plug is performed with a crimping tool. First, using a stripper, remove the outer sheath to a length of 15–20 mm, straighten the cores in the order corresponding to the chosen pinout, trim them to an even length and insert them into the connector as far as they will go. After crimping, the contact blades pierce the insulation and are fixed on the copper. The resulting cord is checked with a tester.

The connection of two lengths of trunk cable is made using a heat‑shrinkable joint. The preparation is carried out stepwise, the cores are joined by twisting or crimping, each pair is insulated with a tube, after which the joint is heated until complete shrinkage. Restoration of the screen and armour is mandatory. If a break has occurred on a subscriber line, the repair often comes down to installing a junction box where the cores are fixed with insulation displacement contacts.

9. Installation: Underground, Aerial and Indoor Routes

Laying of a telephone cable in a communication duct is carried out in asbestos‑cement or polyethylene pipes. The cable is pulled in using a steel draw wire, avoiding twisting and exceeding the permissible pulling force. When laying in the ground, the trench depth for an armoured cable is 0.7–1.2 m. A subscriber two‑core wire is often run overhead from a pole, using a messenger wire suspension. Inside the building, TPVng or ShTLP is laid along skirting boards, fixed with clips at intervals of 30–50 cm. The distance to power cables is not less than 0.5 m, to heating pipes – 1 m. More details on the rules of joint laying with other networks are given in the guide utp communication cable.

10. Selection Tips

To choose a telephone cable correctly, several criteria are used:

• application: for indoor subscriber wiring, ShTLP 4 cores cu is sufficient, for an office network – UTP 5e, for outdoor use – TPPep with a messenger wire;
• number of pairs: one pair, if only a telephone is needed; two pairs for the possibility of connecting a second line or fax;
• installation conditions: armoured for underground, with hydrophobic filling for damp basements, ng‑LS for administrative buildings;
• compatibility with old telephone sets: a Soviet set with a rotary dial works with any copper wire, but for digital system phones a matched impedance is important.

11. Optical Fibre and Copper Lines Today

Fibre‑optic cable is actively displacing copper telephony on trunk sections, but the last mile in many regions is still copper. Providers such as Rostelecom bring optical fibre to the distribution cabinet and then run twisted pair into the apartment. This combination preserves the familiar telephony and provides high‑speed internet. The ordinary urban cable TPPep 10×2×0.5 continues to be manufactured and installed in new residential complexes for intercom systems and backup communication channels.

12. Conclusion

The telephone cable has come a long way from varnished tape and overhead wires to modern symmetrical pairs with hydrophobic filling. Understanding the marking, design differences and installation rules makes it possible to competently deploy a distribution network, connect a subscriber or restore a damaged line. The range of copper conductors – from field “polevka” to shielded multi‑pair – covers any fixed‑line communication tasks. You can view the catalogue on the telephone cable page.