Amp clamp on CATV cable

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goldstar

goldstar

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New Jersey
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Electrical Contractor
I'm just curious as to whether anyone else has run into this. I recently installed (6) CATV lines in someones house as well as ports and splitters. Everything seemed fine with clear pictures on all TV's. When the cable company came out to install boxes for their hi-def services thay advised the homeowner that they used an amp probe on the incoming CATV cable and there was more than 1 amp of current on the line and that their electrician had to correct it. Until that was corrected they would have bars and fuzzy lines running through their TV screens. They were correct.

I would never have thought to do this but I went back to the house and clamped onto the (underground) incoming CATV cable and sure enough I had over 1 amp of current. It went even higher when I removed the wiring going into the house. After re-grounding the outdoor connection port I managed to get the amperage down under 1 amp. I was unable to check whether the water main ground to the breaker panel was clean and not corroded because the homeowner sheetrocked the basement and buried the connection behind a wall. Other than that I can't think of anything else that might cause this problem. Any ideas ?

Thanks,

Phil
 
The CATV shielding (assuming that it's properly grounded at the premises) creates a path parallel to the the service neutral, going back to the POCO transformer source.

Explanation:

Both the POCO neutral and the CATV shielding are bonded to the premises grounding system.

The pole-mounted CATV system shielding (grounded) is also electrically connected to the POCO neutral conductor (grounded) out at the pole ... even if only thru adjacent grounding electrodes at the pole butt.

Since we know current takes all available paths back to the source ... we now have neutral current on the coax shielding.
 
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Yes ... a ground loop is simply current flowing in the grounding concuctor (i.e. coax shielding) because of differences in ground potential at two physically different points.

And since the the ground potential at the pole is different from the ground potential premises by the amount of neutal conductor voltage drop (otherwise there wouldn't be any neutral current flow) that IS indeed a "ground loop."
 
JohnJ0906 said:
Dosen't change a thing-THEIR PROBLEM! I wouldn't touch it. I didn't install it, I didn't get paid for it, I didn't screw it up.
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Yes ... but a faulty neutral can be the cause of the current on the "parallel" CATV conductor.

However, I agree it's less-than-good design by the cableco out at the pole.
 
stickboy1375 said:
just curious. why should the electrician fix the cable companies service?
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The presumption by the cable company is that there is electricity being "induced" onto their CATV cable. Even though I didn't install the electric service I am the electrician for the homeowner. The CATV co. had a bonding wire to a metal conduit coming out of the earth. In their opinion they have bonded their connection port properly. I re-connected the wire to a different location and the problem subsided. However, as I mentioned I was not able to verify that the bonding wire (for the main breaker panel) on the cold water pipe was solid and free from corrosion
 
Well, I'd explain to the homeowner that the GEC connection to the water line has to be accessible and open it up to check the connection.
If it's part of the problem, it needs to be fixed because it can cause other problems as well.
Either that or I would drive 2 ground rods at the service and bond to them.
Just my opinion.
steve
 
There will always be current on the shield of the cable in a code compliant installation. The shield is required to be connected in parallel with the service grounded conductor. The amount of current will depend on the relative impedances of the grounded conductor, the coax shield and any other parallel paths. The is no provision in the NEC that would permit you to completely eliminate the current on the coax. Maybe you could wind the incoming coax in a coil to make an inductor to increase the impedance and decrease the grounded conductor current flow on the shield.
Don
 
Steve,
Either that or I would drive 2 ground rods at the service and bond to them.
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The impedance of the additional ground rods will likely be much greater than that of the coax shield and be unlikely to make any real change in the current flow on the coax.
Don
 
I have done many upgrades to satellite Dishes and have what I call tingle voltage on the shield.Sometimes its more than just a tingle. I have chalked this up to
the consumers faulty electronics. I would unplug every television ,VCR ,audio amp everything that is connected to the coaxes and see if the problem is still there.. If the problem goes away i would bill for a troubleshooting task.
 
I have found this also. it is caused by the consumers electronics in 99% of the cases and the cable co always blames the electricity. seems to me they need to evaluate the way they are doing things. their equipment is super sensitive to voltage on the shield and that is what the shield is there for to block the signal from getting to the conductor. you can find this voltage is induced and causing problems. maybe they will need to go to a triax cable soon.
 
electricguy said:
I would unplug every television ,VCR ,audio amp everything that is connected to the coaxes and see if the problem is still there..
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Did that already. There is no induced current on the lines going into the house. There is, however, induced current on the underground coax coming to the house until you properly ground it at the connection port.

I believe the problem has something to do with the grounding system as a whole but this homeowner buried everything when he sheetrocked the basement (CATV, phone blocks, water main ground, etc.). So, I can only take this problem solving so far.

don_resqcapt19 said:
The impedance of the additional ground rods will likely be much greater than that of the coax shield and be unlikely to make any real change in the current flow on the coax.
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Don, I agree with you on this unless I tie the ground rods back into the main breaker panel. However, the ground rods are only intended to be a suppliment to the water main ground and if that is defective in some way then the rods become ineffective.
 
I have seen this a few times and it is most likely caused by a poor grounded service conductor (neutral) between the transformer and disconnect, or your neighbors service neutral sharing a common transformer via water pipe.

So IMO it is not the CATV problem to fix, but the homeowner to locate and either have a electrician or POCO to fix
 
Dereck,
I have seen this a few times and it is most likely caused by a poor grounded service conductor (neutral) between the transformer and disconnect, or your neighbors service neutral sharing a common transformer via water pipe.
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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
 
Goldstar,
Don, I agree with you on this unless I tie the ground rods back into the main breaker panel. However, the ground rods are only intended to be a suppliment to the water main ground and if that is defective in some way then the rods become ineffective.
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This problem has nothing to do with the grounding electrodes. It is caused because the NEC requires that the coax shield be connected in parallel with the service grounded conductor. The only ways to reduce the problem is to increase the difference between the impedances of the grounded conductor and the coax shield or reduce the amount of grounded conductor current.
Don
 
We have had this occur many times.The current can even run back on the telephone ground and sometimes even melt through this telephone ground wiring if the current is high enough.The solution is generally global and requires the power utility involvement.Good Luck:) :)
 
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