User's Guide

Grounding and lightning protection Chapter 2: Planning considerations
2-16
UNDER DEVELOPMENT
phn-2182_003v004 (Oct 2011)
Lightning protection zones
The ‘rolling sphere method’ (Figure 2-1) is used to determine where it is safe to mount
equipment. An imaginary sphere, typically 50 meters in radius, is rolled over the
structure. Where the sphere rests against the ground and a strike termination device (such
as a finial or ground bar), all the space under the sphere is considered to be in the zone of
protection (Zone B). Similarly, where the sphere rests on two finials, the space under the
sphere is considered to be in the zone of protection.
Figure 2-1 Rolling sphere method to determine the lightning protection zones
Assess locations on masts, towers and buildings to determine if the location is in Zone A or
Zone B:
• Zone A: In this zone a direct lightning strike is possible. Do not mount equipment in
this zone.
• Zone B: In this zone, direct EMD (lightning) effects are still possible, but mounting in
this zone significantly reduces the possibility of a direct strike. Mount equipment in
this zone.
Never mount equipment in Zone A. Mounting in Zone A may put equipment,
structures and life at risk.