What provision must individual conductors of an underground high voltage cable have?

Each individual conductor of an underground high voltage cable must have metallic shielding around it to ensure safety and reliability. Metallic shielding serves several important purposes. Firstly, it helps to contain the electric field within the conductor, which minimizes the risk of electric shock or inadvertent contact with the charged conductor. Secondly, it protects the conductor from external interference, such as moisture or other environmental factors that could lead to insulation breakdown or electrical faults.

The use of metallic shielding also enhances the overall performance of the cable by reducing electromagnetic interference, which is particularly important in underground applications where other equipment may be in close proximity. Additionally, the metallic shielding can provide a ground path that helps in the event of a fault, improving safety for both personnel and equipment.

The other options like insulation and rubber coating do not adequately address the specific needs that come from operating at high voltage underground, nor do they provide the same level of protection and performance that metallic shielding does. Thus, metallic shielding is a critical provision for the safe operation of underground high voltage cables.