OVERALL SHIELD PROTECTS INSTRUMENT CABLE FROM THE EFFECTS OF LIGHTNING

Lightning can cause damage to electrical systems from direct and induc ed effects. A study was conducted to devise means to prevent or contro l the effects of lightning on instrument and control systems. The resu lts indicate that an acceptable approach is to use instrument cables w ith 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 opposit ion, current that nearly cancels out the interfering current on the si gnal conductors. The data show that a twisted pair by itself is not ad equate to protect against lightning. The overall shield is used in con junction with a single point ground system to provide this protection.