1. First, Let’s Speak the Language: Understanding Key Voltages
Before we can select a cable, we need to understand the language of voltage ratings. The IEC standard defines three primary voltages that form the basis of our selection.
| Voltage Symbol | Simple Definition | Why it Matters |
| U₀ | The rated voltage between one conductor and the ground (or metallic screen/sheath). | This value tells you the cable’s ability to handle the stress between its live core and the earthed components around it. |
| U | The rated voltage between any two conductors in the system. | This is the primary “phase-to-phase” voltage rating and is critical for ensuring the insulation between the live wires is sufficient. |
| Um | The maximum voltage the equipment can sustain under normal operating conditions. | This is the absolute ceiling. The cable you choose must have a Um rating equal to or greater than the highest possible voltage of the system it will be connected to. |
In practice, a cable’s rating is typically expressed in the format U₀/U, such as 8.7/15 kV. This concisely communicates both its phase-to-ground and phase-to-phase voltage capabilities. These voltages are directly related. The standard provides a table that links the cable’s rated voltage (U₀/U) to the system’s highest voltage (Um).
Relationship Between Cable and System Voltages
| Rated Voltage of Cable (U₀/U) in kV | Highest Voltage for Equipment (Um) in kV |
| 8.7/15 | 17.5 |
| 18/30 | 36 |
| 38/66 | 72.5 |
| 76/132 | 145 |
| 190/330 | 362 |
Now that we understand these fundamental terms, we can move on to the practical investigation required to select the right cable.