Overall shield protects instrument cable from the effects of lightning

Lightning can cause damage to electrical systems from direct and induced effects. A study was conducted to devise means to prevent or control the effects of lightning on instrument and control systems. The results indicate that an acceptable approach is to use instrument cables with a relatively thick overall metallic shield grounded at both ends. The overall shield functions as a magnetic shield, not because of the mere presence of metal, but because it generates a counter, or opposition, current that nearly cancels out the interfering current on the signal conductors. The data show that a twisted pair by itself is not adequate to protect against lightning. The overall shield is used in conjunction with a single point ground system to provide this protection