A cable doesn’t exist in isolation; it’s part of a larger electrical system. The system’s own voltage characteristics are just as important as the cable’s ratings.

2.1. System Operating Voltages

• Nominal voltage of system: This is the r.m.s. phase-to-phase voltage that is used to designate or identify a system. When you hear someone refer to a “30 kV system” or a “110 kV system,” they are referring to its nominal voltage. It’s the system’s general classification.
• Highest voltage of a three-phase system: This is the highest r.m.s. phase-to-phase voltage that occurs under normal operating conditions. It’s crucial to consider this value because a system’s voltage isn’t perfectly constant; it fluctuates. The Um rating of a cable must be equal to or greater than the highest voltage of the system it will be installed in. This is precisely why the Um rating exists. It’s the cable’s promise that it can handle the system’s highest normal operating voltage.

2.2. Overvoltage Events

Normal operating voltages are one thing, but electrical systems can also experience brief, extreme voltage spikes. These are considered separately.

• Lightning overvoltage (Up): This is the peak value of a very high, short-duration voltage impulse that a cable is designed to withstand. As the name suggests, it’s often caused by a lightning discharge. These events are unidirectional (all positive or all negative) and last for only a fraction of a second. The cable’s insulation must be robust enough not to fail during such a spike.

To see how these different voltage ratings for cables and systems relate to each other in practice, the standard provides a helpful reference table.