Voltage Backfeed from Cable Modem

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Old 10-29-17, 11:41 AM
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Voltage Backfeed from Cable Modem

Hi, I have read just about every Internet forum post on my problem, and I haven't found a solution. I've found solutions to similar problems, but they were not exactly my issue. I am in California.

The symptom of my issue is that my cable modem randomly resets every now and then.

I had my ISP come out and they took some measurements on the line from the pole to my house and some additional measurements on the coaxial cables running through my house. They told me I have voltage on my coaxial line and I should contact PG&E.

I had PG&E come out and they took measurements on the line coming from the pole to my house and did a test with load. They did not identify a problem on the outside of my house. They said he panel was really well done. They told me to contact an electrician.

I had an electrician come to my house, and he took measurements on the inside of the house, and he did not know how to solve the problem.

The most damning measurement is this: When I power the cable modem to the wall outlet, and then I connect an open-ended coaxial cable to the cable modem, there is voltage on the other end of the coaxial cable. There is about 45 volts on the center wire and about 51 volts on the outer shielding.

The utility cable line is not connected to the cable modem in this test, so I do not think it is an issue on their end.

I tested the outlet here and there are roughly 120 volts on the small prong hole and less than 1 volt on the big prong hole when measured with the ground hole.

I used an outlet tester in every outlet in my house and there were no reported problems.

I tried a second, brand new cable modem, and it had similar issues but the voltages were slightly different but not by much.

I tried powering the modem in different rooms, and the problem is unchanged.

I tried unplugging all my electrical appliances and the measurement is unchanged.

I tried turning off all the breakers besides the one powering the modem and the problem is unchanged.

I'm stumped.

Some additional info that may or may not be relevant:
I replaced all the light switches in my house with z-wave smart switches. While trying to fix this problem, I pulled them all out of the recepticle boxes (but left them connected) to make sure there were no shorts when packed in. I also double-checked the wiring to make sure I didn't have any shorts or mixed up any wires and it looks fine.

My house was completely remodeled before purchasing it ~3 years ago (2014).

Once in a blue moon, the wired fire alarm will go off when nothing notable is happening within the house and stops on its own. It happens in a weird way such that it sort of stutters like a DJ scratching a record. When it goes off properly while cooking, it stays on until the smoke is cleared.

On different days, the voltages measured were lower numbers.

The cable modems' power adapters are non-polarized, non-grounded plugs.


Please help me with this problem if you've seen this issue before. Thanks in advance!
 

Last edited by VoltageIssue; 10-29-17 at 12:10 PM. Reason: Added more details
  #2  
Old 10-29-17, 11:48 AM
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Welcome to the forums.

These types of problems come up routinely and start all types of debates and discussions.

They told me I have voltage on my coaxial line and I should contact PG&E.
WRONG.

The cable system is a grounded system. There should be 0 volts on their cable. It is their responsibility to see that their cable is grounded. There should be, by code, a ground connection where the coax enters the premises.
 
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Old 10-29-17, 12:12 PM
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No voltage on ingress coaxial

The voltage is not on the ingress cable. The voltage is on the cable from the cable modem. Normally, this cable would be connected to the ingress cable. I'm saying that there is voltage backfeeding from the cable modem.
 
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Old 10-29-17, 12:18 PM
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So you are saying.... if you plug the cable modem in and check from that to the cable you see voltage.

Have you checked from the modem to a known good ground ?
Have you measure from the coax/cable to a known ground ?

If the cable is grounded..... then the modem problem would be the cable company responsibility. It would not be un-common to see voltage from an unconnected modem to ground as it's only has a two prong plug with no ground. +50v might be excessive. It could also be you have a very sensitive meter.

I have FiOS. My router reboots occasionally too.
 
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Old 10-29-17, 12:26 PM
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I will try to explain the measurement more clearly.

Cable modem power is connected to wall power outlet using the factory wall power plug.

The coaxial terminal of the cable modem is connected to an open-ended cable (for testing purposes).

One end of the multimeter is connected to ground (ground hole in wall outlet).

