Isn't it impossible to use steel armor for single-core cables? Recently, our company installed a new main transformer (66 thousand), which uses single-core cables, each as thick as a bowl, the most

This is not an armored layer, but a metal shielding layer, which is used to radially block water and pass fault current. Generally made of aluminum or lead, this layer will have induced voltage when working and needs to be grounded.

Single-core AC cable just can't be armored with magnetic materials, and non-magnetic materials or non-magnetic structures are possible.