CATV coax melted

[nakulak]

820.100 Cable grounding

If that was my house I would cut the cable at the entrance to the house. then I would

1) see if voltage was present on the outside portion or inside portion
2) if inside, trace back till I found where the voltage source was

it could be a neutral, it could be a bad appliance (faulty box), or it could be a hot wire resting against the cable somewhere. providing a ground path might give a path back to source, but personally I would not sleep at night if I didn't find out where that current was coming from.

 

[LarryFine]

I would check for voltage between the utility side of the cut coax to a ground rod, from the house side of the cut coax to the rod, and a clamp-on ammeter from the house neutral to the rod or other electrode.

 

[rf1031]

I would check for voltage between the utility side of the cut coax to a ground rod, from the house side of the cut coax to the rod, and a clamp-on ammeter from the house neutral to the rod or other electrode.

Do I have to run test lead all the way from fourth floor to ground rod? Since it is new york city and all electrical wiring is BX, could I just check for voltage to j-box or third pin of the closet receptacle? Electrician claims to have already checked resistance of ground and it was unmeasurable zero. So wouldn't anything I might find at ground rod also be present at closet grounds?

I don't own ammeter. Is voltage measurement enough to find problem (even if I can't find out how much current is there)? Can current be present without measurable voltage?

Could voltage/current be present intermittently such that it might not be there when I measure but might still be dangerous?

Thanks

 

[rf1031]

820.100 Cable grounding

If that was my house I would cut the cable at the entrance to the house. then I would

1) see if voltage was present on the outside portion or inside portion
2) if inside, trace back till I found where the voltage source was

Should I disconnect TW's new ground when I check for voltage?

Thanks

Randy

 

[eric stromberg]

Where the coax connects to the equipment, install two "Baluns" back to back. These are the 75 Ohm Coax to 300 Ohm Twinlead impedance matching transformers. If you go from coax to twinlead and then back to coax, you have decoupled the shields and broken the alternate path for current to flow.

Hope this helps

 

[nakulak]

yes - carefully measure voltage to ground after removing the paths to ground - using all safety precautions applicable - please remember the forum rules as this is not a DIY info forum, but a forum for professionals - and check voltages to ground to locate source - (insulate the cut leads as you would any line voltage wire because there is no way of knowing what the source is - it could be unfused poco wiring just as easily as it could be your own wiring, take all necessary precautions and use an appropriate cat IV meter

 

[rf1031]

please remember the forum rules as this is not a DIY info forum, but a forum for professionals

I appreciate that. I am in awkward limbo - lo V pro with hi V on lo V wires. And NEC applies to my work too. Heck, even a pro central vac installer can have code/safety issues.

My concern is that neither the electrician nor the CATV provider have found source of current (although CATV has taken action to eliminate possible cause). But my limited expertise and this forum suggest that danger exists until cause is identified. And if a piece of my equipment is the culprit, I will be professional...
ly liable.

Thanks

Randy

 

[LarryFine]

Do I have to run test lead all the way from fourth floor to ground rod? Since it is new york city and all electrical wiring is BX, could I just check for voltage to j-box or third pin of the closet receptacle? Electrician claims to have already checked resistance of ground and it was unmeasurable zero. So wouldn't anything I might find at ground rod also be present at closet grounds?

I don't own ammeter. Is voltage measurement enough to find problem (even if I can't find out how much current is there)? Can current be present without measurable voltage?

My reason for suggesting checking between the local problem (the coax shield in this case) and a remote real ground (the electrode, for example) is that the local ground (the nearest receptacle's EGC) would likely be at the same potential as other supposedly-grounded items (coax shield, for example).

In other words, you're using the local 'mesh' of wiring, appliances, plumbing, etc. around you on the third floor as your reference. The overheating conductive path, the coax, is connected between the local ground reference and the 'rest-of-the-world' ground reference. ("Welcome, Neo, to the real world.") Something is energizing one end of this conductive pathway.

