The strategic use of standardized voltage definitions is paramount in high-voltage engineering. These definitions, derived directly from Clause 2.1 of IEC 183, establish a common technical language that ensures seamless compatibility between the selected cable, its accessories (such as joints and terminations), and the broader electrical system, including switchgear and transformers.

• U₀: The rated r.m.s. power-frequency voltage between each conductor and screen or sheath. This is commonly referred to as the phase-to-ground voltage.
• U: The rated r.m.s. power-frequency voltage between any two conductors. This is commonly referred to as the phase-to-phase voltage.
• Um: The maximum r.m.s. power-frequency voltage between any two conductors for which the equipment is designed. This represents the highest voltage that can be sustained under normal operating conditions at any time and at any point in the system. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads.
• Up: The peak value of the lightning impulse withstand voltage between each conductor and screen or sheath for which cables and accessories are designed.

To provide clear and unambiguous guidance, cables are designated using the standard convention U₀/U (Um). This format ensures that system designers can readily confirm compatibility with associated equipment like switchgear and transformers.

These definitions form the theoretical basis for matching a cable to its intended application; the next step is to use them to align the cable’s rating with the project’s specific system voltage.