Indirect Effects
  
 
Earth Potential Rise: A direct strike to a protected structure will flow to ground via that structure's earthing system. Due to the high current amplitude and the fast rate of rise of current, both conductor resistance and inductance play a part in raising the local earth potential. This potential rise causes equipment damage by voltage breakdown and transfer at both ends of the cable.

Magnetic Field Coupling: The magnetic field associated with current flow in a direct or distant lightning discharge, will induce currents into exposed conductors causing equipment damage at both ends of the conductors. Conductors running parallel to lightning down conductors are to be avoided.

Electric Field Coupling: The electric field collapse associated with direct or distant lightning strikes is capacitively coupled into exposed conductors causing equipment damage at both ends of the conductors in a similar fashion to magnetic field coupling.

Protection Against Indirect Effects

Protection against the indirect effects of a lightning strike require a systematic approach to the installation of protection devices.

Define Protection Boundary. Define a protection boundary, which can be the perimeter of a building, or a section of that building such as a computer room. In the case of a multi storey building, each floor can be defined as a boundary. Any service, such as power, signal or data lines crossing the boundary must be fitted with suitable protection devices.

Protect the Structure. To minimise the indirect effects of a lightning strike, it is essential to provide adequate structural protection with air terminals and down conductors in accordance with appropriate standards.

Install a Bonded Earthing System. This is critical to minimise differences in earth potentials within the protected area.

Protect the Power Lines. Install power surge diverters or power surge filters wherever power lines cross a defined boundary.

Protect Signal Lines. Install signal and data line protection wherever these lines cross a defined boundary.

Regardless of the type of coupling, the method of protection is the same.