There is a potential between the local ground (3rd floor) and the remote ground (real world) that is causing a current to flow on the coax shield. A clamp-on ammeter on the coax while it's being heated would show this current. Opening this pathway and measuring the voltage across the opening would show why.

Normally, the system neutral and/or EGC prevent this voltage by proper bonding. When this kind of problem exists, most of us reach the same initial conclusion: open neutral. Any conductor must present a low impedance for it to carry the current it should, and not allow an appreciable voltage difference between its ends.

Even the most complex of unusual problems obey Ohm's Law. Troubleshooting is all about applying the basics to the situation. You need to be able to note the symptoms, determine all of the possible causes, and then eliminate them, in groups first if possible, and then the remainders one at a time. Experience is paramount.

When you check for voltage on one wire that should be grounded, as compared to another wire that should be grounded, all you're proving is that the two are well bonded together. Your local shield is at the same potential as your local EGC. But, your incoming coax is melting. The other end is bonded to the real world, the earth.

With a properly-wired feeder, the EGC does not carry current, so should not present a voltage drop, which is why I suggest checking for voltages where there should not be any. Eliminate a group of suspects. So, if the local EGC is hot (re: the real world), one likely location of an open is ahead of the service (the utility side).

Of course, this is a jump of a conclusion, but it's a place to start.

 

[rf1031]

Larry,

That was a great post. I'm quite appreciative (as I am of all posters here) of the attention to detail. The homeowner's electrician will be there tomorrow and I will have a printout of this thread to guide him.

The only question I have is: How do we test between the local (4th floor) and the remote real ground? Should I contact him ahead of time and make sure he brings spool of big-ass (technical jargon is OK here isn't it?) grounding wire to run from top floor to basement?

Thanks again

Randy

 

[ramsy]

How do we test between the local (4th floor) and the remote real ground? Should I contact him ahead of time and make sure he brings spool of big-ass grounding wire to run from top floor to basement?

YA, If you do this be sure to buy a jumper no smaller than 750 kcu, to minimize error in voltage-drop gradients. You might need to rent a crane and get permits to hold up traffic while suspending that cable 4 floors out the window.

If no one else in the building is having this problem, a qualified person might check connectors & splitters for open shields (inductive heating), or objectionable 120v paths traveling along cable shielding inside your dwelling. If other residents are having similar problems, a check before and after the D-Mark, near the building service may be in order.

 

[LarryFine]

How do we test between the local (4th floor) and the remote real ground? Should I contact him ahead of time and make sure he brings spool of big-ass (technical jargon is OK here isn't it?) grounding wire to run from top floor to basement?

First of all, a length of wire isn't a bad idea, but for a simple <1a solenoid-tester voltage test, a spool of #14 is plenty large enough to drop out a window.

I would start with measuring for a voltage between the panel neutral and the interior water supply piping. You'd be suprised how often older systems aren't properly bonded to the plumbing. (How old is this building?)

I'd check for voltage across a gap in the coax where the melting has occured, and it may be time to drop a wire out the window to measure between the apartment's neutral and a neighboring building's electrode or water piping.

 

[rf1031]

First let me say that this house is basicly new construction. Gutted to almost nothing with new electric service put in. Both ground rod and water pipe bonding, work done by a good firm.

Friday, one of their electricians came by and did some testing. He did not do everything suggested here (I printed out this thread for him), but we did find some obvious and revealing ammeter measurements:

There was under .2 amps inside our closet in the coax feed. (It read .5 for a second but he thought it was ammeter turn on surge?) I asked him to go on roof and check coax feed (RG11) from TimeWarner (and other houses) into our rooftop splitter and to also check new ground wire from the splitter to his rooftop conduit. Groundwire had around 4 amps! I think the coax had 3 1/2. Output coax into house was same as in closet.

I assume that the previous condition (prior to adding the rooftop ground lead) would have had that 4 amps flowing through our RG6 to our good earth ground. I also assume that RG6 can melt with 4 amps flowing through it.

