Buried Cable Problem...Help

[chemelec]

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

 

[crust]

I once built a unit that was used to determine where transitions were in a cable. Making the shorts permanent is going to be hard, esp since you dont know where they are. Is this cable in an underground conduit? In any case, you can get the datasheet for the cable and in there, it will have the resistance/unit length. Then you can measure the resistance with a _good_ meter (b/c it will be low) and estimage the distance to the short. Then you can dig up that part and fix it. It would be best if you disconnect the cable at both ends so nothing else interferes with your measurement. You can take several measurements at different time and average them to zero in on the location (within the error of the meter). There are better ways to do this, but the equipment is probably not easily available.

 

[Phasor]

Enquire with your local electricity distributor - they may have an underground testing section, who, for a fee, will find the fault, using Time Domain Reflectometry.

 

[Particle]

Were they in PVC or just burial grade cables? Burial grade has a tendency to fail after 3-10 years, despite its weathering.

 

[chemelec]

More on Buried Cable.

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

 

[gerty]

The two major causes of intermittant cable problems that I've run into were moisture and rodents..I would check both ends where they come from underground to the actual termination. I've seen cases where squirrels have made entry into buildings and wreaked havoc with communication cables,shielded or not..If it's a moisture problem I'd go with Phasors option of TDR. The problem would probably be in some valley or low point on the run..Be interested to see how your fusing operation works out, let us know...

 

[Russlk]

Trying to fuse the wires is chancy, but what have you got to loose? Discharging a capacitor will place a limit on the energy and minimize damage to other wires. If the cap is too large (10,000uF?) it may blow the short open, then you will have to use time domain reflectormetry. Do some experiments with #22 wire to see how it works.

 

[bmcculla]

Profesional equipment to find shorts in transmission lines uses pulse reflections from shorts (time domain reflectrometry which Russlk talked about) to find faults. If you put a series of square pulses into the cable you will be able to read a reflection of the pule from where the short is. You will also be able to read changes in impedance from almost shorts (the return pulses wil be attenuated). If you can measure the speed the signal travels through the cable you will be able to find the location of your shorts. Also if the shorts are intermitent you will be able to make a new reading of shorts each pulse so you can log each time a reflection occurs and where it is.

Brent

 

[chemelec]

Hi Brent, I am Very Aware of that form of detection as well, but it won't detect a short unless it is actually there. These shorts are Very Intermittant.

However this cable Runs our towns Water Supply which has caused our town to go dry 3 times and I don't mean Booze. Including No Fire protection. It is Important to Find and Fix or Eliminate these Intermittant shorts.

Take care........Gary

 

[bmcculla]

Time Domain Reflectrometry should also detect sudden changes in impedance that could indicate a problem spot - just a thought.

Brent

 

[Jay87T]

Phone cable?

is this a 6 pair BSW? If so your local phone company should be able to fix this, I work for BELL here in Canada and we simply locate and fork the BSW looking for the ground. have u checked the cable pairs to ground yet?

 

[chemelec]

Not Sure what you mean by BSW.

The cable is made by "Superior", SEALPIC-FSF.
It says its 6 pairs, but to me its more like just 12 wires as none seem to be paired in any way. Usually when there paired, their eith twisted or tied together, but not in this cable.

This is a Remote Tiny town in Southern BC. and it is difficult and Expensive to get someone else to help.

But we are definately Not Impressed with the Professionals (I use the this term VERY LOOSLY) that installed this cable. And they don't stand behind there work. Anyway, its my job now to attempt to solve it with what equipment I have or can make.

I just thought I would pose the above questions, just to see what others thought.

Brent, I do have an Impedance meter and an Inductance analyzer, but neter are showing anything that looks suspecious. I do not have a Time Domain Reflectrometry. And I doubt anyone in this area does.

Take care........Gary

 

[crust]

I originally assumed this was your house or something. If this is a whole town, then hiring an engineering company to use their TDR to find the errors is probably the best bet. Reading the instrument is one thing, knowning the interpretation and tricks are entirely different. We have the equipment, but we don't provide that type of service, so I cant estimate the costs, etc. There are several places that do that type of work though.

 

[Jay87T]

tools

without the right tools you are not going to be able to do this yourself, if the wire is burried. you will need to locate the wire and fork it to find the marginal grounds on it you can only fork and find a trouble if there are grounds. have u tested it yet for any grounds? one side to ground and one to a side of the pair?

 

[chemelec]

I am an Electronc Design Engineer (Specifically Analogue) and I live in this town so it is to my advantage to fix this as cheap as possible. Population is 485 and All Costs are shared by the residents.

If you read my proposal, all I am really interested in, is comments on the ability to create a permant short on the intermittant ones.

This Cable has No Shorts to ground or the Shield. At least None I've detected so far.

The Sencore LC102, Capacitor Inductor Analyzer is Definately capable of determining Very Precise locations of shorts or open circuits, But Not if there intermittant. Nether will the unit you suggested.

I did some Preliminary tests using my current /voltage drop method, using a Simular wire and creating shorts. I was able to determine the distance within two feet on a 500 ft length. But not when intermittant or a high contact resistance at the point of the short.

Currently we only use 3 of these wires, but I want to Expand the system with both a Reservoir Low Water Alarm via Cable transmission back to the pump hous, than RF Transmission to some persons house as well. Additionally, a Direct Readout of the Reservoirs water depth in Inches that can be read in the pump house. These systems have been designd and tested by me.
But Until I can get this cable Relilable, All that is a Waste of time. I started on this last december, but it was too cold here to proceed. (-20 Celsus)
But were now in the +15 Celsus (60 F) Much warmer to work and dig.

Anyway, Thanks to all for the suggestions.

Take care........Gary

 

[Nigel Goodwin]

Just a thought! - is this cable all in one piece?, or is it a number of shorter pieces joined together?.

Because if the cable is just sat there, and nothing has hit it, you wouldn't really expect anything to go wrong with it (at least not in this short a time). Far more likely that the fault would be at a join.

My only experience with underground cables was with an old cable system we used to run, it was VHF 405 line only, and the main cable runs were coax run under ground. The only problems we had were people driving diggers cutting the cables!!!! - and ground subsidence on one of the estates it ran round. The under ground cables came up to tap-off boxes mounted about 2 feet above the ground, and went back down again. We went to find a fault once, and couldn't find the tap-off box! - the ground subsidence had pulled it off the wall - we found it about three feet under ground!.

Needless to say, I've never considered buying a house there :lol:

 

[chemelec]

Because of the Length, this cable was laid in Two Pieces and they were spliced in a Box. The Splicing Job was Very poory done and the first cold weather we got caused it to short out because of condenstaion. The Iodots taped each wire individually and than taped them into a tight bundle. Moisture got in but with no real heat in this underground box, it couldn't get out again.
I fixed that awhile ago, it is definately OK and this problem is definately not related.

As I orgionally pointed out, my first test indicated one short to be 7 feet from one end. I Suspect All the shorts may be there. And that would probably be where the Conduit going into the ground, ends. But before Digging there, I would like to confirm the other intermittants are at the same location.

Unfortunately I was not there to Supervise the laying of this cable. That would have given me a much better insite to these faults.

Thanks for the Feedback.....Gary