Cable Joint Types & Jointing Techniques

Cable joints are essential components in any electrical system, acting as the critical link between separate cable lengths. They ensure a continuous flow of electricity by securely joining conductors and providing insulation and protection against the elements. Using the correct cable joint for your specific cable type – whether it’s coaxial for your TV, fibre optic for high-speed internet, or high-voltage power cables – is paramount. The right joint guarantees a reliable, long-lasting, and safe electrical connection, minimising the risk of outages, signal degradation, or even dangerous electrical defects or hazards. A poorly chosen or installed joint can compromise the entire system, leading to costly repairs and downtime.

This guide explores cable joints, helping you understand the different types, their applications, and best practices for ensuring robust and dependable electrical connections.

Electrical cable joints are essential components in electrical systems, used to connect, extend, or repair cables. Here are the main types of electrical cable joints:

A married joint, also known as a twisted joint, is created by joining two or more electrical conductors together. This type of joint is typically used for multi-strand cables. The process involves intertwining the wires of one cable with those of another, twisting them together, and then soldering for a secure connection. Married joints are used in applications requiring good electrical conductivity and compactness, but they offer less mechanical strength.

The T joint, or tee joint, is used to connect a branch conductor to a main conductor where the main conductor continues beyond the branch. This type of joint is commonly used in overhead distribution lines to tap electrical energy for service connections. It allows for the distribution of power or signals into multiple pathways, facilitating network expansion.

Straight joints, also known as straight-through joints, are designed to connect two lengths of cable with the same diameter and type. These joints ensure a seamless and continuous electrical path and are critical in applications where cable lengths need to be extended without compromising the integrity or performance of the cable system.

The rat-tail joint, also called a twist splice or pig-tail splice, is a basic electrical splice that can be done with both solid and stranded wire. It involves wrapping two or more bare wires together symmetrically around their common axis. While simple to create, this joint is not mechanically strong and is typically used for wires protected inside an enclosure or junction box.

The Britannia joint is a type of compression joint used to connect electrical cables. It consists of two metal plates bolted together, with a rubber seal between them, providing a watertight seal and electrical insulation. This joint is commonly used in high-voltage power lines and industrial/commercial building wiring.

The Western Union joint, also known as a lineman’s splice, is named after its use in telegraph wire connections. This joint is designed to create a strong connection capable of supporting long lengths of heavy wire. It’s particularly useful in applications where the connection must withstand mechanical stress.

A solder lock joint typically refers to a connection where wires are joined and then soldered together for added strength and conductivity. This method is often used in electronics and electrical work to create secure and conductive connections.

These various types of electrical cable joints serve different purposes and are chosen based on the application, environment, cable type, and required mechanical strength.

For connecting different cable types, use transition joints when connecting cables of different materials, sizes, or insulation types. These joints are crucial when upgrading or expanding existing networks with new cable types.

The voltage rating of the cable joint must match or exceed that of the cable:

• Low Voltage (LV) Joints: Used for cables up to 1000V.
• Medium Voltage (MV) Joints: Typically used for cables between 1kV and 33kV.
• High Voltage (HV) Joints: For cables above 33kV.

For example, a 6.6kV unearthed system requires a cable joint insulated for 6600/11000V to withstand potential earth-fault conditions.

The joint must be able to handle the cable’s ampacity (current-carrying capacity):

• Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating.
• Joints should be sized to match or exceed the cable’s ampacity to prevent overheating and deterioration.

Cable joints must withstand short-circuit currents:

• The short-circuit current rating is the maximum current a component can handle during a fault condition.
• Joints should have a short-circuit rating that matches the cable’s ability to withstand high temperatures during a short circuit lasting up to about half a second.

For copper cables, consider the following:

• Conductor Size: Match the joint to the cable’s cross-sectional area. For example, a straight joint kit S50 is suitable for 50mm² 4-core cables.
• Insulation Type: Ensure compatibility with the cable’s insulation material (e.g., XLPE, PVC).
• Installation Environment: Choose joints suitable for the installation conditions (indoor, outdoor, underground, or submerged).

For fibre optic joints, considerations typically include:

• Fibre Count: Ensure the joint can accommodate the number of fibres in the cable.
• Splice Type: Choose between fusion splicing or mechanical splicing based on the application.
• Environmental Protection: Select joints with appropriate ingress protection for the installation environment.

Cable jointing techniques are essential in electrical and telecommunications industries, with both manual and automated methods used depending on the application. Here’s an overview of the main techniques, comparing manual and automated approaches:

Manual jointing relies on skilled technicians and is often used for low-volume or custom applications. Automated jointing, on the other hand, is preferred for high-volume production and offers several advantages:

• Efficiency: Automated systems significantly reduce production time compared to manual methods.
• Consistency: Automated processes ensure high consistency in wire lengths, terminations, and connections.
• Precision: Automated jointing provides more accurate wire cutting, stripping, and termination, leading to more reliable electrical connections.

However, manual jointing remains relevant for its flexibility, lower initial costs, and suitability for field repairs or installations where power sources may be limited.

Soldering involves using a molten metal alloy to join conductors. It’s often used in electronics for its reliability but can be time-consuming.

Crimping is a popular jointing method that involves deforming a connector to create a secure electrical and mechanical bond. It can be done manually or with automated systems:

• Manual crimping: Offers versatility and flexibility for different wire sizes and connector types.
• Automated crimping: Provides higher speed and consistency, especially for high-volume production.

Compression joints use mechanical force to create a secure connection. For example, the Britannia joint uses two metal plates bolted together with a rubber seal, providing a watertight and electrically insulated connection.

1. Manual Jointing:
   1. Pros: Low initial investment, versatility, portability.
   2. Cons: Lower efficiency, potential inconsistency, reliance on operator skill.
2. Automated Jointing:
   1. Pros: High efficiency, consistency, precision, cost-effective for high volumes.
   2. Cons: Higher initial investment, less flexibility for custom applications.
3. Soldering:
   1. Pros: Reliable connections, suitable for delicate electronics.
   2. Cons: Time-consuming, requires skill, potential for cold joints.
4. Crimping:
   1. Pros: Fast, reliable, suitable for various wire sizes.
   2. Cons: Requires specific tools for different connector types.
5. Compression:
   1. Pros: Strong mechanical connection, often weather-resistant.
   2. Cons: May require specialized tools, less suitable for fine wires.

In conclusion, the choice of jointing technique depends on factors such as production volume, application requirements, and available resources. While automated systems offer significant advantages in efficiency and consistency for high-volume production, manual techniques remain valuable for their flexibility and lower initial costs, especially in field applications or low-volume production scenarios.