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Forum Discussion
Nighthawk cm1100 has voltage coming out of cable line
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 ut...
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.
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.
I just don't understand why the modem is putting out the voltage when it only has a 12 volt input.
Ya, ask for a on site tech. This is there responsibility. They need to check everything out.
> [...] 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.
- 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.> [...] 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.
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.