Which of the following must be provided with supplementary corrosion protection?

Supplementary corrosion protection is essential for certain wiring methods and materials to ensure longevity and safety. When wiring is embedded or encased in concrete, or when it is in direct contact with the earth, it is exposed to moisture and varying pH levels, which can lead to accelerated corrosion. Concrete can hold water that may lead to corrosion of the conductors, while direct contact with the earth creates an environment where oxidation can occur, especially in metal components.

Implementing supplementary corrosion protection, such as coatings, wraps, or using corrosion-resistant materials, helps in mitigating these risks, thereby extending the lifespan of the wiring systems in these environments. Options like contact with energized elements may involve different protective measures, but they do not inherently require the same type of corrosion protection as those that are directly interacting with concrete or earth conditions. This focus on environmental factors is why the answer includes both scenarios a and b, which highlights where additional protection is necessary.