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Author Topic: CATV/RV Grounding Issue  (Read 854 times)

Mike Sokol

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CATV/RV Grounding Issue
« on: March 04, 2017, 07:13:21 pm »

This is the sort of question I regularly answer on my NoShockZone blog about RV electricity. Take a look a see if you can guess what's causing the issue. I'm pretty sure I already know so we'll compare notes in a few days after I formally answer this.


"Vance Cowley
CATV/MATV Installer

I maintain a cable TV distribution system at an RV park. I suspect an RV had an AC electrical problem that found its way into the cable TV system. The RV was parked before the first pedestal that the cable TV tap burned up. The cable  tap and coax wiring for 4 pedestals literally burned up, the center conductor vaporized. We found one RV, 3rd one in line,  where the wiring in his trailer is completely melted and the plastic mounting plate at the pedestal had been on fire.

As far as I know the connectors for the TV system at the pedestals do not run through ground blocks, older system. How can I prevent this from possibly happening in the future? Is this a floating ground issue?

Just before this incident owners in these 4 pads were complaining about being shocked when they touched the outside of their trailers. A non contact voltage probe was used to identify RV that was causing the problem. The non contact probe illuminated 4 feet away from the problem RV. The trailer was disconnected and the problem with the outside shock went away. The RV owner was asked to have an electrician look at his trailer before reconnecting the power plug to the power pedestal. No information on what was found."

Keith Broughton

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Re: CATV/RV Grounding Issue
« Reply #1 on: March 05, 2017, 07:23:39 am »

This is the sort of question I regularly answer on my NoShockZone blog about RV electricity. Take a look a see if you can guess what's causing the issue. I'm pretty sure I already know so we'll compare notes in a few days after I formally answer this.


"Vance Cowley
CATV/MATV Installer

I maintain a cable TV distribution system at an RV park. I suspect an RV had an AC electrical problem that found its way into the cable TV system. The RV was parked before the first pedestal that the cable TV tap burned up. The cable  tap and coax wiring for 4 pedestals literally burned up, the center conductor vaporized. We found one RV, 3rd one in line,  where the wiring in his trailer is completely melted and the plastic mounting plate at the pedestal had been on fire.

As far as I know the connectors for the TV system at the pedestals do not run through ground blocks, older system. How can I prevent this from possibly happening in the future? Is this a floating ground issue?

Just before this incident owners in these 4 pads were complaining about being shocked when they touched the outside of their trailers. A non contact voltage probe was used to identify RV that was causing the problem. The non contact probe illuminated 4 feet away from the problem RV. The trailer was disconnected and the problem with the outside shock went away. The RV owner was asked to have an electrician look at his trailer before reconnecting the power plug to the power pedestal. No information on what was found."

Bad wiring on one trailer created a "hot skin" condition and that was fed into the shield of the CATV system.
With the CATV cable floating. no cct breaker action happened and the cable shieled was "hot"
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Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #2 on: March 05, 2017, 07:36:41 am »

Bad wiring on one trailer created a "hot skin" condition and that was fed into the shield of the CATV system.
With the CATV cable floating. no cct breaker action happened and the cable shieled was "hot"

You still have to account for why 4 other RVs were affected by a single RV plugging in. And what was the wiring failure in the first RV that caused the hot-skin condition? Also, was it a high-, medium-, or low-impedance failure. This is closely related to distributed power issues in pro-sound systems, so it's good study.

Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #3 on: March 05, 2017, 07:46:59 am »

Here's a few more hints that may or may not be relevant to this problem....

Some older campgrounds didn't have "grounded" wiring for the campsite pedestals, so they just added a grounding rod to each outlet pedestal.

Many campgrounds use daisy-chained wiring with 4 to 8 campsites hooked onto a single feeder cable.

RV's are required to maintain separated neutral and ground wiring internally, and be only bonded by the connection they're plugging into for shore power.

Many older RV owners like to DIY their own RV wiring.

Some campgrounds use DIY guys to rewire or add power pedestals at each campsite. They often have no electrical wiring training. 

Keith Broughton

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Re: CATV/RV Grounding Issue
« Reply #4 on: March 05, 2017, 11:18:50 am »

I should have added to "no cct breaker action" was due  to no ground .
What we don't know is, were the other RVs plugged into a same pedestal or was this problem spread out through the camp ground.

The original fault would be a wiring defect that brought hot side of AC to chassis of the RV. Hard to say where that happened.
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Re: CATV/RV Grounding Issue
« Reply #5 on: March 05, 2017, 12:25:35 pm »

I should have added to "no cct breaker action" was due  to no ground .
What we don't know is, were the other RVs plugged into a same pedestal or was this problem spread out through the camp ground.

Since it's generally one RV per campsite, and one Pedestal/Outlet per campsite, it's safe to assume these four RVs were spread across four campsites on four different pedestals. All the pedestals in a campground section are generally daisy-chained together.

Quote
The original fault would be a wiring defect that brought hot side of AC to chassis of the RV. Hard to say where that happened.

Yes, that's most likely the cause. I've seen shorts like this created with wiring laying across sharp metal struts in the chassis of the RV, or wiring that's been pinched between the floor and a metal support beam, or even by a screw drilled into the wall which pierces a wire run. That's a low-impedance/high-current fault which will have full circuit breaker amperage available for the fault current.

