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Section 110.27 of the NEC covers the guarding of live parts. As per 110.27(B), when installing equipment in locations where electrical equipment is likely to be exposed to physical damage, "enclosures or guards shall be so arranged and of such strength as to prevent such damage."
Section 230.50(A) covers protection against physical damage for underground service-entrance conductors. This requirement references Section 300.5, which focuses on underground installations. More specifically, 300.5(4) notes "where the enclosure or raceway is subject to physical damage, the conductors shall be installed in rigid metal conduit, intermediate metal conduit, Schedule 80 PVC conduit, or equivalent. Although this installation looks like it met this rule originally, it was still damaged. Go figure!