Phantom CAT6 Bridge Tap Error 1

[Halteclere]

I am an Engineer overseeing the installation of tolling equipment at a series of new toll sites. Part of the installation includes pulling CAT6 cables from the overhead violation cameras to the processing units in a nearby building.

The camera equipment uses 1000Base-T Ethernet communication, the CAT6 cable is Superior Essex's OSP BBD6 product which is UTP, CAT6 rated, and rated for buried applications. The farthest run is less than 300-feet.

Of 32 cables that have been pulled so far, four cables have developed "Bridge Tap" errors over time. All cables were tested shortly after the installation, and these cables failed 2 to 4 weeks later. We have been using the Fluke CableIQ tester to certify the cabling as 1000Base-T, and to diagnose the problem.

Does anyone have any thoughts on what would cause a cable to fail within a few weeks of installation? All four cables experienced the bridge tap error approximately at the same location in the gantry structures (near the base of the gantry where the horizontal entrance conduit sweeps to vertical). Two cables failed at the 1st toll site while the other two cables failed at the second toll site. We have not yet been able to extract the failed cables for investigation, for the installer tightly bundled them with the other cables. The parts of the cables that we have been able to get our hands on do not appear to have been damaged during installation.

Obviously a true bridge tap has not occurred, but our tester does not delve into the conditions that make it think it sees a bridge tap.

I hope that someone here in this forum can provide guidance into this issue. I am in the process of getting a more advanced diagnostic tool that can give me better insight, but any advice in the mean time will be greatly appreciated.

 

[Mountainbear]

The cable may have rubbed on a sharp edge in the conduit. If your installer was "aggressive" while pulling the cable they may have burned thru since you mention the cables were bundled tightly together, perhaps the conduit is too small. when replacing them use lots of lube, they should not pull hard. Also where your cable changes from horizontal to vertical do you have sweep 90's or is the transition more abrupt? If you are pulling underground it is critical that the cables are not damaged since almost all conduits have water in them.

 

[DTSMAN]

I have seen something similiar.

A bundle of 12+ cables were ran over a steam pipe for a radiator system in an older building during the summer when it was not hot. During the winter network issues developed that our fluke could not find. It kicked our butts. At least you are actually finding an issue and a common point.
Your area may bending against a piece of metal that is charged or something. (Just to throw a guess out there) You say they all fail at the same point, then that point is the common demoniator.

Bo

Remember,
If the women don't find you handsome,
they should at least find you handy.
(Red Green)

http://www.redgreen.com

 

[dagwoodsystems]

The CableIQ in "discover mode" uses a TDR to indicate the distance to the supposed tap. Does the distance shown square with the distance between the test point and this horizontal to vertical sweep point? If not, then that's a clue as to where the false positive tap error is coming from. But as the previous posters suggest, excessive capacitance caused by a breach in the jacket (of an individual conductor and thus now making an electrical connection with the encased EMT) could generate the same error. The sweep point would need to be investigated in that case.

However, a common connection point in a star topology would also falsely indicate a bridge tap. When testing, make sure that any DVRs, splitters, multiswitches or other items at common locations have been fully disconnected before you run your cable tests. If you've got a daisy-chain of CAT6 (which happens on an annoyingly frequent basis) AND the tester is placed somewhere in the middle of this chain, it will also report a bridge tap. Since you are using CAT6 for video (which is totally unnecessary BTW), you should know that using a network tester is only effective if you understand how modern voice/vid/HD or whatever is transported across such cable; i.e. 30fps vid over Ethernet cable doesn't necessarily have the same requirements as true LAN traffic. As such, it's wrong to assume that insufficient conditions for a computer network (as reported by a tester) automatically translates to bad video.

OSP is pretty tough stuff. If you were to ask me, I'd be more inclined to look at the way this stuff is wired rather than panic about breaches in the cable jacket. I'm just sure that somebody tried to take a shortcut or didn't entirely understand how the whole thing was supposed to work.

Tim Alberstein

http://www.dagwoodsystems.com

 

[alamtnman]

another thing to check, how tight is the bundled wire you mentioned.overly cinched tie wraps could cause a false reading too. may need to cut tie wraps loose and test again.

T.R.
RCDD

Honest officer,I know I've been thinking but i ain't thunk yet

 

[Halteclere]

Thanks everyone for their responses. I believe I may have figured out what the problem is, but before I give my conclusion let me provide some responses to the follow-up questions.

The internal column raceway is a 12" PVC pipe, which is the only raceway that the gantry designer could fit into the column plans and maintain structural requirements. The raceway fill is less than 20% and is within NEC code. The only other cables in this raceway are 12-ga UTP 24Vdc power cables and coaxial RF cables with 900MHz signal frequencies. The elbow at the base of the column is a standard hydraulic sweep, which is much smaller than sweeps used for electrical conduits. So as Mountainbear suggested, there is a chance for the cables to be damaged at this transition.

We are using 2MB cameras with 60hz frame rate, requiring the use of 1000Base-T communication. These cables are direct run, and were tested stand-alone and without any other cables or devices connected. How tightly the cables are bundled is a concern, for when toning out the problem cables some of the other CAT6 cables gave off a weaker tone. I don't know how much toning crosstalk indicates signal coupling between cables and is an indication of degraded communication. But the installer duct taped the cables together and there is no way to unbundle the installed cables to see see if anything improves.

But what I have found, at the 3rd installation site, is that a handful of cables had their jackets damaged and subsequently "repaired" with tape. The damaged section of the cables occurred in an accessible area and were in a coil of cable laying in the dirt at the bottom of a ground vault. One of the cables had a bridge tap error occur after a heavy rainfall, and after the damaged area was discovered and the cables elevated off the ground the bridge tap error disappeared after a couple days.

The subcontractor that installed the cables claims that someone else must have done the damage and the secret repair after installation, but other factors lead me to believe that someone on the sub's crew performed the repair during installation (observed location of the cable damages, type of repair tape used, etc). And I know that the gantry assembly that spans the roadway has some rough welds inside that could easily damage a cable jacket.

So though despite the subcontractor's claims of innocence, and that I am not able to observe the location of the cable damage for the first two sites, I am convinced that all these intermittent bridge tap errors are the result of water penetration past the cable jackets. New cables have been pulled to replace all damaged cables, and so far no new issues have occurred.

Again, thanks everyone for your help.