Nighthawk cm1100 has voltage coming out of cable line

[cgsfromkc]

I have had two CM1100 routers with the same issue. My cable line coming from my ISP keeps burning up and ISP says I have electrical issues (open neutral on my electrical) Electrician and electrical utility says everything is fine with the electrical in my house. ISP says that there is anywhere from 38 to 45 volts on the cable line coming from my house. I went back to my modem and checked voltage coming from it. I am getting 38 to 40 volts out of the cable input Jack. I have tested both the inner and outer core and on the outer core is where I am seeing the voltage.

I sent the first cm1100 in for replacement from netgear, and the replacement modem has started showing the same voltage causing my cable line to burn up, yet again. I also checked out a spare modem (cm500) and it is showing the same readings.

I don't believe that this is normal, or if it is then what should I tell ISP to do? I plan on testing their modem when they come to fix my line to see if it is doing the same thing.

Any other suggestions are appreciated.

[FURRYe38]

ISP needs to check the line up to the modem. Have them send a on site tech to check everything outside up to the home, 

Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem. 

[cgsfromkc]

I installed a brand new cable run from the ISP service connection straight to the modem. Where the cable keeps burning up is at the service connection where they have a ground block. That ground block keeps melting as well as the 6 to 8 inches of cable on both sides of the block.

I just don't understand why the modem is putting out the voltage when it only has a 12 volt input.

[FURRYe38]

Ya, ask for a on site tech. This is there responsibility. They need to check everything out. 

[antinode]

> [...] I went back to my modem and checked voltage coming from it.

   "checked" it how, exactly?  "coming from" or going to?

> [...] I am getting 38 to 40 volts out of the cable input Jack. I have
> tested both the inner and outer core and on the outer core is where I am
> seeing the voltage.

   Huh?  To what, exactly, are you connecting your (unspecified)
voltmeter?  Both wires.  "38 to 40 volts" AC or DC?

   If you make the same measurements (whatever they were) when the modem
is not connected to your co-ax cables, do you get similar results?

> [...] the replacement modem has started showing the same voltage
> causing my cable line to burn up, yet again. I also checked out a
> spare modem [...]

   When three different modems all "cause" the same problem, some people
might interpret that as something other than a modem being the _actual_
cause of the problem.

> I just don't understand why the modem is putting out the voltage when
> it only has a 12 volt input.

   You apparently don't understand the problem, or what's causing it.
Without knowing what's connected to what, or what you're actually
measuring, I don't know much more, except that three different modems
are unlikely to be causing the problem, whatever it is.

   Note that the modem power adapter is rated at 12V. 2.5A = 30W.  I
would not expect a 30W power adapter to melt/burn much of anything at a
distance.  If you disconnect the power adapter, do your measurements
change?

> [...] Electrician and electrical utility says everything is fine [...]

   You might need to get someone in-house who knows more.

[My_Username]

I suspect that the issue is exhibited with both modems - because the modem is not the issue.

The two-prong adapters shipped with these modems provide a great deal of isolation and would be inacpable of coupling any significant current to the coax shield.

I am not able to examine your system but I will speculate that the device downstream from the modem on the Ethernet cable is powered by a seemingly proper grounded 3 prong outlet and cord. For whatever reason there is a voltage difference between your household safety ground and the cable companies ground.In such case current would flow from your outlet ground prong, through the cord and your computer ground, through the Ethernet cable, through the modem and out the cable to the melting ground point/rod.

Either the electrician missed seeing voltage on your household safety ground or the cable company has a big leakage current problem. If it's the cable company, the coax ground will still fail even with the modem end disconnected and isolated.

Don't test any farther though. If you are seeing enough current to melt a ground connection you are looking at a shock/fire hazard. Get a cable tech or electrician back out there and print this out before they show up.

Good luck, best wishes and be sure to post the outcome.

[cgsfromkc]

As far as how I am testing the modem. I disconnected the coax cable from the back of the modem. I then took my digital volt meter and put the black lead to a known ground and checked the inside part of the coax Jack of the modem with the red probe. Where I am getting voltage at is the threads of the jack (which I am assuming is the shielded side of the coax). That is where I am showing voltage. This is exactly the same voltage that the ISP had seen 3 months ago when they replaced the ground block last time. When I disconnected the modem from the cable line, they didn't get any readings. That's when I first checked the modem to see if it was possibly putting voltage on the coax. And it was. I sent that modem back to netgear under warranty and now the replacement is doing the same thing. I tested an older modem that I keep for back up on my work bench and it too shows 38 volts on the threads.

I just had power company out and their electrician replaced some of the cabling by the transformer. He said it looked like squirrels had chewed through some of the outer jacket at the pole. He also said that my ISP needs to utilize their own ground rod, and not share the ground rod from my house panel.

I am waiting on the ISP to show up tomorrow to get a new service line run and to get them to install a new ground rod for their equipment.

