neutral on CATV sheaths

[karl riley]

Anyone know how to keep neutral return current off CATV sheaths?

They have to be grounded to the premises grounding electrodes, but are any kind of low-voltage blockers allowed? The idea is to allow lightning and HV power surges to discharge to ground, but keep low voltage neutral off.

Karl

 

[charlie]

Re: neutral on CATV sheaths

Karl, it may not be a problem due to the higher impedance of the sheath and the low frequency of the power. On the distribution system, the CATV and other types of communication cables are required to be bonded to us on a regular basis (4 times per mile).

 

[dereckbc]

Re: neutral on CATV sheaths

Charlie, the lower the frequency, the lower the impedance. Not sure your last statement made since too me about the sheath and low frequency.

 

[charlie]

Re: neutral on CATV sheaths

Dereck, I was talking about two different things in my response. I am thinking about interference of the low frequency on the signal and about the small size of the sheath for greater impedance to the current flow.

 

[dereckbc]

Re: neutral on CATV sheaths

Sorry Charlie, I have bee a little slow since I quit drinking 10 years ago.

 

[hurk27]

Re: neutral on CATV sheaths

Karl
I did a nite club that had this problem when they tried to connect the audio output from a VCR into the mixing board it caused a 60hz hum in the PA system. After doing some checking that is exactly what I found and used a 75 ohm to 300 ohm back to 75 ohm matching balun's to isolate the current where the coax entered the building. I bonded the street side to the service GE, and the building side to a 300 ohm grounding block which was bonded to the service GE. This eliminated the hum.
I have obtained what the cable company called a current block which was a 1 to 1 transformer which did the same thing, from our cable company. But this was when I had a computer with a PCI card cable modem which was grounding through the case of the computer. The owner was tired of getting shocks when he unhooked the cable from the computer. :roll:

 

[hbiss]

Re: neutral on CATV sheaths

Actually current, beyond a certain amount on the coax sheath, whether it's the drop or the hard distribution cable on the street will modulate the RF with 60Hz enough to cause hum bars in the picture. The only way to eliminate this is with frequent bonding along the street to equalize currents.

You don't say what problem, if any you are experiencing. The coax drop shield is basically bonded at the pole to the power neutral and must be bonded to the service ground at the house. Some current would be normal and analogous to the current on the service ground connected to a municipal water system. If the current is excessive look for the usual causes of a high impedance service neutral.

There are isolation devices available that will interrupt the shield and center conductor DC and low frequency AC continuity passing only RF, but these can only be used after the ground block within the premises. They would normally be used to eliminate ground loops in certain AV installations.

-Hal

 

[hbiss]

Re: neutral on CATV sheaths

The owner was tired of getting shocks when he unhooked the cable from the computer.

Wayne, if you think about it there is no reason that there should have been such a large voltage differential between the cable and the computer.

Both the computer, through the branch circuit EG and the cable drop, through the ground block at the service entrance should have been grounded at the same point. There would be no reason then for a voltage between the two unless one or both grounds were missing or improper.

By installing the ground isolator (not to be confused with the ground block I mention above) you may have eliminated the symptom but you let a potentially hazardous condition continue. Next time find out why there is enough voltage to shock on the cable or computer. This is not normal! Most likely you will find a missing ground on the CATV drop or maybe only a 3' ground rod.

-Hal

[ February 21, 2005, 08:33 PM: Message edited by: hbiss ]

 

[karl riley]

Re: neutral on CATV sheaths

Hal and Wayne, your info is good. But it sounds as if there is no way following Code to eliminate the parallel path for neutral between service point and transformer. Right?

Here is why I asked. I have to fly out to a client who has leukemia who wants to get rid of the high fields in her house. From the description, some are due to circuit miswirings (misconnection of neutrals (oops, GCs). But there are also sizeable fields from the water pipe and from the CATV which runs along the outside of the house. I suspect corrosion impedance in the service neutral, but just in case cleaning that up doesn't reduce the CATV current (sounds like one or two amps)I wanted to understand how baluns work. So they don't stop the 60Hz in the sheath outside the house?

Maybe re-routing the cable would be the answer.

Karl

 

[hurk27]

Re: neutral on CATV sheaths

Karl they will acutely block anything below about 100khz. If installed right after the grounding block before the cable splits up its different paths around/into the house the only field you should have is the field from just the drop.

This is what one looks like and as long as the shield is bonded to the service GE before the isolator. I don't see a problem with the NEC?

