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Author Topic: CATV/RV Grounding Issue  (Read 853 times)

Jonathan Johnson

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Re: CATV/RV Grounding Issue
« Reply #20 on: March 06, 2017, 01:35:26 am »

As to the original question: with modern electrical systems, for someone to be shocked, there is almost always at least two failures involved: the primary failure of the electrical insulation, and the secondary failure of the safety system.

(Usually, what I have termed a "primary" failure happens after the secondary failure. I call it a primary failure not because of chronology, but because the insulation is the primary safety device. The EGC or breaker or GFCI is the secondary safety device.)

It would initially appear that the EGC is not properly connected in one or more pedestals, and a fault in one RV is backfeeding throughout the CATV shield to the other RVs.

I wouldn't be surprised if the grounding terminal of the receptacle in the pedestal isn't even connected to anything. The wiring is probably a 3-wire bundle either overhead or underground, two hots and a neutral (no ground). The pedestals may be designed with separate neutral and grounding busbars, and the installer didn't jumper (bootleg) the busbars together. If a ground rod was connected, there's no bond to neutral to provide a return path, and if the soil is sufficiently dry, that doesn't provide a good return path, eiter. With no ground wire in the feeders, there is no connection to the service entrance bonding point, or to other pedestals, so the voltage backfeeds throughout the CATV.

If there is a ground rod at each pedestal, but no bond between pedestals, the CATV cable provides a bond through a bonding point (intentional or unintentional) in other RVs.

So you have this condition: hot to chassis fault in RV #1, chassis to CATV shield bond in RV #1, voltage through shield to RV #2 (and 3 and 4), bonding point CATV to chassis in #2, hot skin #2, voltage on EGC of #2 to pedestal, into ground rod at #2. Because the ground rod at #1 pedestal is too high impedance, current flows through CATV shield to ground rod at pedestal #2, accounting for the damage seen in the CATV distribution system.

This shows the importance of having a continuous INTENTIONAL ground path/bond between power distribution points (and appliances such as an RV) to ensure they are at the same potential (as the soil), as well as an INTENTIONAL bond between ground and neutral to provide a controlled return path for fault currents to increase the likelihood of tripping a breaker in the event of an insulation fault.

EDIT: The proper repair is to install a proper EGC from the bonding point at the service entrance to each pedestal, and keep the neutral and ground separate at each pedestal. But if there is no master service entrance -- that is, if each pedestal is considered a service entrance, then bonding the neutral and ground at each pedestal and installing proper ground rods at each pedestal is the way to go. You probably need a ground rod at each pedestal, whether or not neutral and ground are bonded at the pedestal.
« Last Edit: March 06, 2017, 01:46:37 am by Jonathan Johnson »
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Jonathan Johnson

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Re: CATV/RV Grounding Issue
« Reply #21 on: March 06, 2017, 01:40:30 am »

The arc flash potential is fairly low in this case due to the long distance these pedestals are from the service panel (a lot of series resistance in those hundreds of feet of connecting wire limits the peak amperage). But many RV owners will pull the plug under load (not switching off the circuit breaker) which eventually causes a lot of pitting on the contacts of the plug/receptacle. This reduces contact surface area as well as increases oxidation of the brass, and that can result in overheating and meltdown of the shore power plug's insulating plastic.

Depends on where you are in the park. If you plug in to the first pedestal in the line, and you're near the utility transformer, the arc flash potential could be much higher. But  I do recognize that it's not particularly high, and the pitting damage is a much greater concern -- and equal reason to turn off the power.
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Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #22 on: March 06, 2017, 08:21:34 am »

To paraphrase someone else (read it from Nathan Lively's little book on audio)  "electricity is difficult to explain because it's invisible."

So true. When I was a controls designer building automated packaging machinery back in the 70's, there was a Mechanical Engineer on my team who I considered to be a genius of mechanical design. He intuitively knew everything about bearings, air cylinders, hydraulic systems, tolerance build up, welding, strength of materials, how to move packages on a conveyor line, etc... He was like a walking Google search engine on anything mechanical before the Internet existed. But anytime I threw a drawing on my proposed wiring diagram on the board he literally froze up. Same for troubleshooting wiring in the field. He said he couldn't understand it because he couldn't see it. Now, I could show him a logic chart detailing what each button or switch was supposed to activate and he was fine with that. And he easily understood any sort of timing chart I drew. But electricity was this big unseen mystery to him. 

Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #23 on: March 13, 2017, 07:13:00 am »

I finally got some pics from the campground CATV technician. Looks similar to a few XLR cables I've seen burn up. He's offered to send this to me, so of course I want it for demonstration.

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