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Voltage Feedback Causes TV Signal Loss Due to Possible Grounding Problem

   
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Why Installing and Maintaining the Bonded System Ground Is Very Important: Improper Grounding Can Cause High Levels of Feedback Voltage and possible electric shock. Important Warning about Ungrounded Outlets.


64VAC Neutral to Ground

Electrical Question: I am a Senior Field Technician with a local cable company. I was at a trouble call for intermittent signal on a modem today.

  • I noticed when I unhooked the cable service drop the transmit level of the modem dropped substantially to an expected level based on the splitter configuration. This was my clue of voltage feedback from a TV somewhere in the house.
  • I isolated the modem outside and grounded the TV outlets so the voltage could not build up on the modem line. This resolved my issue but the customer was curious as to why the TV’s were feeding back voltage, I gave the generic response of “Sometimes TVs lose the chassis ground and do this, sometimes it’s a house electrical issue and sometimes there’s really no explanation.”
  • We then went inside and found that all 3 CRT type TVs (old box style from the 80′s and 90′s) were feeding back 45.8VAC. Just for verification I checked the electrical outlets to see what the voltages were. Across the line and neutral I had the expected 121VAC, across line to ground I had 40VAC, and across the neutral to ground I had 64VAC.
  • My question is, when I run into electrical outlets with similar situations what would be best to tell the customer? I typically tell them to consult an electrician, but I am curious about this myself. What causes this type of issue? I have a fair understanding of voltages and electrical wiring and tell the customers that I’m not an electrician but I know enough to know that isn’t right. Any help provided to improve my personal understanding would be greatly appreciated.

This electrical wiring question came from: Terry, in Texarkana, Texas.

Dave’s Reply:
Thanks for your electrical question Terry.

The Importance of a Bonded Ground System

Skill Level: Advanced – Licensed Electrical Contractor, Not Recommended for Homeowners.
Tools Required: Electricians Pouch of Hand Tools.
Estimated Time: Depends on the extent of the project, the grounding method to be used and available access to the project area.
Precaution: If the only ground source for the electrical system will be removed or modified then the electrical service should be shut off until the new ground source can be reconnected.
Notice: Making changes to the Electrical Service Panel and Electrical System should be done according to local and national electrical codes with a permit and be inspected.

Improper Grounding Can Cause High Levels of Feedback Voltage

  • When there is an abnormal amount of voltage that is detected between the neutral and the ground then my first impression is to check the main ground of the electrical system and make sure that it is intact and bonded.
  • The next thing I would check is to make sure the outlets are properly grounded as well.
  • My first clue would be if the home is older and the circuit cables are 2-wire without a ground, and the wall outlets are 3-wire with a ground.
  • After identifying the circuit and turning it OFF  the outlet would be removed and the wiring  would be inspected to see if there is a separate ground wire, and if not then check to see if the grounding method is according to code, or if there is a provision for a ground of any kind.

The Mistake of Installing Ungrounded Outlets

  • One of the biggest electrical mistakes that is made in the home is the when 2-wire outlets are replaced by 3-wire outlets, even when there is not ground wire available, and even worse is when a bootleg ground is put in place.
  • At the job site an electrician could performing specific tests that would reveal the exact problem, however ungrounded receptacle outlets or improperly grounded outlets can cause this kind of problem to the extent of creating a dangerous electrical shock hazard.

Important Warning about Ungrounded Outlets

  • If a home is found to have 3-wire outlets installed where there is no means or provision for a legal grounded connection that is installed according to code then the outlets should be replaced with 2-wire outlets.

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