Although it can't be good to have that kind of juice being permanently shunted through my client's rooftop conduit, I believe this ends my worries that any problem exists inside my client's home anymore. I'll touch base with the electrical contractor and we'll probably get on TimeWarner's case to deal with this.

Should I sleep OK now?

Randy



Randy

 

[rf1031]

Sorry to dwell on this, as I believe that my client is safe, but should I be raising alarms with CATV provider or PoCo? Is someone next door in any danger? Is there anything more I should be doing?

Thanks

Randy

 

[LarryFine]

Sorry to dwell on this, as I believe that my client is safe, but should I be raising alarms with CATV provider or PoCo?

In my opinion, yes. We'd have to test to narrow down the source of the improper voltage, but it is there, and it shouldn't be.

How dangerous is it? Who can say. Besides, they want the cable TV to work, don't they? I'd start with the cable co. for help.

 

[rf1031]

In my opinion, yes. We'd have to test to narrow down the source of the improper voltage, but it is there, and it shouldn't be.

How dangerous is it? Who can say. Besides, they want the cable TV to work, don't they? I'd start with the cable co. for help.

Dangerous for my clients? Or for someone else on the street?

Forgive if this seems grammatically picky, I actually don't know what you mean: When you say "We'd have to test" do you mean "we have to make sure someone tests"? Now that we have established that the current originates outside of the home (we have, haven't we?) I don't think there is anything more I can test.

Are you saying that I need to remain involved in narrowing down the external source of the improper voltage or are you saying that I should remain an actively interested party re. the cable co's diagnosis?

I have involved the homeowner's electrician. I have alerted the cable co. I don't think there's anything else that I can do other than make phone calls. And if the cable co thinks all is OK the way it is, is that the end of it?

Thanks again

Randy

 

[ramsy]

A request for inspection would do the trick. At least the inspection request would prove your good faith in reporting this mater to the Authority Having Jurisdiction (AHJ).

Call your local building department, and keep a hard copy of your formal request for inspection. This tends to shift liability from the property owner to the city, in the event of later fire investigation or injury from such hazard.

 

[don_resqcapt19]

Call your local building department, and keep a hard copy of your formal request for inspection. This tends to shift liability from the property owner to the city, in the event of later fire investigation or injury from such hazard.

If there is not an open permit, they can't come and inspect without the permission of the property owner, and an inspection in no way shifts any liability to the city unless you can prove wanton and willful misconduct on the part of the inspector.
Don

 

[ramsy]

I believe liability suits involving faulty equipment after inspection could name the inspection agency, especially if the record or investigation shows the inspection should not have passed.

If this specific hazard passes inspection, after specific request for inspection by a property owner, then later investigation shows it ignited combustibles or caused injury, I believe the owner's property insurance could not claim owner negligence, and would have little cause to disqualify the claim of this owner, who lawfully acted in good faith.

Without requesting any inspection, the owner could always tell the cable company to fix it, or else the cable will be removed and replaced with Satellite, but I believe showing that it's a hazard that failed inspection lends more credibility to his action.

 

[rf1031]

I understand and appreciate the suggestion to report/document the problem so as to help with insurance claims, lawsuits, etc. But I'd rather not have any fire or injury in the first place.

With that in mind, my bigger questions remain unanswered:
1)Almost 5 amps current exists upstream of cable co's rooftop splitter/tap and also exists on the new ground line from that tap to rooftop conduit, but does not exist on the house side of the tap. Does this prove problem originates outside the home in question?
2)Is my client or his home in any danger?
3)Are his neighbors in any danger and if so, what can/should I do about it?
4)Have I any responsibilities (moral or legal) besides notifying Cableco and homeowner?

Thanks again to all.

Randy

 

[don_resqcapt19]

Roger,

I believe liability suits involving faulty equipment after inspection could name the inspection agency, especially if the record or investigation shows the inspection should not have passed.

That may be true for 3rd party inspectors, but its not for municipal inspectors which is what we have in our area.
Don