Question 1

What are the primary considerations for the placement and spacing of lightning down conductors on a structure?

Accepted Answer

Down conductors should be strategically positioned along the perimeter of the protected object,  ideally close to building corners,  to ensure multiple paths for lightning current to reach the ground.  NFPA 780,  Standard for the Installation of Lightning Protection Systems,  provides guidelines for spacing,  which typically depends on the structure's height and class of protection.  Proper placement minimizes the risk of side flashing and ensures efficient current dissipation.

Question 2

What materials are typically used for lightning down conductors,  and what are the minimum size requirements?

Accepted Answer

Common materials for lightning down conductors include copper and aluminum,  chosen for their excellent electrical conductivity.  NFPA 780 specifies minimum material requirements,  such as a diameter of ⅜ inch for Class I copper conductors and ½ inch for Class I aluminum conductors.  The selection also considers the corrosive environment and compatibility with other system components.

Question 3

How should down conductors be protected from physical damage,  and what are the bonding requirements?

Accepted Answer

Exposed down conductors at grade level must be protected from physical damage up to a height of 6 feet (1830 mm) above grade.  If metal conduit is used for protection,  both ends of the conduit must be bonded to the down conductor to maintain electrical continuity and prevent dangerous potential differences.  Additionally,  down conductors coursed on or in reinforced concrete columns or structural steel columns should be connected to the reinforcing steel or structural steel,  as per relevant standards like NFPA 780.

Lightning Protection Down Conductor Size Requirements | NFPA 780 Standards