The outer sheath of coax cable used in a communication system must be grounded at the buildings premises. It should be grounded close to the cables point of entry into the building. This grounding is required if the coax is exclusive to accidental contact with conductor carrying what minimum voltage?

The outer sheath of coax cable used in a communication system must be grounded at the buildings premises. It should be grounded close to the cables point of entry into the building. This grounding is required if the coax is exclusive to accidental contact with conductor carrying what minimum voltage?