Grounding and Bonding a Rooftop Antenna and Communications Rack
A low voltage contractor is installing a rooftop cellular booster antenna and a small communications rack in a single-story commercial building. The antenna mast is 12 ft tall, mounted on a steel frame on the roof. The rack contains a switch, a router, a UPS, and a patch panel. There is a nearby metallic water pipe that the building owner says is the "main ground." The contractor plans to run a 10 AWG green conductor from the rack to the water pipe and a separate 10 AWG conductor from the antenna mast to a ground rod driven at the roof edge. The electrical inspector has not yet visited. The general contractor wants the system energized Friday.
1. Identify the grounding and bonding deficiencies in the proposed plan.
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Several problems stand out.
1. The water pipe may not be a suitable grounding electrode. A metallic water pipe can serve as a grounding electrode only if it is in direct contact with the earth for a specified length and is not isolated by a dielectric fitting. The contractor has not verified either condition. More importantly, a water pipe is a grounding electrode, not a convenient bonding point for equipment. Bonding to it without confirming it is part of the premises grounding electrode system can create a path that is not actually grounded.
2. The rack bonding conductor size and routing. A 10 AWG conductor may be adequate for some telecommunications bonding, but the size depends on the applicable telecommunications bonding requirements and the size of the premises grounding electrode conductor. The contractor has not determined the GEC size, so the bonding jumper size cannot be confirmed. The conductor should be routed as short and straight as practicable, not run arbitrarily to a water pipe.
3. The antenna mast ground is separate and isolated. Driving a ground rod at the roof edge and running a separate conductor to the mast creates a second, isolated grounding electrode. Two separate grounding electrodes that are not bonded together create a difference in potential between them during a lightning event or a fault. The mast ground must be bonded to the building's grounding electrode system, not left as an island. A ground rod at the roof edge also has poor contact with earth unless it is actually driven into soil, which it is not on a roof.
4. No bonding of the steel frame. The antenna is mounted on a steel frame; the frame should be bonded to the mast grounding conductor so that all metallic parts are at the same potential.
5. Missing intersystem bonding termination. The communications systems should connect to the premises grounding electrode system through a listed intersystem bonding termination, not by field-fabricated connections to a water pipe.
6. Energizing before inspection. The GC's Friday deadline does not override the inspection requirement. The contractor should not energize a system with unverified grounding.
2. Describe the corrected grounding and bonding approach and the sequence of steps before energizing.
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Corrected approach:
- Establish the premises grounding electrode system. Identify the existing grounding electrode conductor and its size at the service. Confirm which electrodes are present (ground rod, water pipe, building steel, concrete-encased electrode) and that they are bonded together.
- Install a listed intersystem bonding termination at an accessible location near the service, or use the existing one. All communications and antenna grounding conductors terminate there.
- Bond the antenna mast and steel frame to the building grounding electrode system with a conductor sized per the applicable requirements, run as short and straight as practicable. Do not drive an isolated roof ground rod. If a ground rod is used, it must be bonded to the building electrode system with a conductor sized for the electrode system.
- Bond the rack to the telecommunications bonding infrastructure, which in turn connects to the intersystem bonding termination. Confirm the bonding jumper size against the GEC size.
- Bond metallic conduit, cable trays, and the UPS enclosure to the same system so there is no potential difference across the rack.
- Verify continuity end-to-end with a bonding meter before energizing.
Sequence before Friday:
- Issue an RFI or field memo documenting the as-found conditions and the proposed corrections.
- Confirm the GEC size and the intersystem bonding termination location with the electrical contractor.
- Perform the bonding work and continuity tests.
- Request the electrical inspection; do not energize the booster or rack until the grounding is inspected and accepted.
- Record test results and photographs in the closeout package.