Trystan Casey
Electrical cables are used to transmit electrical energy and signals, and they come in a variety of designs and materials. The third part of an electrical cable is the shield or armour, which is an optional layer that protects the cable from external interference and possible external aggressions. This part of the cable can be made of metal and helps to isolate the signals passing through the interior of the cable, ensuring the cable's safety and longevity.
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What You'll Learn
Electrical cable shielding
Metal Screens
Metal screens are electrical metal shields applied to electrical cables to isolate the signals passing through them from external interference. They are particularly useful in industrial, engineering, or manufacturing settings where electromagnetic interference (EMI) is prevalent due to electric motors, switches, and high levels of electrical power. By using screened cables, the transfer of data or power is insulated and protected from this "electrical noise," ensuring reliable performance.
Armours
Armours refer to the mechanical protections of electrical cables. They shield the cables from external threats, such as physical damage, animals, or environmental factors. Armours are typically made of layers of galvanised steel wires or two layers of metal tape, providing mechanical strength and protecting the cable from moisture, corrosion, dirt, and dust.
Sheaths
Sheaths are another form of shielding used in electrical cables, providing an outer protective layer. They are designed to prevent moisture ingress and safeguard the cables from external influences like chemical or electrochemical attacks and fire. Lead alloy is commonly used for cable sheaths, especially in submarine and damp soil environments, due to its ability to withstand internal pressures. However, lead sheaths are susceptible to corrosion and electrolyte damage when laid directly on the ground. To mitigate this, layers of fibrous material like paper, hessian, or polyvinyl chloride are applied to protect the cable from corrosion.
Braid, Spiral, and Foil Shields
In addition to the broad categories of shielding mentioned above, there are three common types of shields used in multi-conductor cables: braid, spiral, and foil. A braided shield is made of interwoven conductive wire and offers flexibility in terms of materials and wire sizes. Spiral shields, on the other hand, are constructed by helically wrapping conductive wire around the core of the cable. Foil shields are metallic foils, often aluminium laminated with a polyester backing, wrapped helically around the cable with an overlap. These different shield types can be combined, such as stacking foil and braid, to maximise shielding effectiveness for specific applications.
Overall, electrical cable shielding plays a critical role in maintaining signal integrity, protecting against external interference, and ensuring the safe and reliable transmission of electrical energy in various environments and conditions.
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Cable screens
There are several types of cable screens, each with its own advantages and disadvantages. Lapped wire screens, for example, are made of wires helically wound onto the cable. They are flexible but have poor screening effectiveness and are inductive at high frequencies, limiting their use to audio applications. Single braid screens, on the other hand, consist of a wire woven into a braid, providing a metallic frame covering the cable. While this type of screen offers good coverage and reasonable high-frequency performance, it adds significantly to the weight and stiffness of the cable.
The performance of cable screens depends on their construction and the type of shielding used. Shielding can be unshielded, foil shielding, braid shielding (outer layer only), twisted pair, or twisted pair with individual shielding in quads. The type of shielding chosen will depend on the specific application and the level of protection required from external interference.
Proper termination is critical for the effectiveness of cable screens. A common method is a 360-degree peripheral termination, which uses a conductive gland and ferrule to clamp over the cable screen. This ensures that the screen makes full contact with the connector shell. Another type of termination is the pigtail connection, commonly used for data cables due to its ease of assembly. However, at high frequencies, the pigtail inductance can degrade performance.
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Cable armour
There are several different types of cable armour, including:
- Steel Wire Armour (SWA): This is a hard-wearing power cable used for the delivery of mains electricity. It is commonly used in underground systems, power networks, and cable ducting.
- Aluminum Wire Armour: This type of armour is used in single-core cables and is suitable for electrical power networks, underground installations, and indoor and outdoor applications.
- Wire Braid Armour: Also known as basket-weave armour, this type of armour is used when light and flexible protection is needed. It is constructed like a fibrous braid, with the metal knitted directly over the cable as the outer covering.
- Steel Tape Armour (STA): STA cables are suitable for buried laying and can withstand external mechanical forces, but they cannot withstand large tensile forces.
