Abstract: A safety system for sensing the position of a load being carried by a crane, relative to the boom thereof. It comprises a self-powered radio transmitter positioned on the end of the boom of the crane, and an on-off switch to be engaged by a load-carrying block or hook whenever the load is in close proximity to the boom.The system further comprises a radio receiver within the cab of the crane, such receiver being connected in circuit with suitable warning devices to energize the same upon receipt of a radio signal from the transmitter.