When I was a kid, my two older brothers were standing in the front door watching a massive electrical storm through the screen door. I was afraid to get too close to the doors or windows because of what sounded to me like earschplittenloudenboomers. Each bolt of lightning shook the house and rocked the neighborhood. They were
both leaning against the door jam with their hands resting on the aluminum weather stripping when suddenly, a bolt of lightning struck the power line in the alley behind our house. Both brothers howled like wolves and nearly jumped out of their fur. The energy of the lightning strike enveloped the house and they got a good shock. As far as we know, it wasn’t severe enough to cause any damage, but they were always a bit goofy so it’s hard to tell.
Can that happen if you’re standing on a metal catwalk and lightning strikes the building?
That was the question on the mind of a reader, “a sound technician with over 35 years of experience, but with limited electrical engineering knowledge,” who wrote: “I have a contract to provide theatrical sound, stage-lighting and video services at a university. Recently, the university allowed multiple cell phone companies to install equipment on the roof of the theatre. The cell phone tower equipment installed by one company includes a conduit from the roof
to the stage area catwalks on the inside of the building. The conduit is bonded via a decent gauge wire to the exposed steel of the building. (The cell phone company) also installed a heavy wire from inside the same conduit to the
exposed steel of the building. My concern is, what happens in the event of a lightning strike or other electrical issue from their equipment? The building's steel is exposed throughout the catwalk area above the stage and the steel most likely is also connected to our double-purchase rigging system steel. Are we at risk for a shock in this situation? Does it matter if the heavy wire is for lightning suppression versus just a heavy ground wire for their installation?”
You might want to turn off the Sponge Bob Square Pants episode and think about this. After all, the voltage between clouds and the earth can reach as high as 10 MV to 1000 MV and the current from a lightning strike can range from a few thousand amps to 200 thousand amps. Get it right and standing on the catwalk during a thunderstorm is not likely to present a problem; get it wrong and it can ruin your whole day.
The short answer to his question is that it should be safe to stand on the catwalk during a thunderstorm because the cell phone tower is bonded to the building. The reason for bonding them is to do our best to make sure all of the metal in the building structure, the cell tower, and the electrical system remain at the same voltage potential regardless of what might happen. Otherwise a lightning strike or ground fault could create a difference in potential that could be very dangerous.
Depending on how tall the cell tower is and how often lightning occurs in the area, there may or may not be a lightning protection system on the building. In either case, there is an electrical grounding system which is separate from the lightning protection system. The general idea behind both is to provide a low impedance path for fault current or lightning discharges. In the case of lightning, the low impedance path keeps the current away from non-conductive parts of the building which would sustain more damage than does the low impedance path. Without it, the lightning discharge could potentially damage materials, start a fire, and/or electrocute anyone with the misfortune of touching two objects at different potentials.
The electrical grounding system keeps all of the bonded parts at the same voltage potential during a ground fault or a lightning strike. This is what generally makes it safe by making sure that one part of the structure becomes energized
while another part doesn’t. It also causes a massive amount of current to flow in the event of a ground fault, which insures that the circuit breaker will trip very quickly and remove the voltage from the grounding conductors.
Standing on the catwalk while fault current or lightning discharge current is flowing isn’t enough to hurt you. Much like a bird sitting on an electrical power line that is conducting massive amounts of current, you won’t be injured because there is no 0V reference relative to you. A high voltage has no significance unless it is referenced to another voltage point. That’s why you can’t measure any voltage with one test lead; you can only measure voltage between two points. If, on the other hand, the bird were to straddle the power line and a grounded transmission tower it would be cooked because the tower is connected to the earth, making it a solid 0V reference. Now the high voltage has real meaning.
Bonding the cell tower to the building steel is part of the NFPA 70 (normal"">National Electrical Code) requirements for radio and television equipment. Article 810.21 says that masts and structures supporting antennas have to be grounded to an electrode or other suitable grounding connection point. In many cases that means the building steel, provided it has a solid connection (meaning low impedance) to earth. Otherwise the electrode could be a water pipe within five feet of its point of entrance into the building or a grounding rod. The Code also says that the grounding conductor should be protected from physical damage (thus the conduit) and that both ends of the raceway should be bonded to the grounding conductor or to the same electrode to which the grounding conductor is connected.
