| Electronics > Beginners |
| Locating a failed buried wire using a TDR |
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| Muttley Snickers:
I have both the Tektronix 1503C and a TS100 TDR here along with a variety of other testers and tracers which you would be welcome to borrow all but for the distance. Using a TDR an experienced operator will be able to detect and determine the type of fault or faults present and the distance to such faults provided they know the cables velocity of propagation or can calculate it by means of the existing cables physical length. A TDR cannot display where a cable fault or junction is located only the distance to it based on certain parameters and as previously mentioned where a cable incorporates junctions or is saturated at multiple points determining the exact location of faults will be a tricky process. Half the battle for these types of jobs is determining the location and direction of the original cable trench and any branch feeds. Based on the information provided in the first post I strongly suspect that these cables are now saturated and in need of replacement, even if the current faults were located and repaired sooner or later similar problems are likely to return elsewhere, also If you do need to dig a new trench and replace these cables be sure to place them in proper watertight conduits. |
| CJay:
--- Quote from: Muttley Snickers on April 02, 2019, 09:35:28 am ---A TDR cannot display where a cable fault or junction is located only the distance to it based on certain parameters and as previously mentioned where a cable incorporates junctions or is saturated at multiple points determining the exact location of faults will be a tricky process. Half the battle for these types of jobs is determining the location and direction of the original cable trench and any branch feeds. --- End quote --- What Muttley said, it's not likely to be an easy job for a TDR because your cable has failed once already and it's possibly damaged in several places as there's no guarantee it failed in the same place, it's going to be further complicated due to water ingress and mechanical factors, sharp bends, direction changes, even the moisture level in the ground can make a difference to how the TDR 'sees' the cable if the insulation is compromised. Personally, I think I'd bite the bullet and replace the lot, if I could afford to I'd also run a conduit so the cable can be pulled and replaced with relative ease next time. |
| David Hess:
With a 100 MHz oscilloscope you have almost enough for a basic TDR. Connect a pulse generator to the pair you want to measure through a resistor of about the estimated impedance and connect the high impedance x10 oscilloscope probe in parallel to the pair. This will not be calibrated but the reflections and approximate relative impedance will be apparent. A small 1 kilohm variable resistor will allow adjustment but is not really necessary. The pulse generator needs to have a fast edge to deliver good resolution. The propagation delay will be about 1.5 feet per nanosecond. Any logic output will be sufficient like the sync output on a function generator. The linear output of the function generator will likely be too slow. A low frequency square wave makes the least confusing source. The half period just needs to be longer than the propagation delay which will not be a problem. Distance calibration might be possible using one of the good pairs. Or make the measurement from both ends. I agree with Muttley that simply finding the fault will not be helpful because the damage is likely to require complete replacement. |
| cosmicray:
My current attempt to solve this issue goes like this ... I know the open wires show resistance between one another, both from the CD box and at the load end. The resistance is high enough to tell me it's not a dead short, but low enough to suggest a breach of the insulation somewhere. My recollection is that I was seeing a 10k-15k resistance between the wires. The calculated current flow is going to be way below what it would take to trip the CBs, but there should should be some flow. Enter one other piece of test gear I had lying around, a Simpson Electric Amp Clap. The lowest scale on the Amp Clamp will read up to 5 amps AC current on the 2.5 volt AC scale on a Simpson 260. As a test (at a different location), I tried using jumper clips between the amp clamp output and the O-scope. Sure enough, the scope is sensitive enough to see reasonably low levels of AC voltage (which the clamp is capturing from current flow). I have dug several inspection holes over the buried wire path. The plan is to measure the leakage current at the feed FB box, then move down the line and see if similar values can be seen at each inspection hole. The Amp Clamp should save me from having to breach the wire's insulation to look at resistance and/or voltage. If I hit a wire location that shows no leakage current flow, then I need to look back up the line for a possible reason for the wires to have failed. At least this is the plan. I'll check back in a few days with progress. |
| Doctorandus_P:
I like the idea of the spark plug with AM radio and it just might work. I also have an UT210E current clamp which has a resolution of 1mA and both AC and DC ranges. It also has a NCV measurement (as lots of other DMM's have) and you can also try using that for locating the open connection. |
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