The standard defines several key voltage ratings to ensure compatibility between cables, accessories, and system equipment like switchgear and transformers.

Voltage Definitions (Clause 2.1)

• U₀: The rated root-mean-square (r.m.s.) power-frequency voltage between each conductor and the screen or sheath for which the cable and its accessories are designed.
• U: The rated r.m.s. power-frequency voltage between any two conductors for which the cable and its accessories are designed.
• Um: The maximum r.m.s. power-frequency voltage between any two conductors that can be sustained under normal operating conditions at any time. It represents the highest voltage for the system.
• Up: The peak value of the lightning impulse withstand voltage between each conductor and the screen or sheath for which the cable and its accessories are designed.

Cables are designated by the format U₀/U (Um) to provide clear guidance on their compatibility with associated equipment.