The selection of an appropriate high-voltage cable requires a detailed analysis of numerous operating, installation, and design factors.

4.1. Operating and Service Conditions (Clause 3.1)

The following information must be considered to determine the appropriate cable type:

• Nominal voltage of the system.
• Highest voltage of the three-phase system.
• Lightning overvoltage.
• System frequency.
• Type of earthing and the maximum permitted duration of earth fault conditions.
• Environmental conditions where terminals are specified (e.g., altitude above 1000 m, indoor/outdoor installation, expected atmospheric pollution, use in SF₆ switchgear).
• Maximum rated current, specified for continuous, cyclic, or emergency/overload operation. A load curve is essential if cyclic loading is considered.
• The expected symmetrical and asymmetrical short-circuit currents.
• Maximum time for which short-circuit currents may flow.

4.2. Installation Conditions (Clause 3.2)

Detailed data on the installation environment is crucial for cable selection.

Underground Cables:

• Installation conditions (e.g., direct burial, in ducts).
• Requirements for armour, anti-corrosion, flame-retardant, or anti-termite properties.
• Depth of laying.
• Thermal resistivities and types of soil (e.g., sand, clay).
• Minimum, maximum, and average ground temperature at the burial depth.
• Proximity to other load-carrying cables or heat sources.
• Number, material, and internal diameter of ducts or pipes.

Cables in Air:

• Minimum, maximum, and average ambient air temperature.
• Type of installation (e.g., direct laying on walls, racks, grouping of cables).
• Details of ventilation for indoor, tunnel, or duct installations.
• Whether exposed to direct sunlight or a specific fire risk.

4.3. Conductor Size Selection (Clause 5)

The choice of conductor size must account for the following factors:

1. Maximum Temperature: The temperature occurring in the cable under specified continuous, cyclic, or emergency load conditions. IEC Publication 287 is referenced for current rating calculation procedures.
2. Mechanical Loads: Forces imposed on the conductor during installation and throughout its service life.
3. Electrical Stress: A small-diameter conductor may result in unacceptably high electrical stress in the insulation.

Conductor sizes should be chosen from standard series, such as those in IEC Publication 228.

4.4. Termination Design (Clause 6)

The design of cable terminations is dependent on:

• Withstand Voltages: The required power-frequency and impulse withstand voltages, which may differ from those of the cable itself.
• Atmospheric Pollution: The degree of exposure to pollution determines the minimum creepage distances and the type of insulators to be used for cable sealing ends.
• Altitude: At high altitudes (above 1000 m), the electric strength of air is reduced due to lower air density. To ensure reliable performance, air clearances must be increased by a suitable amount.