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Intermittant Buried Cable Problem

Discussion in 'Electronic Basics' started by Gary Lecomte, May 24, 2004.

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  1. Gary Lecomte

    Gary Lecomte Guest

    i have a Buried cable that is about 1/2 mile long. It contains 6 pairs
    of #22 AWG wires wrapped in a shield and filled with gel. But some
    wires are shorting out with others on occassion. And not necessarly
    just pairs.

    I believe I have Possible solutions to the problems below, but I would
    like other opinions before giving mine.

    1) How do you make these intermittant shorts, Permanent so they can be
    more easily found?
    2) How do you determine their location?

    Digging up and/or replacing this cable is Not an option. It cost
    $10,000 and is only two years old.

    Any Legitmate Suggestions Appreciated.......Gary
     
  2. KR Williams

    KR Williams Guest

    High-current. Maybe you'll even see the smoke. ;-)
    A used TDR can be had for much less, and is a cool toy too. ;-)
    A Few Legitimate Suggestions Offered.
     
  3. Jim Thompson

    Jim Thompson Guest

    The installer doesn't guarantee his work, or did YOU install it ?:)

    ...Jim Thompson
     
  4. Jerry G.

    Jerry G. Guest

    Did the installer, or you put in a cable that was rated for underground use?
    Also, was the cable put in to a non-corrosive type of piping that is rated
    for underground cable?

    There are standards for these cables. If they are not met, the settling of
    the ground will eventually tear or damage the cable. There is movement of
    rocks, and the earth itself that has a lot of force against things. There is
    also plant growth that can root around and tear a cable, and small animals
    that may borough through the ground, and chew on a cable.

    There are really no easy shortcuts for this. If you have an opened
    connection, there are some sophisticated instruments, that can approximate
    where the break is. These use the technique of sending a pulse down the
    cable, and measuring the reflected open end pulse coming back. Then with
    knowing the exact characteristics of the cable, some calculations can be
    made to determine the approximate location of the open or break. With an
    underground cable, the result would be undeterminable, because it would be
    hard to predict the exact characteristics effect of the ground on the cable.
    Even the moisture in the ground at the time would effect the readings.

    If the cable was in a pipe, then this may be more exact and give a better
    chance to analyse, because the characteristics of the pipe on the cable
    would be a fixed value to work with.

    I have a strong feeling you are going to have to lay in a new cable. Also,
    you would be very wise to do the job properly using the proper rated piping
    that is able to withstand ground and rock movement, and earth pressure.

    --

    Jerry G.
    =====


    i have a Buried cable that is about 1/2 mile long. It contains 6 pairs
    of #22 AWG wires wrapped in a shield and filled with gel. But some
    wires are shorting out with others on occassion. And not necessarly
    just pairs.

    I believe I have Possible solutions to the problems below, but I would
    like other opinions before giving mine.

    1) How do you make these intermittant shorts, Permanent so they can be
    more easily found?
    2) How do you determine their location?

    Digging up and/or replacing this cable is Not an option. It cost
    $10,000 and is only two years old.

    Any Legitmate Suggestions Appreciated.......Gary
     
  5. Robert Baer

    Robert Baer Guest

    ....and use a pipe larger than the cable, all bend radii about 10 times
    cable diameter (or more), to make it easy to pull cable for replacement.
     
  6. CWatters

    CWatters Guest

    Perhaps you could fit some kind of data multiplexer at each end and use just
    a few pairs - swap them around until you find a reliable set.
     
  7. Mjolinor

    Mjolinor Guest

    At that cost I assume it is proper stuff so my first thought would be where
    did they joint it at install and dig there.

    If there are no joints then you have to wonder if fixing it is sensible
    becasue it obvioulsy isn't up to the job so even if you isolate the problem
    and cure it I would have thought that there is a fair possibility of it
    going wrong again.

    Out and return on half a mile will be about 5 us so put a pulse into a good
    pair that is shorted at the far end, put the same pulse into a shorted pair
    and the external trigger of an oscilloscope put the two traces of the
    oscilloscope on the two return wires you will receive two pulses that are
    time seperated from the trigger and each other in proportion to the distance
    from the source to the short. In theory :).
     

