20 amps on the TV cable?

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  #1  
Old 12-20-04, 11:16 AM
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Question 20 amps on the TV cable?

Could someone please help me to understand an electrical phenomenon which I recently encountered… I was presented with a problem where the house lights would dim when a 110v space heater was turned on in the room. I assumed that either 1) the circuit was overloaded causing the house wiring to heat up and add resistance to the circuit, or 2) there was a bad connection on the neutral which returns to the center tap of the transformer forcing the imbalance to flow through ground. A set of current readings on the transformer legs and ground yielded the following:

Tap “A” = 20 amperes
Center Tap = 0 amperes
Tap “B” = 0 amperes
Ground Rod = 0 amperes
Shield on the incoming TV cable = 20 amperes and hot to the touch!

The imbalance was apparently finding it’s way through the TV cable shield and back to the center tap (the TV cable shield is bonded to the #4 bare to ground). This wouldn’t be too difficult to understand if the neutral had a bad connection at the service entrance and the TV cable shared a ground with the center tap at the pole… but they are fed from different directions, from different poles and from different sides of the road!
I disconnected the incoming TV cable which arced and suddenly the lights in the house brightened and the dimming problem went away. A recheck of the currents showed that the 20 ampere imbalance was now flowing through the neutral.
That leads to two questions…
1) What the %*#&@ was going on?
2) When I reconnect the TV cable is it going to happen again?
 
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  #2  
Old 12-20-04, 02:06 PM
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It could be a bad TV also...
 
  #3  
Old 12-20-04, 02:37 PM
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Hint for the future: if you measure 20A going through a conductor, _don't_ simply disconnect it. As you noted, there was quite an arc.

If a conductor is bonded to ground, and carrying current to ground, then its voltage will be quite low; near ground potential. But if you then break the connection to ground, the voltage of the conductor will shoot up to whatever is supplying the current. A conductor could easily go from near zero voltage with 20A going through it, to 120V or more, depending upon the source of the voltage.

It sounds to me like you do have a bad neutral connection 'somewhere'. Your neutral current was returning to the transformer via the cable TV shield, to the pole, thence through the grounded messenger wire that goes from pole to pole, presumably around the block from where the CATV cable was grounded to where the transformer was.

I am guessing that somewhere there is either a thin break in a conductor, or else a bit of oxide corrosion in a splice. The oxide layer was sufficient to act as an insulator at very low voltage. The CATV bond kept the neutral at very low voltage, so the oxide layer was enough to break your neutral connection.

When you opened the CATV bond, the voltage on the neutral shot up, burned through the oxide layer, and the current started flowing properly in the neutral.

You should get the power company to check all of the connections from the transformer out to your main disconnect, including the connections in the meter. Somewhere there is a bad connection. This connection will only degrade with time.

-Jon
 
  #4  
Old 12-20-04, 03:36 PM
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What winnie said and also check your ground on the cable tv line. It should connected to your electrical system ground and not have it's own ground rod. Also check the bond to your waterline. I have heard of the same situation happening with the water pipe acting as the return path thought the neighbors house and out their neutral wire. Experienced plumbers carry jumper cables and connect across a main line pipe before they cut it just for what you experienced with the cable wire.
 
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