IEC 61312-1 established the general principles for protecting structures and their contents against the lightning electromagnetic impulse, the electromagnetic field and the surge currents that a lightning stroke produces even when the structure itself is not struck. The document explained the composition of the lightning current waveform as it appears at different parts of a building, distinguished the surge current that enters through the external protection system from that induced in internal cabling, and described the concept of a lightning protection zone in which the electromagnetic environment is progressively reduced from the outside towards the most sensitive equipment. On this basis it set out the strategy of coordinated surge protective devices, installed at the boundaries between zones, whose energy handling capability and voltage protection level are matched to the exposure of each zone and to the withstand of the equipment. Guidance was given on earthing and bonding, on the routing and shielding of cables and on the layout of equipment inside a structure. The material has since been incorporated into IEC 62305-4.