3. Choosing the Right Armor: Cable Insulation Levels
A cable’s insulation is its primary defense against electrical failure. The “toughness” of this insulation is selected based on how the electrical system responds to an earth fault—a situation where a live conductor accidentally comes into contact with the ground. IEC 183 defines three categories based on how quickly the system can clear such a fault.
| System Category | Description of Fault Condition | What this Means |
| Category A | Earth faults are cleared as rapidly as possible, always within 1 minute. | The system has excellent protection. The cable is not subjected to prolonged fault stress, so standard insulation levels are sufficient. |
| Category B | The system can remain operational for a short time (e.g., up to 1 hour) with a fault. | The system has good protection but allows for temporary operation during a fault. The cable insulation must be tougher to handle this extended overvoltage stress. |
| Category C | Includes all systems where faults may persist for a long time without being automatically cleared. | These systems have limited or no automatic fault clearing. They require the most robust and durable cables to avoid insulation damage from prolonged overvoltage stress during earth faults. |
With the right level of insulation chosen, our focus shifts from the cable’s protective layers to its current-carrying core.