Voltage on Cable Line

Cable companies are required to install a grounding interface somewhere on your coax before it enters you house. This is primarily for lightning protection. I've seen them connected to electrical boxes, cold water pipes (assuming metallic, conductive pipes) and driven copper or copper-coated ground rods. Any grounding problems (leaks?) beyond that point are your problem and must be solved at your expense.

If your electrical system ground is open or defective, you could have a dangerous situation that could lead to electrocution or fires. A qualified electrician should be called in to check this, if in fact, you are convinced that this is the problem. There are inexpensive testers that can at least verify an open ground problem at the electrical outlet.

The cable modems i've seen either have separate wall wart power supplies or just two prong (non-polarized) ac plugs. There might be other types (with 3 prong grounded plugs) that I am not aware of. There should not be any current flowing from your cable modem to the shield of your incoming coax. The safety ground (third prong on the AC plug) should not have anything to do with the signal ground (coax shield) on an incoming signal cable.

Beachcomber

On 2 Jan 2007 13:19:56 -0800, snipped-for-privacy@gmail.com Gave us:

Check to make sure the connection to your HOUSE ground rod/water pipe is good and tight.

If it is a ground rod, slowly dump a couple gallons of water on it, and see if the problem goes away. Also, the FAULT wire in your electrical outl;et (third wire/green) and the tie point back at your distribution panel. You should have an electrician check those though if you are not electrically inclined or an electrician, etc.

That is the best ground for the structure you can get typically. ALL the connections in the service/distribution panel should be checked for tightness too, and this should ONLY be done by an electrician.

Your current ground rod may be a fudged, shortened version for that reason then. It may be the cause of the problem. An electrician has access to a tool that pipes a sound wave down the rod and can tell the tester how many feet of ground rod there is. It stems from the cable industry installers cutting corners, and making cable companies liable since it does not comply with code.

Check the cable on your house drop, or lock box if it is an apartment. Hard line cable has a 40 Volt DC feed on it, and it should get trapped out by the pole taps, but your voltage could be coming down the coax. Not as likely, but possible.

This is Usenet. You ask in these forums... YOU need to come back to these forums to gather your replies.

So. Many Usenet posters prefer to remain anonymous, particularly as it relates to email.

If you want answers from queries made in this forum, you should expect to get your replies here as well. The information is not merely for you.

Cable companies will always find someone else to blame when they find they cannot deliver the goods. Most likely there is nothing wrong with your electrical ground.

On Tue, 2 Jan 2007 20:11:04 -0500, "shark45" Gave us:

Do you have any communications experience at all?

Either the cable installation has a poor ground back at the ground block, or a bad fitting was cut, or the house wiring has grounding issues.

Learn to snip too, twit.

Ok, thanks to everyone for posting so quickly.

The thing is - the voltage isn't coming IN through the coaxial cable. If I hook up the modem to house AC - via non-polarized 2 prong plug, incidentally - and measure the center pin of the coaxial in the BACK of the modem, not connected to anything else except house power, to ground, I get between 7 and 13 volts. There's no way the problem is coming IN through the cable because the power is coming out of the back of an unconnected modem.

That being said, the ground rod is connected solidly - I checked that. I have no house drop - the electrical in my neighborhood is below ground. But that doesn't matter because the voltage cannot be coming in through the coax - see above.

Tonight I tried connecting the modem to a UPS system, but the voltage coming out of it is only 85 VAC, so I can't get a reliable answer from that. I also connected to several other outlets, including a surge protector on my TV, and I get the same results.

However, I ran the following test - I connected an old surge protector I had with coaxial protection on it. I plugged the cable modem into house power. The cable in the back was NOT connected to the incoming cable feed. But I fed it through the surge protector and measured voltage on the other side and it was negligible. AND that seems to have made my internet connection better.

I can't see how a house ground problem can be causing these problems. But I'm not an electrician. I'd appreciate any insight you all have on this. I don't doubt that it can still be the equipment the cable company is providing, but it can't be the incoming cable feed.

And by the way, it was a courtesy request to CC a copy to my e-mail. It certainly doesn't take any more time or effort to CC an e-mail address on a posting. No need to get snippy about it.