  8. Before getting too carried away with fancy test equipment or not so fancy
    shovels, keep in mind that the vast majority of cabling problems occur at
    the very ends or someplace where extra hardware is attached (such as T's and
    the like). Typically the ends of the cable are easy to access, so before
    doing anything else I would (at the very least) inspect the end connections
    to make sure they are not the cause of your problems.
     
  9. Gary Lecomte

    Gary Lecomte Guest

    Thanks to the Honest Answers so far.
    To Further Answer your questions, as well as my idea:

    1) This was a Direct Bury, Except where it comes out of the ground,
    where they used a 2" conduit at the ends to better protect it. Not
    Sure how long these are,
    going into the ground.

    2)It was installed by a Professional Electrical Contractor and a good
    layer of sand was laid down first, followed with the cable and than a
    Foot more sand on top. I Extremely Doubt any rocks have damaged the
    cable.

    3)There are no trees close to this cable, and this is in a Very Dry
    part of the Country. This is a Mountain Area with Very hard, Stable
    base under this cable.

    4) I have a "Sencore", LC102 capacitor Inductor Analyzer. And
    according to the manual, if these shorts were not intermittant, I
    could determine the location of these shorts with an Inductance test.
    Unfortunately you need a 25 foot piece of cable to get a reference
    Inductance Value. And the town sent back all the Extra cable. Damm.

    In my inital tests, I used an Ohm Meter, but was coming up with
    Varying Readings because of probable Contact Resistance at the point
    of the short. So I than introduced a 1 Amp, Constant Current Source.
    Measuring the voltage at the entry point and calculating the
    resistance verses the resistance of the wire, divided by two Because
    its a Round Trip, I determined one short to be 7 feet from the end.
    Repeated tests that day concured this. However a month later, that
    short No Longer Existed! I believe this must Temperature Related,
    causing Small Amounts of Contraction and Expansion.
    In all Probability I would think all these shorts are at the same
    point, But I need to Confirm it before digging this up.

    My New Plan is to use a 150 volts or 300 volt @ 100 mA Current Limited
    supply.
    In hopes of Arcing and Fusing these bad spots. Maybe also a Cap to
    Give a High Current Burst of power but Hopefully, not enough to damage
    the other wires.

    Does This sound Reasonable?

    Thanks.....Gary
     
  10. First off, you should be posting this to comp.dcom.cabling. You should
    get a cable tester that can do TDR and find out how far the
    intermittents are from the ends. This might give you a good idea of
    what the problem is. If you have it buried, I would guess that you
    might have a rodent problem. But it could be something else such as
    tree roots. In any case, if you have spare pairs, you're in luck. Just
    hope that the spares don't also go bad.
     
  11. Jerry G. wrote:
    [snip]
    Depends. It it's in a conduit, then pulling a new cable may be the
    cheaper option. If it's direct buried, then finding the breaks and
    repairing them will probably be cheaper.
     
  12. Dbowey

    Dbowey Guest

    Gary posted, in part:

    << My New Plan is to use a 150 volts or 300 volt @ 100 mA Current Limited
    supply. In hopes of Arcing and Fusing these bad spots. Maybe also a Cap to
    Give a High Current Burst of power but Hopefully, not enough to damage the
    other wires.

    Does This sound Reasonable? >>

    ---

    That is on the right track, but there is a piece of eqpt that is built for that
    purpose. It's called a "Megger" or a breakdown test set. It will generate
    voltage high enough to solidify a swinging short. Once that is done you can
    measure the resistance from each end, to the short and get within a foot of the
    short.

    A TDR is probably the most simple method, though, because it does not require a
    solid short; it will show distance to the discontinuity in the path.

    The contractor who laid the cable should have tested it before turning it over
    to you. I hope he is participating in your effort. If this is a fairly new
    placement, he should be doing all the work and covering the costs.

    Your description of the sand bed, etc., sounds like they did it right in that
    regard, but I suspect they may have damaged the cable by pulling too much load
    from the end.

    Do you know if the cable is continuous or spliced in the path?

