A thorough analysis of all service and installation conditions is mandated by Clause 3 of IEC 183 and is essential for determining the appropriate cable design. These environmental, electrical, and mechanical parameters directly influence the cable’s performance, expected service life, and overall system safety. Each factor must be precisely documented in the project specification to ensure the selected cable is fit for purpose.

4.1. Operating Conditions Specification

To create a complete specification, you must define and document the following critical operating factors:

1. Nominal and Highest System Voltage: The primary electrical parameters of the system (U and Um) which the cable will serve.
2. Lightning Overvoltage (Up): The expected lightning overvoltage level, which determines the required impulse withstand capability of the cable insulation.
3. System Frequency: The standard operating frequency of the electrical system (e.g., 50 Hz or 60 Hz).
4. System Earthing Type: The method of system earthing (e.g., solidly earthed, resistance earthed, unearthed) and the maximum permitted duration of earth fault conditions, both for a single occurrence and annually.
5. Maximum Rated Current: The current load profile, specifying if operation is continuous, cyclic, or includes defined emergency or overload conditions. A detailed load curve is essential if cyclic loading is a factor in determining the conductor size.
6. Short-Circuit Currents: The expected symmetrical and asymmetrical short-circuit currents that may flow through the cable, along with the maximum duration for which these currents are expected to persist.
7. Termination Environment: The specific environmental conditions at the cable termination points, including altitude (especially if above 1000 m), whether the termination is indoor or outdoor, the expected level of atmospheric pollution, and if termination will occur within SF₆ switchgear.

4.2. Installation Conditions Specification

General Installation Data

The following general data regarding the cable route and installation method must be specified:

• Route Details: The total length and detailed profile of the cable route.
• Laying Arrangements: The specified formation (e.g., flat, trefoil).
• Bonding Method: The specified method of connecting the cable’s metallic coverings (sheath, screen, armour) to each other and to earth.
• Special Conditions: Any unique installation conditions, such as laying cables in water, which require individual consideration.

Underground Cable Installations

For projects involving cables installed underground, the following specifications are required:

• Installation Method: Specify if the cable will be directly buried or installed in ducts, troughs, or pipes.
• Protective Layers: Detail the required composition of the metallic sheath and armour, including any special properties needed (e.g., anti-corrosion, flame-retarding, or anti-termite).
• Depth of Laying: The specified burial depth.
• Soil Conditions: The thermal resistivities and types of soil along the entire route. This information should be based on measurement and inspection where possible.
  • Practical Insight: Assumed book values for soil thermal resistivity are a common source of error leading to under-sized cables or unnecessary de-rating. Whenever feasible, insist on field measurements along the proposed route, especially where soil composition is known to vary.
• Ground Temperature: The minimum and average ground temperature at the depth of burial.
• Proximity to Heat Sources: Details of any proximity to other load-carrying cables or external heat sources.
• Duct/Pipe Details: The number of ducts or pipes, their internal diameter, the spacing between them, and their material.

In-Air Cable Installations

For projects involving cables installed in air (e.g., on racks, in tunnels), the following specifications are required:

• Ambient Air Temperature: The minimum, maximum, and average ambient air temperature to be assumed.
• Installation Type: The type of installation (e.g., on walls, on racks, in tunnels) and the grouping arrangements of the cables.
• Ventilation: Details of the ventilation system for indoor installations.
• Sunlight Exposure: Specify whether the cable will be exposed to direct sunlight.
• Special Conditions: Any unique conditions, such as areas with a significant fire risk.

The comprehensive data gathered on service conditions, particularly regarding earth fault duration, directly informs the next critical decision: selecting the appropriate cable insulation level.