If there is a lightning protection system then the game changes a bit. A lightning protection system has four main parts: (1) a system of strike termination devices (called air terminals) on the roof and other elevated locations; (2) a
system of grounding electrodes; (3) a system of conductors connecting the air terminals to the grounding electrodes; and (4) transient voltage surge suppressors (TVSS). The air terminals are mounted at the highest point on a building in intervals of no more than 20 feet. If it does its job right then the lightning protection conductors will be the preferred path for current due to lightning strikes and it will conduct it through the grounding electrodes and into the earth.
The NFPA 780-2008 Standard for the Installation of Lightning Protection Systems spells out specific requirements for lightning protection systems. One of the requirements is that the lightning protection system, electric service, communications, antenna system grounds, and underground metallic pipe systems all to be interconnected, again, so that they will all stay at the same voltage potential. NFPA 780 also requires that the building is bonded to the lightning protection system for the same reason.
Ideally, the resistance of the grounding system or lighting protection system to the earth should be 0 ohms; however, in practice that just not possible because even the largest conductors have some resistance, even if it is small. So we do the best we can to make the resistance as low as possible.
The NEC says that if the resistance to the grounding electrode is more than 25 ohms it has to be supplemented with another grounding electrode that is bonded to the first. The combination of the two will lower the resistance to ground. In many instances we try to make the resistance to ground less than 25 ohms. The NFPA and IEEE recommend a ground resistance value of 5 ohms or less.
Lightning protection systems are supposed to be inspected every year although there is no enforcement. But you can perform a visual inspection of the grounding and bonding to ease your mind. Things to check for are loose or broken connections in the lightning protection system and the electrical grounding system. Check for corrosion or burned conductors. Pull on components to make sure they are well connected and tighten clamps and splices with a wrench. Check the transient voltage surge suppression units to see if the indicator lights warn of a failure.
Also, grounding rods have a limited life due to corrosion. Moisture, salt, and high temperatures contribute to the degradation of electrodes. The life expectancy of a driven rod or a grounding plate is only five to 10 years. A concrete
encased electrode is expected to last 15 to 20 years while a building foundation electrode is expected to last 20 to 30 years. There are also specially engineered electrodes such as an advanced driven rod, which looks a bit like a masonry drill bit, and an electrolytic electrode, which is a hollow copper shaft filled with natural earth salts and desiccants to draw moisture from the air, forming an electrolytic solution that seeps into the soil and keeps it moist and conductive. The life expectancy of the advanced driven rod is 15 to 20 years and that of an electrolytic electrode is 30 to 50 years.
A formal inspection of the lightning protection system is more time consuming and involves specialized test equipment. The 3-point test requires two auxiliary grounding rods to be driven into the ground. Voltage is then applied between the grounding electrode under test and the second grounding rod. An ammeter reads the current generated between them while a volt meter reads the voltage between the electrode under test and the third grounding rod. The result is the resistance in ohms between the ground electrode and the surrounding earth.
There are also ground resistance meters, such as the Fluke 1630 Earth Ground Clamp meter. It allows you to test the integrity of a grounding electrode by clamping the meter around the electrode or grounding conductor instead of using auxiliary stakes.
The main idea behind a lightning protection system is that we want to be able to control where the current from a lightning strike is flowing and it shouldn’t be through you or anyone else. If the lightning protection system, electrical
grounding system, and bond are all installed and working properly then more than likely there will be no problems.
There is a lot to know about grounding and lightning protection systems. A good book to read about grounding is called "Soares Book on Grounding and Bonding," which is published by the International Association of Electrical Inspectors. In the meanwhile, keep your eye out for air terminals (you’ll see plenty of them at the highest point
in any outdoor electrical substation) and watch for ways in which electrical grounding systems and lightning protection systems are bonded to electrodes.