17.2.3.4 Self-protecting system
It is normally agreed that in self-protecting systems, metallic structures are electrically
continuous, tightly sealed to prevent the escape of liquids, vapours or gases, of a thickness
adequate to withstand direct strokes (that is, at least 5 mm thick), and encase
highly dangerous material. When these conditions are met, other than ensuring adequate
bonding and grounding, an ELPS based on an air termination system for lightning interception may not be required at all [12]. Tanks or vessels with
non-metallic roofs cannot be considered to be self-protecting.
Irrespective of the above, it should be observed that the self-protecting criterion
varies from one place to another as it depends on several parameters of a typical
lightning flash in that region, such as the habitual variation in the charge of a discharge
and the duration of an event. In this way, for a severe lightning discharge, 5 mm thickness
might not be adequate to remain undamaged. Thus, the self-protecting criterion is
always determined by local conditions and regulations.