Thank you for your question regarding towers and lightning protection systems, it is our pleasure to help.
The short answer is that “yes”, in certain cases you can use the steel tower legs as part of the down conductor system for a Lightning Protection System (LPS); based on the height and construction of the tower, and given that the metal is thick enough. Please refer to US lightning standard, NFPA 780 Standard for the Installation of Lightning Protection Systems, Article 126.96.36.199 & 188.8.131.52 (and some other articles as well), as there are some fairly complicated formulas required to determine if your tower is eligible. If you are using the European Standard, BS EN 62305:2006, you will have many more requirements including impedance checks and arc-flash verifications, to name only a few.
Whether you install a copper down conductor or not, you must install grounding electrodes and bond them to the base of the tower in accordance with Chapters 4 and 7. This is primarily to protect the concrete foundations from being overheated when the energy from the lightning flows through it. Overheated concrete can crack when the water inherent in its chemistry begins to boil and expand.
Another issue you should consider is the electromagnetic fields that are generated on a metal tower when it is struck by lightning. Copper down conductors reduce these fields thereby helping to protect sensitive electronic systems. Of course, there are serious Human Safety implications when choosing to use the steel structure as a down conductor. Steel towers that do not have copper down conductors are far more dangerous to personnel that may be working in or around the tower, as the Step & Touch Voltage hazards will be far greater.
It is the official position of E&S Grounding Solutions to always recommend copper down conductors for Lightning Protection Systems (LPS) on all structures, regardless if the code allows it or not. Forcing all of the energy from a lightning strike into the structure you are trying to protect, is counterproductive. After all, the goal of a Lightning Protection System (LPS) is to protect the structure, the equipment, and the personnel associated with the tower form the effects of both direct and indirect lighting strikes. Copper does this better than steel.
We hope you find this information useful. If you should have any further questions, please feel free to call one of our engineers and they will be glad to discuss your project with you free of charge.
The Engineering Team at E&S Grounding Solutions