General Order 64-A
Supply Lines And Communication Lines Under Structures
In the case of trolley contact wires run under steel structures, properly insulated trolley troughs or equivalent protection shall be installed to prevent contact between the trolley pole and the structure in case the trolley slips from the contact wire. This does not apply where pantagraph is used or where the clearance is sufficient to eliminate the possibility of such accidental contact. No clearance is specified between trolley contact wires and structures under which they pass.