A significant evolution in the standard is the move from two to three system categories to better account for modern cable dielectrics, varying overvoltage performance, and the increased likelihood of extended overvoltage during earth fault conditions in new systems.

System Categories (Clause 4.2)

• Category A: Systems where earth faults are cleared as rapidly as possible, in any case within 1 minute.
• Category B: Systems that, under fault conditions, operate for a short time with one phase earthed. This period should generally not exceed 1 hour, although longer periods may be tolerated if specified in the relevant cable standard.
• Category C: Includes all systems that do not fall into Category A or B. A note advises that if an earth fault is not automatically and promptly eliminated, the extra stress reduces cable life. In such cases, it may be more economical to classify the system as Category C, even if it could technically operate under Category B conditions for short periods.