Hong Kong Regulations

USE OF ELECTRIC POWER ON THE TRAMWAY RULES - RULE 9

When the return is partly or entirely uninsulated a continuous record shall be
kept by the company of the difference of potential during the working of the
tramway between the points of the uninsulated return farthest from and nearest
to the generating station. If at any time such difference of potential exceeds
the limit of seven volts, the company shall take immediate steps to reduce it
below that limit.