The Efficiency of Nonmagnetic Shields on Control and Communication Cable

Five formulas proposed at various times by King, Lyons, Schelkunoff, and Terman to calculate the magnetic field shielding efficiency of nonmagnetic shields have been compared. Data from two metal sheaths, one welded corrugated tube, three tape shields, and one braided shield show that King´s low-frequency formula is the only one applicable at frequencies such that skin depth is greater than or equal to shield thickness. When skin depth is less than shield thickness, the mathematically equivalent full- frequency formulas of Lyons and Schelkunoff are suitable; they are inapplicable at lower frequencies. When skin depth is less than or equal to 0.5-inch shield thickness, the mathematically equivalent Terman´s radio frequency or King´s asymptotic formulas are suitable.