A noteworthy example of the fallacy of the ‘area-of-protection’ theory is to be found in the case of the explosion at the powder magazine at the Victoria Colliery, BurntclifFe, Yorkshire, which was struck by lightning and destroyed on August 6, 1878. The instance is also instructive as showing how important it is that copper conductors should possess the highest possible conductivity--i.e. be made of the best and purest copper.
The magazine was an oblong building of brick, nine feet long, five feet wide, and six feet high (internal dimensions), and it had a uniform thickness of three bricks. At one end was a heavy iron door, and at the other a lightning conductor, consisting of a copper-wire rope seven-sixteenths of an inch in diameter. The point of the terminal rod was about thirteen feet above the top of the building, and a similar length was carried into the ground and terminated in clayey soil. The conductor was fixed to a pole distant about two inches from the end of the building opposite to that in which the iron door was fixed. _It was not connected with the iron door in any way._ At the time of the explosion the magazine contained about 2,000 pounds of gunpowder.
Major Majendie, H.M.’s Chief Inspector of Explosives, in his official report ascribed the accident to the fact of the iron door being unconnected with the lightning conductor, and in doing this he was doubtless right, but only to a limited extent. The author of this work visited the colliery shortly after the explosion, and found that the conductor--the weight of which was about one pound per yard--had been fastened to the pole, which was about twenty-one feet high, by two glass insulators, and that the conductor was not connected with the building. On testing the copper rope which formed the conductor, its conductivity was found to be only 39·2 instead of 93 or 94 per cent. The conductor, therefore, was but little better than if it had been made of iron, and, even supposing it had been made of good copper, it was of too small a size. It should have been of double the weight, and _not_ insulated from the pole. In order to be thoroughly efficient it ought to have been brought down the pole, carried through under the roof, down the iron doorpost, and so into the ground.