Every high-voltage cable has a set of fundamental voltage ratings that define what it can safely handle. These are the cable’s intrinsic properties, determined by its materials and construction.

1.1. Defining the Ratings

The IEC 183 standard specifies three primary ratings you must know:

• Rated r.m.s. power-frequency voltage between each conductor and screen or sheath (U₀)
  • Definition: The rated voltage between any single conductor and the “earth” layer of the cable (the metallic screen or sheath).
  • In simple terms: This tells you the strength of the insulation between a live wire and the grounded part of the cable. It’s the “conductor-to-earth” rating.
• Rated r.m.s. power-frequency voltage between any two conductors (U)
  • Definition: The rated voltage between any two conductors in a multi-conductor cable.
  • In simple terms: This rating typically corresponds to the Nominal voltage of the system (which we’ll define next), such as a “30 kV system.” This is the “conductor-to-conductor” rating.
• Maximum r.m.s. power-frequency voltage between any two conductors (Um)
  • Definition: The highest voltage that can be sustained between conductors under normal operating conditions at any time and at any point in a system. It excludes temporary voltage variations due to fault conditions and the sudden disconnection of large loads.
  • In simple terms: This is the absolute maximum voltage the cable can tolerate continuously. It accounts for normal system fluctuations, but not for extreme, short-term spikes from events like faults or major switching operations.

1.2. How the Ratings Work Together

These three ratings are almost always presented together in a standard format: U₀/U (Um). This notation provides a complete picture of the cable’s capabilities and its suitability for a given electrical system.

Now that we understand a cable’s intrinsic ratings, let’s look at the characteristics of the electrical system where the cable will be used.