It sounds like you might have a bad neutral (grounded conductor) between the power company transformer and the power entrance into the house. Either a bad connection or the wire itself is bad. That wire carries current just like the "hot" wires do. The current can't flow on the neutral so it's trying to follow your coax instead. It's trying to flow back to the utility's transformer. It's a dangerous condition if my guess is right. Time for an electrician unless you're familiar with such things. You might ask the electrician about a 4 wire service from the utility's transformer to your house. It's a safer set up.

Dean

On 2 Jan 2007 18:14:46 -0800, snipped-for-privacy@gmail.com Gave us:

I meant the house drop for the cable. Where it attaches to the house should be a "P hook" (typically). There should be an "expansion loop" of additional cable, and at some point between the attachment and the entry into the structure should be a grounding block, and ground wire. Sometimes it is in the attic or the inside the house side of the entry point, but not usually.

THAT ground point should be your house ground, OR its own ground rod OR the cold water service.

On 2 Jan 2007 18:14:46 -0800, snipped-for-privacy@gmail.com Gave us:

Sounds faulty.

On 2 Jan 2007 18:14:46 -0800, snipped-for-privacy@gmail.com Gave us:

The word for today is baseline noise. You could also have an open shield in the coax run at SOME point in your house or as far as the taps on the splitter outside.

The cable co here used to place a trap on the cable modem segment of the lines, I assume to cut any digital noise from getting back into the cable channels... I dunno.

They don't use it any more though.

They did have to come place a specific splitter in our building's lock box that "passes DC" so I could get digital phone service. My Net hooks have been screwed up ever since. I get in streams fast, but a lot of retries on outgoing packets. Not like that before.

On 2 Jan 2007 18:14:46 -0800, snipped-for-privacy@gmail.com Gave us:

As I said, I like to keep my email to myself, and sending you one means revealing mine, even if only to you.

It isn't snippyness so much as it is an obligation for respect to the forum. You asked here. It isn't asking too much to expect you to complete the dialog here.

Dude... I have been calm the last few days. You should see me when I am snippy...

Dean:

Yours is the post that makes the most sense to me. I'm not an electrician, but if I understand correctly, the ground exists in case there is a problem with the neutral - that way the current has a way out other than through me or something else I don't want current flowing through. So if the neutral is bad, the current is looking for a way out, and some of it is finding the incoming coax when it's connected and going out that way.

So assuming that the neutral could be bad, it could either be a bad connection or wire inside my house, or could it be the feed from the power company to my house? For obvious reasons, I'd prefer the power company to come out and fix something outside my house rather than me paying an electrician to fix something inside.

If it probably is inside my house, what would I look for in tracking it down? It's in all the outlets, so it has to be at the main service panel somehow, right? Making it either the power company's feed or something bad inside the fuse box, if I'm guessing right.

Thanks alot for your help.

Matt

Not quite. The ground rods at a house are there mainly for lightning protection. If the neutral (grounded conductor) between the house and utility is bad, a bit of current can flow through the ground rod, through the soil and back to the utility company's transformer. Some could be flowing through your coax now. The rest could flow through a person given the right circumstances. Current will flow through all available conductors, not just the one with the least resistance. That's why this could be dangerous.

More likely from the house to the utility. The neutral (grounded conductor) and equipment ground are connected together in your panel. All three wires going from the house to the utility actually carry current. A problem downstream of your panel would probably cause a light or something to not work. It shouldn't cause stray voltage because the neutral and equipment ground are separate downstream of the panel.

The problem might be the neutral connection as it enters the panel. I would suggest not messing around in the main panel. It would be too easy to make a mistake. There is also an arc or flash hazard in addition to the electrocution hazard. I think burns cause more injuries to electricians than the shocks do. My memory fails me right now at what voltage the flash hazard starts. Electricians are cheap compared to doctors. Good luck.

Dean

Measuring voltage cannot tell you where the electricity is coming from. You know this. There is a 5 to 13 volt difference between cable and AC electric. Why? This must not happen because cable and AC electric must share a common earthing electrode. AC electric at the mains breaker box and cable must be at 0 volts. Again, this because cable shield and breaker box ground wire both meet at a common earthing electrode.

The purpose of that electrode serves many functions as defined by so many. It is for safety from a failed neutral. For safety so that a human is not electrically shocked. For surge protection.

Therefore inspect the cable where it enters the building. A grounding block (or splitter) is wired to the earthing rod. A thicker bare copper wire leaves breaker box to connect to that same earthing rod.