If someone out there can check their modem the same way I checked mine and let me know if they are getting the same readings i would appreciate it. I am having the ISP bring me a modem and will check theirs as well to see if it is showing voltage.

I'm not sure if I can post a picture of my testing, but when I get someone else to take pictures I will try.

[antinode]

> [...] I disconnected the coax cable from the back of the modem. [...]

   What else is still connected to the modem?

> [...] I then took my digital volt meter and put the black lead to a
> known ground [...]

   "known" by whom?  Tested how?  This whole story smells of faulty
ground/neutral.

> [...] and checked the inside part of the coax Jack of the modem
> with the red probe. [...]

   "checked"?  Actual meter reading?  On which scale?

> [...] "38 to 40 volts" AC or DC?

   Still wondering.

> [...] Where I am getting voltage at is the threads of the jack (which
> I am assuming is the shielded side of the coax). That is where I am
> showing voltage.

   Voltage (also known as "potential difference") is not measured "at"
_a_ point; it's measured _between_ two points.  (Because it's a
potential _difference_.)

> [...] I tested an older modem that I keep for back up on my work bench
> and it too shows 38 volts on the threads. [...]

   Again, "tested" _how_, exactly?  When it's connected to what?

> [...] He also said that my ISP needs to utilize their own ground rod,
> and not share the ground rod from my house panel.

   Again, this whole story smells of faulty ground/neutral.

> The two-prong adapters shipped with these modems provide a great deal
> of isolation and would be inacpable of coupling any significant current
> to the coax shield.

   I'm with him.

> Don't test any farther though. [...]

   There's wisdom in that suggestion.

>    You might need to get someone in-house who knows more.

   Still my belief.

[cgsfromkc]

I am trying to get a picture of my testing methods. I took the modem to my work bench that has a GFCI that my electrician installed for me. So I am assuming that should suffice for a known ground. I don't have any cable from the ISP attached. I am solely testing from the ground to the coax jack. And I am observing the readings on my meter.

[cgsfromkc]

Here is the connection from the isp

[antinode]

> [...] picture [...]

   Ok.  AC volts.  Unlikely to be generated by the modem.

> [...] I am assuming that should suffice for a known ground. [...]

   It should, unless your house has ground/neutral problems.

   Does the power switch on the modem have any effect?

   What do you get if you eliminate the modem, and just grab the red
probe with your hand?

   One problem with a good-quality (high-impedance) meter is that a
little capacitive coupling can give you readings which are true but
not significant.

[cgsfromkc]

I have the same readings if power button on modem is off or on. If I disconnect the ac power pack for modem it is 0.112 volt on both spots. If I touch the red probe it is about 0.052

It does the same thing with my other modem, the cm500. Reads around the same voltage in the same scenarios.

[My_Username]

I still suspect the modem is just a bridge between the Ethernet ground and the coax shield/threads.

Unplug all connections to the modem and remove it completely.

Measure the AC voltage between the *cable* shield and a ground or chassis on the computer that the modem *was* connected to.

As previously stated the power adapter may have a few pF capacitance between AC line and its output. If all capacitances were symmetrical and your meter had infinite input impedance you would see half of your line voltage. Something less than that is normal. Regulatory agencies test that any leakage current is non-hazardous to humans in all conditions, let alone hazardous to grounding hardware.

Run your AC voltage test again with any normally functioning wall-warty supply in the house and you will get similar results. It means nothing in and of itself.

[My_Username]

Another modem test you could run:

Repeat your modem thread AC Voltage test with a 1k Ohm (1000 Ohm) resistor connected between your red and black meter leads. This essentially converts your AC Voltmeter into an AC milliAmp meter. How many AC volts do you read? The number will indicate your AC leakage current in milliAmps. I really don't suspect it will be enough to melt a ground connection.

[My_Username]

Are the cable jackets burned from the cable ground point toward the house or toward the cable company?

[My_Username]

Call a professional.

[cgsfromkc]

I will try with the resistor if I can find one around my bench.

The jacket on both sides of the ground block is burned up. Although it looks like the side toward the utility pole is burned up more.

I have the ISP tech scheduled for Saturday. Hopefully they will figure it out this time. I will have them redo the entire line from the utility pole to my house, as well as, install a separate ground rod as the electric company suggested.

[My_Username]

Taking the cable system to the same ground as the utility should yield minimum voltage between cable and utility grounds. That said two ground rods should work fine as well. I don't believe at all that the modem could be causing this. It's good you have someone coming out.

I am a retired electrical engineer having worked in the switchmode power conversion field for 40 years. Voltages to 30kV, currents to 6 kA. Not on the same device. 🙂 Anyway - I have seen a few ground problems.

[cgsfromkc]

Just wanted to give an update. Spectrum tech came out and replaced the line from the utility pole to the house. He said that they never install a ground rod and that they tie into the electrical ground. He is hopeful that what the electric utility did at the pole will take care of the issue.

I will see in about 3 to 4 months if it happens again.


Thanks for all the help.

[FURRYe38]

Sounds good. Hopefully you won't.