13 Feb Fault Finder Mission: El Capitan Canyon Campground
Between two buildings filled with telecommunications equipment stands an organic farm, a service road, an expansive ocean view, and one kilometer of undocumented conduit. Red flags in the dirt marked its location. We were on a mission: Find a wiring fault buried up to 8 feet underground somewhere along a 2,700 foot 25 pair copper telephone cable.
On the cable, only 7 of the 25 pairs are capable of carrying a reliable digital phone signal. Most of the lines toned all the way through, so we suspected a single fault where most of the conductors became stripped, cut, and/or shorted from a single event. Perhaps the fault was caused by rodents, construction work, or earth movement and likely occurred within the pull box itself.
Previous scouting missions revealed the path and depth of the cable. Measuring distance on Google Earth showed an estimated distance. Walking the line revealed a few pull boxes. But we found no splices on the line and the pull boxes we found were too far apart – meaning a non-zero number of pullboxes were grown over or buried somewhere between the phone room and the outbuilding punch-down block.
Maintenance folks considering digging spot holes between pull boxes to cut into the cable for spot testing using a tone generator and ohm-meter. A destructive and time consuming process. And it would introduce more splices into the cable creating the potential for future issues with those splices.
Additionally, the functioning phone lines are in use 24/7 for dial-up telemtry, DSL networking, PBX telephones, and analog fax. How can we test the cable while avoiding downtime?
Enter Arc Chaser.
The Arc Chaser Testing & Monitoring SSTDR is an easy to use, hand held cable testing instrument with few buttons, a large LCD screen, and the ability to scan inside cables – while the cable is in use.
A Time Domain Reflectometer (TDR) is used to test for cable length, faults, junctions, and other impedance changes. Often compared to radar, TDR devices send a pulse down the cable while checking for reflections. Those reflections tell a story. For example, say a reflection is received 50 nanoseconds after the transmit pulse. The TDR will observe the time delay and convert that into distance displayed in feet or meters.
Typical TDR testers require that a cable under test be disconnected on both ends for a proper test due to signal interference and, on high voltage cables, to avoid damage to the tester itself.
The Arc Chaser fault finder is unique among TDR devices as it includes Spread Spectrum Time Domain Reflectometry (SSTDR). A technology that allows the signal pulses and reflections to be observed on live cables. Spread spectrum signalling prevents interference caused by operating equipment such as telephone, networking traffic, even high voltage power lines. The Arc Chaser can monitor live wires up to 600 Volts.
The practical upshot of testing with the Arc Chaser SSTDR is… no downtime. We tested the buried cable while the office was in full swing mid day during the week. No overtime needed. Comparing known good, live, pairs on the cable next to the damaged pairs, we were able to confirm the length of the cable.
Now that we knew the length of the cable, it was a simple matter of connecting the Arc Chaser to one of the faulty pairs to activate the automatic test.
The Arc Chaser reported an open AND a short condition at 1,309 feet from our test point at the outbuilding. Testing from the phone room revealed the same type of fault at a distance of 1,410 feet. Adding these two numbers together confirmed the length of the cable, 2,719 feet which is nearly identical to the full estimated and TDR measured cable length.
We found our fault! No digging. No downtime. And quite certain of the results. With these test results, maintenance can now dig for the fault or cut into and bypass it entirely.