<P> Ideally, the underground part of the assembly should reside in an area of high ground conductivity . If the underground cable is able to resist corrosion well, it can be covered in salt to improve its electrical connection with the ground . While the electrical resistance of the lightning conductor between the air terminal and the Earth is of significant concern, the inductive reactance of the conductor could be more important . For this reason, the down conductor route is kept short, and any curves have a large radius . If these measures are not taken, lightning current may arc over a resistive or reactive obstruction that it encounters in the conductor . At the very least, the arc current will damage the lightning conductor and can easily find another conductive path, such as building wiring or plumbing, and cause fires or other disasters . Grounding systems without low resistivity to the ground can still be effective in protecting a structure from lightning damage . When ground soil has poor conductivity, is very shallow, or non-existent, a grounding system can be augmented by adding ground rods, counterpoise (ground ring) conductor, cable radials projecting away from the building, or a concrete building's reinforcing bars can be used for a ground conductor (Ufer ground). These additions, while still not reducing the resistance of the system in some instances, will allow the (dispersion) of the lightning into the earth without damage to the structure . </P> <P> Additional precautions must be taken to prevent side - flashes between conductive objects on or in the structure and the lightning protection system . The surge of lightning current through a lightning protection conductor will create a voltage difference between it and any conductive objects that are near it . This voltage difference can be large enough to cause a dangerous side - flash (spark) between the two that can cause significant damage, especially on structures housing flammable or explosive materials . The most effective way to prevent this potential damage is to ensure the electrical continuity between the lightning protection system and any objects susceptible to a side - flash . Effective bonding will allow the voltage potential of the two objects to rise and fall simultaneously, thereby eliminating any risk of a side - flash . </P> <P> Considerable material is used to make up lightning protection systems, so it is prudent to consider carefully where an air terminal will provide the greatest protection . Historical understanding of lightning, from statements made by Ben Franklin, assumed that each lightning rod protected a cone of 45 degrees . This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building . </P> <P> A modeling system based on a better understanding of the termination targeting of lightning, called the Rolling Sphere Method, was developed by Dr Tibor Horváth . It has become the standard by which traditional Franklin Rod systems are installed . To understand this requires knowledge of how lightning' moves' . As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path . The maximum distance that each step may travel is called the critical distance and is proportional to the electric current . Objects are likely to be struck if they are nearer to the leader than this critical distance . It is standard practice to approximate the sphere's radius as 46 m near the ground . </P>

An earth conductor provided on the top of the transmission line provides protection against