The other end of the multimeter touches the open-ended coaxial cable (connected to the cable modem). That is when it shows a voltage (both on center wire and on outside shielding).
 
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Old 10-29-17, 12:29 PM
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The cable modem plug is not grounded. It is not polarized. The cable company said the voltage coming from the cable modem should be close to zero and the voltage is causing the reboots.

I should also note that even when the cable modem is off (via switch on the modem) but still connected to the wall, this voltage reading exists.
 

Last edited by VoltageIssue; 10-29-17 at 12:39 PM. Reason: More info
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Old 10-29-17, 12:39 PM
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That's my point.... it's THEIR modem. They are responsible for getting it to work correctly. Your only obligation is to supply them with 120vac to that receptacle.
 
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Old 10-29-17, 12:43 PM
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It is not their modem. It is a modem I bought separately that is a much higher rated modem. And I tested this with two high-end modems just to reduce possibility of modem error.

It could be possible that all modems have this backfeed and backfeeding is not the cause of the resets. There seems to be consensus on the internet that voltage should not be there.

When I go to the office tomorrow, I will test the same measurement in my modems with the power there.
 
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Old 10-29-17, 12:52 PM
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Ok.... it's your modem. I didn't see that mentioned.

To eliminate or reduce the meter sensitivity.... connect a small wattage lamp across what you are measuring. If it lights.... there is a major problem. If there is no or just a minute amount of voltage measured on the meter.... there is no problem.

I would use a meter set to ua (microamps) to measure the leakage.
 
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Old 10-29-17, 01:19 PM
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I tried running a 110v 15w lamp in series with the cable modem and multimeter (wall power -> cable modem power -> cable modem coax -> lamp -> multimeter -> wall ground). The multimeter still read 42v, but the lamp did not light up. Should I expect it to light up even though the lamp is rated for 110v?

I don't think I can safely test the current because my multimeter (Innova 3320) is only rated for 200mA.
 
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Old 10-29-17, 01:45 PM
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Put the meter in parallel with the lamp. The lamp will dissipate and residual voltage.

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Old 10-29-17, 04:42 PM
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When connecting the components in parallel as your diagram illustrates, the lamp does not light up visibly and the voltage drops to 0.72 volts.

What does that information provide?
 
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Old 10-29-17, 04:54 PM
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It shows a very tiny amount of leakage from the cable box. Many times small capacitors are used from the two power leads to the metal chassis for noise suppression.

I would consider that normal and not the issue causing a re-boot problem.
 
  #14  
Old 10-29-17, 05:19 PM
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My cable modem is also a 120Vac direct feed and not grounded. I would expect near 60V ac on the shield to a true ground, due to the Y caps inside the modem power supply area. But, as noted, this should have little current potential. Like less than .5mA or so.

PS; as a kid, I use to put one lead of a DMM on my sister, the other to ground. And got over 30V or so. Voltage tests without knowing the impedance can be misleading.
 
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Old 10-29-17, 06:04 PM
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As "PJmax" mentioned already, I'd say that is normal. Your cable tech probably did not know what he is talking about (which is not uncommon).

The reason your cable modem reboots is most likely have nothing to do electricity at your home.

Is your cable modem actually rebooting or just loosing internet connection and back?
Are you able to access modem's log/event page?
You can access cable modem's web interface by typing 192.168.100.1 on your web browser, but log page may or may not be accessible depending on your modem's model and how it is provisioned by cable company.

If your modem is actually rebooting, you will see repeating logs with wrong date (logs created while the modem is booting up).
If you have bunch of T3 time-out that means your modem is loosing cable signal.


I once had issue with internet connection, but TV worked fine. First technician who came out and made new F connectors and replace splitters then said it is working fine. Few hours later, lost internet connection again. Another higher tier technician came out and he found that center pin was broken at the tap on the pole.
He tested by installing 75 ohm terminator on one end and measuring resistance (ohm) at the other end with multi-meter.
Because RF signal still can transmit over small distance, just testing for the signal strength may not be enough.
 
 

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