An example of a mid-impedance/mid-current fault would be an electric hot water heater with a leak in the heater element. That will provide about 1 to 2 amps of current to the chassis.

A high-impedance/low-current fault would be an appliance with a leaky transformer plugged into an interior outlet in the RV. Leakage can be anywhere from a few mA up to a few hundred mA. Some will trip a GFCI, while others won't.

Everything plugged into an outlet exhibits SOME leakage to their chassis. For instance, a Crown iTech amp will leak up to 3 mA and still pass final testing. A common Surge-Strip will leak 1 to 2 mA. A double-insulated piece of gear will have to have less than 1 mA leakage (IIRC less than 0.7 mA) to pass UL listing.

But this all begs the question. How does ONE RV with a wiring problem that creates a line-to-chassis fault-current affect several OTHER RVs in the same wiring loop. And how do they all get a "hot-skin" voltage?

Hint: I call this a "Reflected Hot-Skin Voltage".

John Roberts {JR}

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Re: CATV/RV Grounding Issue
« Reply #6 on: March 05, 2017, 12:38:08 pm »

RPBG?


JR
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Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #7 on: March 05, 2017, 01:09:47 pm »

RPBG?

Probably not. Here's another hint. I've seen a few campgrounds where all the pedestals in a section had their EGC "grounds" connected together, but there was a lost connection back to the service panel's Neutral-Ground bonding point. So any voltage induced on one pedestal's ground wire would be "Reflected" to all the other pedestal "ground" wires in the area. So this becomes a floating ground wire interconnecting several pedestals and the RVs connected to them. Any single RV with a line-to-chassis leakage will induce a voltage on its ground wire, which then shows up at multiple RVs at several other pedestals in the area. Of course, if the daisy-chained EGC wire was actually "Bonded" back to the service panel, then NO voltage would be possible on an RV chassis. In a properly bonded EGC the fault current would sink back to the bonding point. If it was a high-current fault, it would trip a 20-amp circuit breaker. If it was a mid-current fault, it would just dump a ampere or two into the service panel bond and we wouldn't know about it. If it was a low-current fault, no problemo since that's what the EGC is supposed to deal with.

Now, think about what happens when you have four RVs connected together via a common EGC ground wire that's not bonded at the service panel. When you hook up a CATV wire that ties the chassis back to the service panel, and plug in a single RV with a direct line-to-chassis short, you can dump ten's of amperes of current through the CATV wiring. I've seen this a bunch of times and it always mystifies the local electricians.

So the next question is, how do we test for this sort of fault at a campground?

John Roberts {JR}

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Re: CATV/RV Grounding Issue
« Reply #8 on: March 05, 2017, 02:40:32 pm »

Probably not. Here's another hint. I've seen a few campgrounds where all the pedestals in a section had their EGC "grounds" connected together, but there was a lost connection back to the service panel's Neutral-Ground bonding point. So any voltage induced on one pedestal's ground wire would be "Reflected" to all the other pedestal "ground" wires in the area. So this becomes a floating ground wire interconnecting several pedestals and the RVs connected to them. Any single RV with a line-to-chassis leakage will induce a voltage on its ground wire, which then shows up at multiple RVs at several other pedestals in the area. Of course, if the daisy-chained EGC wire was actually "Bonded" back to the service panel, then NO voltage would be possible on an RV chassis. In a properly bonded EGC the fault current would sink back to the bonding point. If it was a high-current fault, it would trip a 20-amp circuit breaker. If it was a mid-current fault, it would just dump a ampere or two into the service panel bond and we wouldn't know about it. If it was a low-current fault, no problemo since that's what the EGC is supposed to deal with.
That reminds me of my kitchen before I added a ground wire back to my panel. The ungrounded 3-wire outlets and power drops connected all the appliance chassis grounds to each other but the whole shebang was floating.

A bad overvoltage clamp device in a faulty power strip energized my entire kitchen appliances through the common ground (only low mA but enough to feel).
Quote
Now, think about what happens when you have four RVs connected together via a common EGC ground wire that's not bonded at the service panel. When you hook up a CATV wire that ties the chassis back to the service panel, and plug in a single RV with a direct line-to-chassis short, you can dump ten's of amperes of current through the CATV wiring. I've seen this a bunch of times and it always mystifies the local electricians.

So the next question is, how do we test for this sort of fault at a campground?
Line to ground through a 20A fuse. If the fuse doesn't blow, Houston we have a problem.

JR
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Mike Sokol

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Re: CATV/RV Grounding Issue
« Reply #9 on: March 05, 2017, 02:52:53 pm »

That reminds me of my kitchen before I added a ground wire back to my panel. The ungrounded 3-wire outlets and power drops connected all the appliance chassis grounds to each other but the whole shebang was floating. A bad over-voltage clamp device in a faulty power strip energized my entire kitchen appliances through the common ground (only low mA but enough to feel). Line to ground through a 20A fuse. If the fuse doesn't blow, Houston we have a problem.


Exactly right. And this is the same sort of thing that can happen in a distributed power system for sound systems. If there's a failure in the EGC Ground back to the Service Panel's bonding point, any single piece of gear with a significant line-to-chassis fault current will energize the chassis of everything with a ground plug. Then if you touch the mixing console, amp chassis or microphone and an earth-grounded object at the same time, your body will be in the middle of the fault path. If it's a low-current fault, then you feel a tingle. If it's a mid or high-current fault and your hands are wet, that's when you can die.
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