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Tv affects operation of 3-way light switch - please tell me why

Tv affects operation of 3-way light switch - please tell me why

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  #1  
Old 08-22-13, 09:27 PM
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Tv affects operation of 3-way light switch - please tell me why

I am a landlord and general contractor. I was called to a property in response to a call that a light operated from two locations (3 way switches) would not turn off. After checking all the seemingly obvious causes I was dumbfounded. I asked the tenant if she had made any changes in the room.

She stated she had just moved her home entertainment system into that room. All of the various components of this system were connected to a surge protector and plugged into an outlet that shared power to the three way switches.

I unplugged the surge protector and the three way switches worked fine (turn light off and on from either switch).

I unplugged all components of the entertainment system from the surge protector and started plugging them in one at a time. I ultimately discovered the plug from the HDTV was the culprit.

Every time that plug was inserted into an outlet, the light remained on regardless of the position of either 3 way switch. Playing around for a while, I further discovered if a put an adapter on the 3 prong HDTV plug that modified it to a two prong, ungrounded plug (removing the ground prong) it worked fine.

Although I've figured out what caused the problem, I have no concept of what is happening. I hate to conclude that magic is the cause of this problem. Can anyone help with an explanation of this phenomenon???
 
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Old 08-22-13, 09:35 PM
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Welcome to the forums.

No....it's not magic. It sounds like someone switched the neutral (white wire). When you plug the grounded plug in...... the neutral is getting connected to ground.
 
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Old 08-22-13, 09:58 PM
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Thanks PJ; but, if that was the case, wouldn't the problem exist when anything was plugged in? Several components of the entertainment system could be plugged in and turned on and the light worked fine. It was only when the TV was plugged in that the problem occurred.
 
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Old 08-22-13, 10:05 PM
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Duke, are the two 3-way switches regular hard-wired, 3-terminal switches, or are they "smart" switches or a master/slave pair?
 
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Old 08-22-13, 11:16 PM
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Nash, they are regular hard-wired 3-way switches. I'm absolutely positive they are correctly wired and they have worked flawlessly for more than two years. I keep thinking the ground plug on the TV must be getting some amperage from the TV through some sort of faulty circuitry - but even if that is the case I can't comprehend why the light fixture would remain on.

Thanks for taking the time to read and respond to my question.
 
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Old 08-22-13, 11:45 PM
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You could take a voltmeter and check at the receptacle. Check from the smaller pin (hot) to ground. When switches are on you should have power. When switches are off you should have no power.

My guess...... regardless of where the switches are you will always have hot.
Let us know.

If you always have hot..... the neutral is being switched.
 
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Old 08-23-13, 10:01 AM
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they are regular hard-wired 3-way switches. I'm absolutely positive they are correctly wired and they have worked flawlessly for more than two years.
The wire connected to the common terminal on one switch is the ungrounded conductor from the panel and the wire connected to the common terminal on the other switch is the ungrounded feed going to the lights? Neutral is never connecter to a switch?
 
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Old 08-23-13, 10:14 AM
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But in 3-way wiring a white is not always a neutral but instead an ungrounded conductor that wasn't properly redesignated. Someone changing a switch out could have had what appeared to be two whites and accidentally used the wrong one... maybe?
 
  #9  
Old 08-23-13, 08:50 PM
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Hypothesis

1. Perhaps at some time in the past somebody constructed a 3 way switch setup using the bare wire in a Romex cable because there were not enough wires. (This is improper.)

2. The bare wire was also left connected to the green screw of a receptacle and not to the other ground wires with the result that the receptacle was not truly grounded. (If the receptacle was grounded then the problems you report would have shown up a lot sooner.)

3. The antenna coax cable was still attached to the TV and the antenna system was properly grounded.

4. When the TV was plugged in, the bare wire used in the 3 way switch setup and the receptacle were grounded via the TV cord ground wire and the coax antenna cable going to the antenna or satellite dish components. This grounded the receptacle frame (yoke) and simultaneously caused the incorrect behavior of the 3 way setup.
 
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Old 08-24-13, 06:38 AM
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To test the above hypothesis, unplug the TV. Then unscrew the antenna coax from the back of the TV and also unplug the other audio and video cables. Plug in the TV and retry the light switch tests.

Repeat: Unplug the TV before touching anything behind the TV. This problem (if present in your 3 way switch wiring) will make those connections live with 120 volt power.
 
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Old 08-24-13, 09:56 AM
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I follow what you're saying..... but instead of going thru all that a check for switched hot. If the hot wire was switched like it was supposed to be, then there would be no power at the light and nothing could make it turn on .
 
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Old 08-24-13, 10:50 AM
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Allen, I think you may be "on the mark" with this hypothesis. I'll check it out and let you know for sure. I see you're from NH - I grew up in Claremont. Thanks so much for taking the time to read my post and to respond.
 
  #13  
Old 08-24-13, 01:59 PM
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PJ's answer is correct. The 3-way switch system is not interrupting the hot feed to the lights. If it was, only flipping the switches would affect the lights.

The 3-way switches are not wired correctly.
 
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