Amp clamp on CATV cable

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JohnJ0906 said:
I would think that this isn't a rare occurance, so what would the fix be?
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An isoltation transformer, aka a ground-loop breaker.

180-075L.jpg


http://www.partsexpress.com/pe/showdetl.cfm?Partnumber=180-075
 
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What irks me is that this is obviously something that should be addressed by the cable company, and they try to foist it off on the electrician. Between them and the phone company, I am real tired of having the finger pointed at me for their issues.
 
JohnJ0906 said:
Between them and the phone company, I am real tired of having the finger pointed at me for their issues.
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John, you're not being paranoid; it really IS your fault. ;)
 
don_resqcapt19 said:
Dereck,

This happens even without a grounded service conductor problem. The shield is connectect in parallel with the grounded conductor. This is required by the code.
Don
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Don I agree, never meant to imply it could be eliminated, but it can be minimized to far less than 1-amp. Current on the shield is the nature of the beast, but high amounts to me indicate a neutral is starting to open and finding alternate path back to the source. CATV companies are fully aware of the problem and most I have dealt with use 1-amp as pass/fail point.
 
Larry,
820.93 Grounding of Outer Conductive Shield of a Coaxial Cable
The outer conductive shield of the coaxial cable shall be grounded at the building premises as close to the point of cable entrance or attachment as practicable.
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You can't use that device to eliminate the current on the coax shield between the building and the pole or underground distribution pedestal.
820.93 Grounding of Outer Conductive Shield of a Coaxial Cable
The outer conductive shield of the coaxial cable shall be grounded at the building premises as close to the point of cable entrance or attachment as practicable.
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Don
 
Dereck,
Don I agree, never meant to imply it could be eliminated, but it can be minimized to far less than 1-amp.
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I just looked at the DC resistance of a coax with a copper shield and the resistance for the shield is listed at 6.5 ohms/km. The DC resistance of #2 copper is 0.634 ohms/km. If there is 10 amps of grounded conductor current in this example about 0.9 amps would be on the shield, assuming that they run to the same location. If the coax has an aluminum shield the DC resistance would be about 11 ohms/km so you would have about 1 amp of current on the shield with 20 amps of grounded conductor current. Of course as the house service size is increased the size of the grounded conductor would increase and the resistnace would decrease. With a 4/0 copper grounded conductor and the coax with the copper shield about 3% of the grounded conductor current would flow on the shield and about 1.8% if the shield is aluminum.
Don
 
Don I don?t dispute your numbers, just reinforcing what you are saying that there will be some current flowing on the coax shields and CATV draws the line at 1-amp.

In addition I am just passing on a troubleshooting tip;
If you are getting hum lines on the analog channels, a good place to start is looking at the current on the coax, and if it seems high, suspect the service neutral is opening up. Might even be you neighbors. If you notice hum bars and some lights are bright while others are dim, you more than likely have a service neutral problem.
 
Dereck,
My only point is that unless you take some action to change the relative impedances of the two paths you could easily have more than one amp of current on the shield without any type of problem on the electrical system. Is there some type of device that you can use to increase the impedance of the coax and still make the installation code compliant?
Don
 
don_resqcapt19 said:
... You can't use that device to eliminate the current on the coax shield between the building and the pole or underground distribution pedestal ...
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Don:

Just for the sake of argument ... why not?

The coax shielding running from the pole would still "grounded"; albeit the shield conductor would now have a higher impedance at the "premises end".

What (if any) rules prevent adding impedance devices to reduce objectional shield current? How would this be any different from the home-made coil inductor you suggested in an earlier post.

Isn't the CATV house drop (prior to the demarc/ground block) governed by NESC anyway?

[Edit to add] Thanks for working-out and posting the relative impedances and current flows between the coax and service conductors ... it helps explain a lot!

Jim
 
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Jim,
What (if any) rules prevent adding impedance devices to reduce objectional shield current? How would this be any different from the home-made coil inductor you suggested in an earlier post.
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I am seeing the isolation transformer as a open, not an increase in impedance. The code requires the conductive shield to be grounded at or near the point of entrance to the building. I am not sure what the NESC says about this installation on the street side of the building. Maybe the cable company could install one on their end, but if they could I think they would be doing it as a standard practice. I am not even sure my idea of an inductive coil would be code compliant.
Don
 
don_resqcapt19 said:
... I am seeing the isolation transformer as a open, not an increase in impedance.
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Aha !!! Now I undersand your reasoning.

I agree that an "open" in the house drop coax shielding would be illegal ... I was thinking of Larry's posted device as a simple inductor.

Thanks! ... and thanks again for your "comparative conductor" post (#27).

Jim
 
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My experience with noisy coax has been that the problem origintates within the house, after the exterior grounding block. Often, a powered sub-woofer completes the ground loop. Don't overlook the AV system itself when troubleshooting these issues.

The ground breaker I posted (and they also have them for RCA-plug-equipped interconnects) are usually used at the cable tuner box, not outside the premises.
 
Larry,
The ground breaker I posted (and they also have them for RCA-plug-equipped interconnects) are usually used at the cable tuner box, not outside the premises.
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I don't see any issue with using them at that location in the circuit. I was under the impression that the current in the original post was between the house and the outside cable trunk line.
Don
 
Just to clarify things, this was an underground installation. With all cables attached there was approx. 1.5 amps of current induced onto both sides of the CATV cable. When I detached the cables into the house the induced amperage on the street side raised up to approx. 2.5 amps. Once I grounded (or I believe I did) the termination port and reconnected the interior cables I managed to get the amperage on either side down to somewhere less than 1 amp.
 
Is this an underground dip from a common utility pole shared with POCO or is it completely underground service from both CATV and POCO?

If both are undergound services, there should be very little if any current on coax shields.
 
LarryFine said:
My experience with noisy coax has been that the problem origintates within the house, after the exterior grounding block. Often, a powered sub-woofer completes the ground loop. Don't overlook the AV system itself when troubleshooting these issues.
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i too have seen this to be a problem. when using a surge suppressor that includes a coax connection, this contributes to ground loops.
 
goldstar,
When I detached the cables into the house the induced amperage on the street side raised up to approx. 2.5 amps.
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I don't understand. How is there any current flowing on an open circuit?
Don
 
Dereck,

These are completely underground services in an underground area. The POCO, telephone and CATV services are in the lawn approx. 30' from the residence.

don_resqcapt19 said:
I don't understand. How is there any current flowing on an open circuit?
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Don,

I don't know how to explain this but I removed the CATV cable entering the residence and clamped onto the incoming CATV line and read about 2.5 amps until I properly grounded the connection port. At that point I reconnected the CATV cable to the residence and the amperage diminished to less than 1 amp. I don't remember what the reading was before I connected the cable into the house.
 
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