When lightning strikes a structure, it causes a current pulse to flow, and that pulse has an extremely fast rise time- making it appear to be a high-frequency signal in the MHz range. A pulse of 200,000 amperes flowing through a steel tower's impedance may allow a voltage gradient of several thousand volts to appear along its length. That is why all conductive metal objects (ladders, window frames, HVAC ductwork, etc.) within six feet of a down conductor must be cross-bonded to the lightning protection down conductor to prevent side-flashes.
Extensive research on lightning conducted years ago by Dr. Phillip Krider at Kennedy Space Center, FL and at Tucson, AZ, proved that magnetic forces generated by the lightning current caused the current to flow mostly along the outside corners of a square or triangular tower. Relatively little current flows through the bulk steel of a tower. That is why you will find the heavy copper down conductors placed along the outside corners of such towers. The details of this design can be found in NFPA 780, Standard for the Installation of Lightning Protection Systems. While it is true that the lightning protection system must have its own grounding rods and interconnecting system, it is also true that the lightning protection grounding system and the building electrical and communication grounding system MUST be bonded together to create one grounding system. The NEC makes this requirement very clear in Article 250.106: "The lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system." Failure to follow this requirement to the letter can have disastrous and possibly tragic consequences. Readers of this thread may be surprised to learn that Article 810 of the NEC applies to Radio and Television antenna systems, and to Amateur Radio equipment. It should be mandatory reading for any Ham who seeks to erect an outdoor antenna system. This Article covers grounding systems, surge protection, building penetrations, and similar topics. Definitely good information. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of wd8chl Sent: Friday, April 10, 2009 8:22 AM To: [email protected] Subject: Re: [Repeater-Builder] Re: Motorola R56 grounding Eric Lemmon wrote: > Bob, > > The reason is quite simple. If a fault (say, a short to the case) occurs in > a piece of equipment that is grounded per the NEC, the fault current- which > may be 100 amperes or more- flows from the service through the circuit > protection device (fuse or circuit breaker) to the fault, and returns via > the equipment grounding conductor (green wire) to the service. <snip for brevity> The problem with all this is that, for lightning protection at a tower site, the ground for the arrestor should not run along the same path as the AC conduit, because it will be a much HIGHER impedance. So while you need a proper ground within the service wiring, there also needs to be a good separate ground for lightning protection. Ideally, a bulkhead plate where the cable enters the building, but that's not always possible, especially at older sites. Next best is to mount the arrestor on the cabinet where the feedline enters, and run a large ground wire to the site ground in as short a run as possible, minimum bends, etc. And I think lightning grounds is what R56 is more concerned with, more than service grounds.
