Lightning strikes

Any tall ruin will attract lightning, and the risk increases in open situations, where the ruin is at a relatively high elevation. Lightning produces between 10 and 100 kA in a period of less than one-hundredth of a second, and in a poor conductor has a significant heating effect. Trees hit by lightning, being poor conductors of electricity, sometimes split due to the tracking path being partly internal, where there is a reserve of moisture, and instant vaporisation into steam creates an explosive force. On occasions the heat generated will also set the tree afire. Hence, stainless steel reinforcing rods will heat up with a lightning strike, and if epoxy grouted in position, the heat generated will melt the epoxy. If the reinforcing rods are in a group then the current will flow through all rods, simultaneously generating a powerful magnetic field, which will pull the rods violently together through the melting epoxy. In a worst case scenario, this sudden force could split the top and bottom column sections longitudinally into several fragments. In a lesser case scenario, the discharge of current at the base is likely to track across the surface of the stone pedestal, and if marble, some electrolysis may occur with consequent surface damage.

Conductors should be external to the monument in question, and solid copper for preference, as any hollow formation will collapse inwards. In sandy soil the best earthing is a buried copper grid, but this may need to be quite extensive and may be impracticable in an archaeologically sensitive context.

Omission of a conductor will sooner or later lead to lightning causing some damage, as was caused by a strike in 2003 to a conical roof to Cesis Castle in Latvia (Figure 2.14), which had been replaced in the 1890s. Little damage has been caused in this case, as the vertical transmission of the electrical energy has dissipated into several of the sloping timber props. Although nominally dry, residual moisture in the timber has been vaporised from those parts of the timber retaining the most moisture, such as the central pith, and at the cross beam intersections.

Both of the props nearest the camera show the splintering effect caused by the sudden release of energy as moisture turns instantaneously to steam.

Figure 2.14 Evidence of lightning strike, Cesis Castle, Latvia.

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