Registered: Mar 2001
Location: Minneapolis, MN USA
Originally posted by greywolf
The NEC says "The grounding conductor shall be connected to the nearest accessible location as follows: (a)The building or structure electrode system as covered in 250-50. (b)The grounded interior metal water piping system as covered in 250-104 (a). (c)The power service accessible means external to enclosures as covered in Section 250-92 (b). (d)The metallic power service raceway. (e)The service equipment enclosure, or (f)The grounding electrode conductor or the grounding electrode conductor metal enclosures."
The meter enclosure, as part of the raceway, being bonded to the main building ground rod is typical and good practice. It can cause problems in very dry areas though and the longer the ground rod, the better. Using the water main as a ground rod can have its own problems with the advent of plastic pipe or the pipe run being shallow. The 2002 NEC adds that, if the water pipe is used for ground, it must be all metal and the equipment ground wire must be connected to the pipe within 5ft of the pipes entry to the building. The electrical service panel must also be bonded within that 5 ft.
You are correct that the water supply can have "its own problems." However, it is difficult to imagine the use of PVC as a water main for an apartment building. I've never seen such a thing. Of course, in Minnesota, water pipes are never run shallow, as they must be safely below the frost line. Larger mains are typically cast iron and smaller ones copper. Maybe they polyethelene tubing in Virginia. But if there is a problem getting a good ground with a water main that may run hundreds of feet underground, how can it be possible to get a good ground with a ground rod which is typically 8 to 10 feet? Builders often use really poor soil to backfill a building, and this can exacerbate the problem.
You can also install a ground rod today and have a very reliable ground and come back 10 years later and have a poor one.
As you mention, the effectiveness of a driven ground rod is highly variable, not just with the depth, but the soil conditions, and moisture content (which also varies independently).
Utilities do not have to abide by the same rules as contractors do. If the apartment building were in Minnesota, the contractor would almost certainly locate the service entrance near the water main. If possible, the ground would be connected on the street side of the meter, and if not, the meter would be jumpered with the appropriate guage grounding conductor.
Who knows what was done in this case?
The main issue with ground loops, however, is not the quality of the ground, but whether or not there is a single grounding point to which all secondary grounds are bonded.
Perhaps there is only one ground here, but the present case sure sounds like a ground loop related issue. I can't think of any other possibility given that attaching the coax to a grounded device seems to cure it.
"Ignore 'em, m'dear. They're beneath your dignity." W.C. Fields
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