Radio waves can induce currents in electrical firing circuits used for blasting in quarries, and for similar industrial purposes, and could cause premature detonation if sufficiently strong. The paper describes an experimental and theoretical study of the currents in typical circuits, and a set of maximum-current curves is presented. These curves form the basis of a set of safe field-strength curves contained in the British Standard guide to the prevention of inadvertent initiation of electro-explosive devices by radio-frequency radiation.