Hong Kong Regulations

USE OF ELECTRIC POWER ON THE TRAMWAY RULES - RULE 5

Where any rails on which cars run, or any conductors laid between or within 1
m of such rails, form any part of a return such part may be uninsulated. All
other returns or parts of a return shall be insulated, unless of such
sectional area as will reduce the difference of potential between the ends of
the uninsulated portion of the return below the limit laid down in rule 9.
(L.N. 156 of 1977)