When Test Sets Do Not Perform to Expectations —
Field technicians have a multitude of test equipment that is used to identify and locate faults in the copper network. Many of the test sets have multi-function features. If the fault or faults in the cable are resistive in nature, such as unwanted shorted and grounded cable pairs or crossed battery from other cable pairs in the same compliment, the test set has a resistive fault locate function (RFL) and a Time Domain Reflectometer (TDR) function to help you locate these problems.
If the fault or faults in the cable are capacitive, such as an open cable pair or an unwanted lateral that is interfering with bandwidth, these issues can be identified and located with the open meter or TDR function.
If the fault or faults in the cable are in a buried section, the field technician has a combination cable locator to locate the path and the depth of the cable or cables and earth and earth frame that is used to pinpoint shield to earth and conductor to earth faults.
Most times, but not always, these test sets do the task that they were designed to do. For example, when using the RFL feature the field technician identifies the fault or faults with the digital multimeter (DMM) function on his test set.
Proper use of the DMM indicates the type of fault and whether an RFL, open meter, or TDR, measurement can be obtained. If there is a short or ground on the cable pair, it must be more solid than 30 megohms or the RFL feature will not function. If the fault changes in value and the DMM shows rolling numbers in either ohms or crossed battery, the root cause of the fault is water. It could be water in a jack, in a splice, or in the cable, but the root cause is water.
If both sides of the cable pair indicate a fault, then a separate good pair is needed to measure the distance to the fault. If one side of the pair is clean, either tip or ring, a single pair hookup can be used. The technician can strap the pair at the far end and measure the distance to the fault or the distance from the far end back to the fault.
Sometimes when using a single-pair hookup, the distance to the fault is displayed, but the distance to the fault and the strap to fault indicates "stars" instead of footage or meters to the fault. There are 2 reasons for this: first, the field technician hooked up the wrong leads of the test set putting the good lead on the faulted conductor and putting the fault lead on the good pair; or, second, the pair is transposed in a splice along the lead. A good rule to follow is to use the single-pair hookup for convenience (and it is accurate most times) and the separate good pair for accuracy. This is most important when the faults are in a buried section and must be dug up with a shovel.
Sometimes when fault locating in a section of buried plant and measuring several faulted conductors, the distance to the fault measurement changes. This is usually because there is more than one fault in the section.
For example, in a buried 1,200-foot section of air-core buried cable, there is sheath damage at 200 feet affecting 15 customers. Rather than allowing the tech to find the root cause, management instructs them to transfer damaged pairs to other good pairs. Over a period of time, water enters the cable sheath at the damage and runs down the cable filling up an old trouble splice at 300 feet with water. If a cable pair is faulted only at the damage, the distance to fault would measure 200 feet. If the pair is faulted only at the wet splice, the distance to fault would measure 300 feet. If the pair is faulted at both the sheath damage and the wet splice, the distance to fault would measure somewhere between 200 and 300 feet and different footages would show depending on the severity of each fault.
To combat this, anytime that I use the RFL feature, I use the REPEAT feature several times. If the footage to the fault stay the same, usually there is only one fault on that conductor and I can use the measurement. If the footage to the fault changes, any measurement will be between the two measurements.
When fault locating cable faults I rely heavily on the multi-function test set, and many times I get valuable information when the test set does not function as expected. Learn and trust your meter, and know what to do when you suspect something is off in the readings.
I hope my column helps you do your job better and if it does, let me know! If you have an interesting topic you’d like me to comment on, also let me know. Contact me at [email protected] or 831.818.3930.