47. Where there is reason to believe that a load being lifted or lowered on a lifting appliance weighs more than twenty tons its weight shall be ascertained by means of an accurate weighing machine or by the estimation of a person competent for the purpose, and shall be clearly marked on the load:
(a)indicates clearly to the driver or person operating the crane when the load being carried approaches the safe working load of the crane for the radius of the jib at which the load is carried; and
(b)gives an efficient sound signal when the load moved is in excess of the safe working load of the crane at that radius.