3. Making the Connection: The Voltage Relationship Table
3.1. Understanding Table I
The following table, taken from the standard, clearly links the rated voltage of a cable with the nominal system voltage it’s designed for and the highest voltage it must be able to withstand.
| U₀ (kV) | U (kV) | Um (kV) |
| 1.8 | 3 | 3.6 |
| 3.6 | 6 | 7.2 |
| 6 | 10 | 12 |
| 8.7 | 15 | 17.5 |
| 12 | 20 | 24 |
| 18 | 30, 33 | 36 |
| 26 | 45 | 52 |
| 36 | 60, 66 | 72.5 |
| 64 | 110, 115 | 123 |
| 76 | 132, 138 | 145 |
| 87 | 150, 161 | 170 |
| 127 | 220, 230 | 245 |
| 160 | 275, 287 | 300 |
| 190 | 330, 345 | 362 |
| 220 | 380, 400 | 420 |
| 290 | 500 | 525 |
| 420 | 700, 750 | 765 |
3.2. How to Interpret the Table
This table is a practical tool for matching a cable to a system. It removes guesswork by showing standard, compatible pairings.
For example, look at the row for a cable rated 18/30 kV.
- The U₀ is 18 kV (conductor-to-earth).
- The U is 30 kV (conductor-to-conductor).
- The table shows this cable is intended for a system with a nominal voltage (U) of 30 kV (or 33 kV).
- It also shows the cable can handle a highest system voltage (Um) of 36 kV. This ensures it can safely operate within the system’s normal fluctuations.
While these voltage ratings cover normal operation and predictable variations, it’s also crucial to understand how a cable is chosen based on the system’s behavior during a fault.