Tower down conductors offer little lightning protection Lightning-simulator test results indicate that a down conductor connected to a towertop lightning rod adds no significant improvement to the protection offered by a lightning rod bonded to the tower
Numerous grounding standards require that a copper down conductor be run along the tower length to a ground rod or to the grounding system. Perhaps those who wrote the standards thought that copper was a better conductor than the tower, or that because the tower is made of steel (ferrous material), its additional inductance would impede the lightning strike energy from flowing to ground. Regardless of the reason, such standards still exist.
These standards address two types of installations, one with the top lightning rod grounded to the tower, and one with the rod insulated from the tower. With the insulated rod, it is impossible to prevent electrical energy from arcing across from the rod to the tower. The instantaneous voltage drop caused by the down conductor’s inductance is measured in hundreds of kilovolts, more than enough to allow an arc from the rod to the tower. With the rod grounded to the tower, not much energy flows on the down conductor compared to the tower itself.
To test this theory, we placed a 10-foot section of Rohn No. 25 tower over our Big Bertha lightning simulator. A 10-foot section of insulated No. 2/0 cable was bonded to the surge input side of the tower. The other ends of the tower and cable were monitored to measure the current carried by each. The setup was “shot” with 5,000V. The results showed that the tower section carried four times the current carried by the cable. Only 19.7% of the current traveled the cable, and 80.3% of the current traveled the tower. When bare copper wire is used for the down conductor, rain, which is slightly acidic (pH 5.5-6.0) will remove some copper ions on contact. When these ions drip onto the tower, the galvanized coating (zinc) washes away, resulting in rust and decreasing the life of the tower. In a subsequent test, a 10-foot section of Times Microwave LMR-1200 coaxial cable was bonded to the same tower section over the lightning simulator. This coax has a larger diameter than No. 2/0 cable. The test result showed an advantage in using a coaxial cable instead of a grounding conductor. The coax carried 30.9% of the current, and the tower carried 69.1% of the current. This result shows how coaxial cable can perform a double duty–as a useful transmission line and as a conductor for lightning surge current.
Remember, though, that the tower-mounted lightning rod is only one part of a comprehensive protection system that should include surge suppression devices and proper grounding techniques.