And can buy here: Naval supply

And Hal you are right the cable came into the house on the opposite side from the service and the home owner didn't want to pay us to do the grounding that he thought the cable company should have done. So as far as I know he had them do it. But at the night club there was a enough current still flowing in the cable shield that even after bonding to the service GE it still cause a very bad hum in the PA.

 

[karl riley]

Re: neutral on CATV sheaths

Thanks, Wayne. What I am thinking is that the offending field is from the CATV drop outside the house, and that the balun is not allowed to interrupt the sheath's grounding to the SE point.

What I am wondering is that since the grounding of the sheath is for the purpose of carrying current under significant voltage, why can't some device block low voltage neutral return current and let through voltage due to ground faults, lightning and HV surges?

By the way, since I am in the last stages of getting my book revision ready for printing, I want to get this right before I finalize the section on magnetic fields due to parallel paths on CATV sheaths.

Karl

 

[hbiss]

Re: neutral on CATV sheaths

Karl, the more I think about this- I don't see anything in the code that would require that the coax drop shield be continuous from the tap on the pole to the house, only that it be bonded at the point of entry.

We have a similar situation with shielded TELCO drops. It is usual practice to bond only at the pole or pedestal, the building end is left floating. The metallic splice case at the building end as well as the protectors are bonded to the service ground, but there is no connection to the shield. The reason is to keep whatever is outside outside.

With a CATV coax drop it is a slightly different situation in that there is no protector from the center conductor to ground at the house as there is with the copper pairs of a TELCO drop. Only a ground block is required to ground the shield to the service ground.

The requirement to ground only the shield may be because the DC resistance at either end of the drop, (due to the baluns in the tap and the TV or cable box) from center conductor to shield is very low, effectively shorting the center conductor to the shield and then to grounded for DC and low frequency AC. Thus grounding the shield would keep these voltages off the center conductor also.

My thinking here is that there really isn't anything to prohibit using a ground isolation device ahead (on the street side) of the ground block at the house. As long as you have that ground block that bonds the shield at the point of entry code would be met.

-Hal

 

[hurk27]

Re: neutral on CATV sheaths

Hal I was rethinking this also and I think your right. The intent of the code is to limit the current from a lightning strike or contact with higher voltages from entering the house. so if a isolation device is placed ahead of the grounding block it wouldn't matter, as the grounding block would still prevent the higher voltages from entering the house. just the drop will no longer have the neutral current on it. This would work even at the house end of the drop, just have the isolators on the street side of the grounding block.

 

[dereckbc]

Re: neutral on CATV sheaths

Karl, Wayne, Hal, It is possible to use a balum or some isolation technique in the drop somewhere to break the continuity thaus the return current. But I am not sure how much luck you would have with the local CATV operator to install one, let alone if they stock one to install. I am sure they have thier own rules.

 

[karl riley]

Re: neutral on CATV sheaths

This makes sense to me. Let's see if anyone else has an idea on this.

Would the isolator illustrated work?

The cable guys want to cooperate in this case and want info.

Karl

 

[hurk27]

Re: neutral on CATV sheaths

Hi Dereck
I have received allot of devices from the cable/phone company's over the years as a professional some do give a little common courtesy that we try to give back in return. AS I said in a previous post I have acquired a couple of these 75 ohm isolators from the cable service man before, after explaining the problem and they were more than happy to give them up not having to do the tech work themselves in-spite of the price tag I have found at suppliers. I have also had a phone service tech supply me with tunable RFI filters so we can install them when we have a problem house ( how about a whole neighborhood of them)

[ February 23, 2005, 12:20 AM: Message edited by: hurk27 ]

 

[dereckbc]

Re: neutral on CATV sheaths

Wayne shows you how much I work between a CATV company and a home owner, however I do know a little about phone companies and RF, and yes we can provide RFI filters for POTS, but DSL, problems-problems-problems

 

[hbiss]

Re: neutral on CATV sheaths

Karl, I think if you installed an isolator before the ground block it would be YEARS if ever until the cable company discovered it! Even if they do I can't see why they would object.

-Hal

 

[karl riley]

Re: neutral on CATV sheaths

Thanks Hal, but I want to see if the cable company will install it. I will know in a couple of weeks. I'll post the results when I get back.

Karl

 

[hbiss]

Re: neutral on CATV sheaths

I can hardly wait to hear what they are going to tell you! :roll:

-Hal