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Cisco STB Ground Leak

I have FiOS triple play with a Cisco 335HDC STB and a Cisco 435HDC DVR.

I was re-arranging the coax connections in my home as I was moving my router. While doing this, I got a nasty shock from the coax. After some careful debugging and measurements with my voltmeter, I found that both of my Cisco STBs are putting out 20-40VAC on the coax ground. Since the ONT has a good ground connection, it is shunting the leakage current to ground when the coax is connected.

I verified this by having nothing but power connected to the STBs and then measuring the voltage on the coax and chassis against ground and neutral. I measure 20-40VAC, enough to explain the nasty shock I got.

Has anyone else ever seen anything like this? I will probably ask Verizon for replacement boxes, as this is a significant safety hazard. I am curious if others have seen (or felt) this.

Comments

armond_in_nj1

@gs0b wrote:
... I got a nasty shock from the coax ... measurements with my voltmeter [show that] both Cisco STBs are putting out 20-40VAC on the coax ground. Since the ONT has a good ground connection, it is shunting the leakage current to ground when the coax is connected ... I verified this by having nothing but power connected to the STBs and then measuring the voltage on the coax and chassis against ground and neutral. I measure 20-40VAC ... I will probably ask Verizon for replacement boxes ...

Certainly it's a good idea to have the boxes checked by either a tech or the factory, or to have them replaced when they show a dangerous defect. However I am puzzled by two other separate questions.

First, did you have any evidence of stray current prior to "rearranging" the coax? IOW what happens when you revert to your original wiring layout?

Second, it seems too much of a coincidence that both boxes are now showing this behavior. One maybe, but both? I would rather take a close look at your 110 AC supply, which itself could have somehow developed a ground leak.

Just thinkin' out loud, of course.

gs0b

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armond_in_nj wrote:

First, did you have any evidence of stray current prior to "rearranging" the coax? IOW what happens when you revert to your original wiring layout?

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FYI, I'm an EE, so I know what I'm doing. 😉

I hadn't checked for leakage current before doing this work, as I didn't expect there to be such a problem and didn't go hunting for it. I was simply moving my router from an upstairs room to my basement, so it could be right next to the ONT (it's inside). I had to disconnect the coax from the ONT to add a splitter to feed the router and the TVs, while I had removed a splitter from the old location. While adding the splitter near the ONT, I was holding both the coax from the ONT and the coax from the STBs. A nice shock caught my attention, which then led me on a "leakage hunt" with my voltmeter.

Of course, I've checked for proper ground and neutral connections at the outlets in my home and checked for leaks from other AV equipment. Everything else is fine. I know it's the STBs because I can measure a voltage between the coax/chassis ground relative to both neutral and ground, with nothing other than power connected to the STBs. (Of course, like most North American homes, neutral and ground are the same in my home - and yes, I've checked my panel for proper ground bounding).

I agree that it is odd that both STBs show this problem. My theory is that a surge somehow got into ground (say, from a nearby lightning strike) and that the MOVs in the STBs shunted it but got damaged and are now leaking. Since I can't open the STBs, I can only eyeball the MOVs through the vents in the chassis. I don't see any obvious signs of damage.

I will be calling Verizon one of these days to see what they say, although I'm not sure how the phone techs will respond to my issue. I also want to watch a few more things on my DVR before it gets swapped. 🙂

Finally, the reason I was moving my router is I got a rev I, 802.11n model about three months. My old 802.11b/g router had been in the basement and worked fine. The new one could not cover the house with WiFi, so I moved it upstairs. I recently bought +9dBi antennas in an attempt to get it working from the basement, which is why I was moving it back to the basement. The antennas didn't make enough of a difference, so I've already moved the router back upstairs. No change in the leakage. I didn't notice any leakage shocks three months ago; but maybe I didn't hold the coax and ground at the same time.

Thanks for your thoughts.

armond_in_nj1

@gs0b wrote:
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@armond_in_nj wrote:
... did you have any evidence of stray current prior to "rearranging" the coax? ... it seems too much of a coincidence that both boxes are now showing this behavior ...
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... I hadn't checked for leakage current before doing this work ... While adding the splitter near the ONT, I was holding both the coax from the ONT and the coax from the STBs. A nice shock caught my attention ... I know it's the STBs because I can measure a voltage between the coax/chassis ground relative to both neutral and ground, with nothing other than power connected to the STBs ... I agree that it is odd that both STBs show this problem ... I've already moved the router back upstairs. No change in the leakage ...

No "eureka" ideas have occurred to me following your post, other than to note that the issue evidently appeared following changes in the physical wiring of your network.

A rather primitive and perhaps obvious view is that physically messing with connections could possibly have had an adverse effect on the integrity of the connections themselves (a.k.a. broken wires, cracked ceramic insulators, damaged coax connectors in the STBs, etc., etc.). If you change out the boxes and find the issue has disappeared, I suppose we'll both be relieved.

gs0b

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armond_in_nj wrote:

A rather primitive and perhaps obvious view is that physically messing with connections could possibly have had an adverse effect on the integrity of the connections themselves

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I'm sure that the connectors aren't the issue for two reasons.

First, I didn't touch the coax connectors on the STBs. I disconnected the end of one cable at the wall outlet for the DVR to move the splitter. The other STB is in another room that I didn't even go into; until later I measured the leakage voltage at the distribution splitter in my basement after isolating each coax run. Then, I worked my way back to each STB to make sure the leakage wasn't a result of a fault in the in-wall wires. When I finally got to the other room, I had checked everything along the way before I removed the coax cable and measured the leakage directly on the STB. In other words, I was very methodical (after I got the shock, that is 🙂

Second, if the STB design is such that a damaged coax connector can induce 20-40VAC, I'd be very surprised that it would have passed UL, Verizon and other safety approvals. My best guess is that the something has failed in the internal power supply allowing a small leakage from hot to chasis ground. Since this is not a grounded appliance, the chassis is floating, thus exposing this leakage voltage to the chassis and coax ground. Since the ONT has a solid ground connection tied to the coax, it ends up as the path to ground for the leakage current.

Anyway, I'll post back when I get the STB's swapped.

Thanks for your thoughts; this is a bit of a brain twister and I appreciate the ideas.

Pstreicher11

Here's what I would do. I would go to your nearest Verizon store and ask a tech to let you measure a STB that they have in the store. First explaining what you have explained to us here and trying to get a tech or rep that is cooperative and can understand what it is you simply are trying to troubleshoot and prove.

It seems you might have a unique situation as I too don't believe there should be any appliance out there in consumer land that would have a voltage potential like this. Although, who knows. Now you got me to want to go home after I get off work and check out one or all of my boxes with my meter. 🙂 If I do and I find anything I will surely let you know here.

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