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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 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.
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.
20 Replies
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.Ya, ask for a on site tech. This is there responsibility. They need to check everything out.
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.
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.
Are the cable jackets burned from the cable ground point toward the house or toward the cable company?
Call a professional.
- 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.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.
