This paper examines the responses of four different types of shielded conduits to a direct lightning strike. The conduits are buried in the ground and run between two shielded enclosures, one of which is struck by lightning. An induced current flows on the conduit and this induces a voltage on the inner wires within the shield. Using a simple resistive model, this voltage is estimated and the effectiveness of the shielding provided by the various conduits is determined. A key element in the calculations is the transfer impedance of the conduits and an experimental program for determining these quantities is described. From this study, requirements for lightning surge protection devices can be developed