    Don
     
  13. Gary Lecomte wrote:
    (snip)
    If this test result is at all valid, I would bet a dollar that the
    cable has been cut by the bottom end of the conduit. After the next
    time you have made a test, try pulling up, hard (or push down), on the
    conduit, and see if the resistance changes.
     
  14. Speaking of contractors and subcontractors. I've spent this morning
    undoing a mess that a sub did, and I am _not_ a happy camper. The
    idiots used one of my 4-pair telephone lines as a pull line to pull in
    three cat5s, and left both ends dangling. And to make matters worse,
    they used CMG non-plernum cable thru an obvious plenum! Guess who's
    gonna be back out pulling plenum cable? Yeah, that's my revenge. Bastards.
     
  15. Dbowey

    Dbowey Guest

    Watson posted:

    << Speaking of contractors and subcontractors. I've spent this morning
    undoing a mess that a sub did, and I am _not_ a happy camper. The
    idiots used one of my 4-pair telephone lines as a pull line to pull in
    three cat5s, and left both ends dangling. And to make matters worse,
    they used CMG non-plernum cable thru an obvious plenum! Guess who's
    gonna be back out pulling plenum cable? Yeah, that's my revenge. Bastards. >>

    --

    I wouldn't stop there. I would advise the contractor of what the sub did
    wrong, and if I got any lip from the contractor, I would explain to the
    customer, the deficiencies of the Contractor and the subs. I believe we owe
    that to people who hire us.

    Don
     
  16. That sucks. I've heard worse though. A major cabling contractor
    in my area hired a sub upstate New York for a 1500 drop job. The main
    contractor went up there to certify and checkout job and found 40% failure out
    of 1500 drops. And to make it better, they mislabled a lot of jacks and
    patch panel locations!!!! LOL Talk about a quagmire! LOL

    I dont know who to blame, the sub or the main cabling contractor for not
    overseeing the project better...

    LOL
     
  17. the Wiz

    the Wiz Guest

    Go to a telephone supply house and get a cable fault tracer. Put the signal
    generator on the bad pair and follow the cable until the level changes - that's
    where the fault is.

    You can also configure a good oscilloscope to make TDR measurements. Not as
    convenient as a des-gned-for-the-purpose TDR, but it works. I've used it to
    measure the length of coax cable run between floors of a building - impossible
    to measure otherwise.

    More about me: http://www.jecarter.com/
    VB3/VB6/C/PowerBasic source code: http://www.jecarter.com/programs.html
    Freeware for the Palm with NS Basic source code: http://nsb.jecarter.com
    Drivers for Pablo graphics tablet and JamCam cameras: http://home.earthlink.net/~mwbt/
    johnecarter [email protected] mindspring dot.dot com. Fix the obvious to reply by email.
     
  18. Dave

    Dave Guest

    I agree with the principles already mentioned:
    1. Check the obvious
    2. Check the easiest (cable ends, conduit)
    3. Time domain reflectometry will reflect a small pulse when the cable
    impedance changes (i.e. the cores get closer, or resistance changes).
    Hire one.
    4. I don't recommend using high power to burn the bad joints - it may
    damage the rest of the pairs. Unless of course, you don't mind
    repairing the complete set of pairs. What if the complete cable is
    overheated as a result of this?
    5. If the repair is 'mid cable run', then you will need some spare
    cable to make a repair, becuase you might not have any slack.
    6. Determine the cause of failure (you don't mention if you are
    overloading the conductor and heating the insulation).

    I hope that's enough,
    Best regards,
    Dave
     
  19. colin

    colin Guest

    when i worked on a TDR high voltage power line fualt detector equipment i
    heard about the traditional way of determining the position of fualts in
    long hi power cables. (for shorts anyway)

    the cable is buried in sand and an extremely high current is put through the
    cable for a very short period.

    the magnetic field produced by the current makes the cable jump and the sand
    is disturbed but only for the lenght of the cable up to the short.

    Colin =^.^=
     
  20. I remember seeing this conversation about a month ago. I would like to
    know how you resolved this, if you did by now...

    Jerry G.
     
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