Joining cables

Because underground cables can only be made in limited lengths, high precision joints are used to connect the lengths together. These joints must establish both an electrical connection between the conductors and a connection between the insulation and screening of the cable structure.

A considerable amount of development has gone into the design and testing of these jointing systems, which involve mechanical joints between conductors and chemical bonds between polymer insulators and polymer dielectrics.

Specialist tools are used for clamping the cables into precise orientations and precision cutting the insulation and protective layers to expose the conductor and the various layers ready for installing the different elements of the joint.

This photo shows the cuts required to correctly join cables:

Cable cutting tools and a cable subjected to precision cutting

Even if the cable is laid directly in the ground, each cable joint is protected in a jointing bay.

This jointing bay connects cables at Madrid's Barajas airport, where cables were used to replace overhead lines which might have interfered with aircraft approaches.

Jointing bay

The cables to be joined are prepared and joined in a temporary building set up around the cable ends. The building contains an extremely clean and well-controlled environment in which the complicated process of installing the cable joints can be carried out.

Temporary cable jointing building

Inside of a cable joining room

Another important type of joint for an underground cable is where the cable interfaces with an overhead line project.
An overhead line can be seen coming from the left hand side and terminating on the pylon. A protective tent has been erected around the equipment being installed to join the two systems together.

An underground cable being joined to an overhead line on a pylon

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