The selection of a cable’s insulation level is not an absolute choice; it is directly tied to the electrical system’s fault management strategy and its ability to clear earth faults. IEC 183, in Clause 4.2, provides a clear framework for categorizing systems to ensure the selected cable insulation can withstand the voltage stresses imposed during these fault conditions. The project must be classified into one of the following three categories:

1. Category A: This category includes systems where earth faults are cleared as rapidly as possible, and in any case, within 1 minute.
2. Category B: This category comprises systems that, under fault conditions, are designed to continue operation for a short time with one phase earthed. This period should generally not exceed 1 hour, although a longer period may be tolerated as per the relevant cable standard.
3. Category C: This category includes all systems that do not fall into Category A or B.

Action Required: Project documentation must state the system’s designated category (A, B, or C) and provide a corresponding justification based on the documented capabilities of the system’s fault-clearing protection schemes.

Once the system category and corresponding insulation level requirements are determined, the final compliance checks involve the physical conductor and its system terminations.