The type of cable armour used depends on the specific application and environment in which the cable will be used. Cable armour can increase the life of the cable, improving its performance, reliability, and safety.
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Cable insulation
There are many kinds of wire and cable insulation materials available, and performance varies depending on the use case. The three main insulation materials are plastic, rubber, and fluoropolymer. Plastic insulation can be further divided into two categories: thermoplastic and thermosetting insulations. Thermoplastic insulation is any material that does not undergo chemical transformations during the application. The most common thermoplastic insulation is PVC, which has a maximum service temperature of 70°C and is known for its resistance to flame, moisture, and abrasion. Thermosetting insulation, on the other hand, undergoes chemical transformation during extrusion, resulting in a more stable compound with a higher maximum operating temperature of 90°C. Common thermosetting insulation materials include XLPE and EPR.
Polyethylene (PE) insulation is another popular option, known for its exemplary electric qualities. It is often used in coaxial and low-capacitance cables and is suitable for operating temperatures ranging from -65°C to +80°C. However, PE insulation is stiff, hard, and inflexible, and it is flammable. Polypropylene (PP) insulation is similar to PE but has a wider temperature range of -30°C to +105°C.
Insulated wires and cables also come in different colours, which is beneficial when multiple wires are running together, as it helps differentiate between them. Insulated wires are also corrosion and crush-resistant and offer protection against wetness. Some insulated wires, such as HALAR and KYNAR, are highly resistant to chemicals and are suitable for use in water and damp environments.
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Cable conductors
Conductors are available in various forms, including solid wires or multiple bare wires arranged around a central wire. The number of wires in a conductor follows a specific pattern, with 7, 19, 37, 61, 91, etc. The size of the conductor is denoted by a combination of these numbers and a letter representing the diameter of the individual wires, such as 7/A, 19/B, 37/C, and so on.
The choice of conductor material depends on the specific application. Copper conductors are preferred for their excellent electrical conductivity and are used in energy cables, signal cables, and communication cables. On the other hand, aluminium conductors are 70% lighter than copper, making them suitable for power transmission installations, both indoors and outdoors.
To enhance the properties of conductors, they can be coated with nickel to improve corrosion resistance and expand the operating temperature range. Tinning is another technique used to improve water resistance, increase conductivity, and facilitate soldering. Tinned conductors are more expensive but offer longer lifespans, making them ideal for electrical and electronic equipment across various industries.
In addition to copper and aluminium, other materials used in cable conductors include steel and high-strength alloys. Steel-core aluminium conductors are utilised in high-voltage overhead line cables, while alloys provide an option for challenging applications where durability is a priority.
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Frequently asked questions
An electrical cable consists of multiple wires encased in a single covering. The four layers of an electrical cable are:
- Conductor: Channels the electrical flow.
- Insulation: Surrounds the conductor and insulates it from external contacts.
- Screen: Electrical metal protection that isolates the signals that pass through the interior of the cable from external interference.
- Armour: Mechanical protection that protects the cable from external damage.
The third part of an electrical cable is the screen. It is an electrical metal protection that isolates the signals that pass through the cable from external interference.
There are various types of electrical cables, including:
- Three-wire cables: These are commonly used in modern electrical installations and consist of three insulated conductors: the live, neutral, and ground wires.
- Control cables: These are flexible cables used for small household appliances, signalling and control systems, and power supply connections.
- Coaxial cables: These prevent interference in cables that carry high-frequency, multi-megahertz signals.
- Fiber-optic cables: These align fibres to allow light to pass through, enabling quicker connections and disconnections.
The materials used in electrical cables can vary depending on the specific application and requirements. Commonly used materials include:
- Copper: A highly conductive material that is commonly used for conductors due to its durability, flexibility, and compatibility with various plating options.
- Aluminium: Offers lower energy conductivity but has the advantage of being lighter weight compared to copper.
- Insulation: Materials such as polymer, plastic, or elastomer are used to insulate the conductor. Thermoplastic and thermosetting insulations are also commonly used.
