IEC 62305-4 is the part of the lightning protection series that deals with the protection of electrical and electronic systems inside a structure. The series as a whole assumes that a structure may be struck and that the resulting current will be diverted to earth by the external protection system, but a stroke still produces a severe electromagnetic environment inside the building: the current flowing in the down conductors creates a magnetic field, and the potential rise of the earthing system drives surge currents into every conductor that leaves the building. Part 4 provides the framework for managing this environment. It introduces the concept of the lightning protection zone, in which the severity of the electromagnetic disturbance is progressively reduced from the point of entry to the location of the most sensitive equipment, and it sets out the strategy of coordinated surge protective devices installed at the boundaries between zones, with their energy handling capability and their voltage protection level selected so that the equipment behind each boundary is not overstressed. Earthing, bonding, shielding and cable routing requirements complete the document.