Types. Lightning protection systems are either primary, secondary,
or a combination of primary and secondary.
Primary. Design primary protection to prevent damage from direct
lightning strokes by diverting any charges from structures through a low
resistance path to earth.
Secondary. Design secondary protection to prevent metal parts of
buildings, building contents, or other types of structures from accumulating
electric charges that can cause sparking or flashover. Sparking or flashover
is likely to occur when metal objects are proximate. In the event of a
lightning discharge, the potential of independently grounded metal objects can
change with respect to nearby objects generating flashover between the
Combination. The installation of a primary and a secondary
protection system for the same structure is not always required. A secondary
static ground system providing an interconnection of metallic masses within a
building or on piers and wharves may also be required with a primary lightning
protection system. When a structure is equipped with both primary and
secondary systems, interconnect all grounds.
Primary Systems. Design protection based on 100 ft (30.5 m)
lightning strike arc. Design either primary or secondary type and determine
mast locations or grounded aerial conductors and their heights. Mast
locations or grounded aerial conductors and their heights influence the type
of masts along with mast foundation requirements and the location of the
ground counterpoise. Primary protection shall consist of lightning masts or
grounded aerial conductors as described in paras. 2.2.1 and 2.2.2.
Lightning Masts. Lightning masts (freestanding air terminals)
placed around a facility and connected to a buried ground counterpoise (see
Figures 1 and 2).
Grounded Aerial Conductors. Overhead conductors spanned above a
facility and connected to a buried ground counterpoise.
Secondary Systems. A secondary system generally consists of a
buried ground counterpoise to which all metal parts, including reinforcing
steel of the building or other structure, are connected (see Figures 3, 4, 5,
6, and 7). An equipment ground bus may be utilized for the grounding of the
building contents. The ground bus shall not form a loop. Connect ground bus
to ground counterpoise as shown in Figure 3.
Air Terminals. Points (lightning rods) mounted on the salient parts
of facilities and connected to the ground counterpoise may be used for
protection for certain specific applications in a secondary system.