Rooftop Antenna, Separate Ground Rod, and Coax Shock
A two-story 8,000 ft² commercial building has a new 40 ft amateur radio antenna mast mounted on the roof. The coax enters the building through a 1 in. PVC sleeve and runs 45 ft to a radio room. The installer drove a separate 8 ft copper-clad ground rod at the mast base, connected the mast and coax shield to it with 10 AWG copper, and ran a 10 AWG copper conductor from that rod to a metal cold-water pipe 30 ft away. The building's electrical service is 400 A, 120/208 V, with a concrete-encased electrode and a 2/0 copper service grounding electrode conductor. The radio room has a 120 V receptacle 12 ft from the coax entry. The owner reports intermittent hum and a small shock when touching the coax connector and the metal equipment rack. The local AHJ has adopted the Michigan Electrical Code.
1. Evaluate the grounding electrode system for the mast and coax. Is the separate ground rod acceptable as installed?
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A separate ground rod is not prohibited, but it must be bonded to the building's grounding electrode system. The installation as described creates a second, loosely connected electrode. The cold-water pipe may be part of the grounding electrode system only if it is metal, qualifies under 250.52, and is effectively bonded; a 10 AWG conductor is not the correct bonding jumper for an electrode system. For radio and television equipment, Article 810 requires the mast and lead-in grounding conductor to be bonded to the building grounding electrode system, and the bonding conductor to the electrode system is generally required to be at least 6 AWG copper. The 10 AWG antenna grounding conductor may be acceptable for the antenna lead-in itself, but it is not adequate as the electrode bonding jumper. The inspector should verify the water pipe is part of the grounding electrode system and that all electrodes are bonded together per 250.50.
2. What is causing the hum and shock, and what is the correct bonding fix?
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The hum and shock indicate a potential difference between the coax shield, the equipment rack, and the building's electrical grounding system. The separate rod and the building service electrode are at different earth potentials, and the 10 AWG connection to the water pipe is too small and possibly not a proper electrode bond. The fix is to bond the mast, coax shield, and equipment rack to the same grounding electrode system using an intersystem bonding termination or an equivalent listed connection. The bonding conductor from the antenna system to the building electrode system should be sized per Article 810 and 250.66, typically 6 AWG copper or larger for this service. The equipment rack must be bonded to the same point. Do not leave the isolated rod as the only reference and do not use the coax shield as a grounding conductor. After bonding, verify no measurable voltage between the coax and rack.
3. What role does the intersystem bonding termination play, and how should it be connected?
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An intersystem bonding termination gives communications, antenna, and other low-voltage systems a single accessible point to connect to the building grounding electrode system. It avoids the isolated-rod problem and provides a consistent reference for the coax shield, telephone, and data grounds. The IBT must be connected to the grounding electrode system with a conductor sized per 250.66, not with a 10 AWG jumper. It must be accessible and listed for the purpose, and the connection must be made with listed fittings. The antenna grounding conductor can terminate at the IBT if the IBT is properly bonded to the electrode system. The inspector will check that the IBT is not merely attached to a separate rod and that the equipment rack is bonded. The exact conductor size depends on the service conductors, so the installer should use the 250.66 table rather than guessing.