Effect of Shield Wires on the Lightning-Induced Currents on Buried Cables

The effect of a shield wire, which is a bare conductor buried above an insulated metallic cable, on lightning-induced currents on the cable has been studied using the three-dimensional (3-D) finite-difference time-domain (FDTD) method. In the 3-D FDTD model, a 1-km-long insulated cable is buried at depth 0.6 m in 1850-Ω·m ground, and a 1-km-long shield wire is installed 0.3 m above the cable (at the depth 0.3 m from the ground surface). A 1-km-long vertical lightning return-stroke channel, attached to a 30-m-high tower or flat ground, is located 25 m away from the cable and the shield wire. The validity of this model has been shown by comparing the FDTD-computed lightning-associated currents on the cable and the shield wire with the corresponding measured currents associated with rocket-triggered lightning. The FDTD-computed results show that the presence of the shield wire reduces the induced current on the cable and the voltage across the outer insulation layer of the cable, as expected, but the series resistance of the shield wire has a negligible effect on this reduction. The presence of 30-m-high strike object slightly reduces the cable current. The ground permittivity and the return-stroke speed have negligible effects on the cable current, but the higher the ground resistivity, the higher is the cable current. The increase of bonding points between the cable sheath and the shield wire increases the cable current, but decreases the voltage across the outer insulation layer of the cable.