Although not related to your problem, that earthing rod is solid (cannot be shaken) and is 8 feet into earth. Put up on it and the rod should not rise.

Cable cannot be earth via a water pipe (or faucet) connection. It must be earthing where AC electric and telephone are also earthed; a rod that you (or your agent the electrician or builder) should have installed.

Now, implied is that voltage is in modem. Therefore you can measure voltage between modem and cable center conductor? You can measure same voltage from modem to cable shield connector? You can measure same voltage from modem to ground screw on wall receptacle? You can measure the same voltage from modem to cold water pipe? Based upon how I read your posts, all reading should be that same voltage.

A bad neutral must not create the problem you are seeing because (again) all three utilities (as demanded by code) must connect to the same earthing electrode.

OK, let's say all utilities do connect to the same electrode. This experiment is best done with an extension cord or some other long wire. Measure voltage from modem direct to the earthing electrode. Does a voltage exist? Do same from cable center conductor (the end that would connect to modem) to earthing electrode. Any voltage? Cable shield to electrode. Voltage? Grounding screw on wall receptacle to earthing electrode. Voltage? In each case, no voltage (less than 1 volt) should appear. For more thorough responses, list what each voltage measures.

Back to that neutral wire problem. If problem exists, then > Dean:

Without fully understanding your problem, I have some suggestions.

1. Run your modem power through an isolation transformer.

1. Run your signal through a line transformer.

That is, avoid your problems with multiple grounds rather than track them down.

Bill

-- Fermez le Bush

On Wed, 03 Jan 2007 06:27:10 GMT, Salmon Egg Gave us:

Show me a single cable installation that has a "line transformer" on it.

I cannot because I am not up on the subject. There certainly are broadband (pulse) transformers available with much greater bandwidth than is required for typical DSL. Transformer exist that use transmission line transformers for high frequencies that simultaneously use iron cores to support low frequencies.

Bill

-- Fermez le Bush

Where "ground" is what.

If measured to power system ground, the modem is not connected to the power system ground and will float. If measured with a digital meter, the meter impedance is very high and the meter can show a phantom voltage produced by a very low leakage current.

If measured to the coaxial connection around the center conductor there shouldn't be 7-13V.

A meter that is not "true RMS" will read wrong on non-sinusoidal waveforms, but if I am thinking right should read high. If you just connected the modem, the UPS may need to have a larger load to produce the right voltage (a guess).

If you use a plug-in suppressor all interconnected equipment (like computer and printer) should be connected to the same suppressor, or interconnecting wires, like LAN should go through the suppressor. Other external wires like phone, CATV, ... also should to go thorough the suppressor. A plug-in suppressor works by clamping the voltage on all wires to the common ground at the suppressor.

Verify that the ground block at the cable entry is connected to the grounding system for the power. It can't (in the US) just be connected to its own ground rod.

If you have underground metal water service, that is a better ground than a rod. If at least 10ft of underground metal pipe it has to be included as a grounding electrode in the US.

-- bud--

On Wed, 03 Jan 2007 14:24:15 -0600, Bud-- Gave us:

Not true. We were REQUIRED to drive OUR OWN 8 ft rod whenever we did a post wire on an apartment complex.

This was Time Warner (Warner Amex CUBE system at the time)

Single home drops MAY be attached at the house tie point, or a ground rod CAN be driven. It is for lightning protection, and the problems being discussed in this thread are not related to that.

That alone points toward the issue being with HIS power wiring, and NOT the cable system.

Matt, one thing to consider is that your voltmeter may be reading RF rather than AC particularly if it is a $3 drugstore special.

if this is the case there may be nothing intrinsically wrong with you house wiring or grounding.

you might try repeating the measurements with a .01 uF (or thereabouts) capacitor across the meter and see if it makes a difference.

if so a different approach may be called for.

i cannot speak for all cable installers, however the ones who wired one of my facilities were completely and totally incompetent. it took many months and many calls to get most of the issues resolved and the modems to be generally reliable. after switching to TI and turning off cable life was much better.

my recommendation is to politely but firmly and repeatedly insist on an inspection by the "head end technician". this is what the cable industry calls the chief engineer. with luck this will be an experienced and knowledgeable individual armed with advanced test equipment. if he says everything is OK from our end there is nothing more we can do then you can procede with troubleshooting the house grounding/electrical system.

in the end you may have to use a 'cable ground loop isolator' but see what the head end tech recomends first.