Around 1976, a fine gentleman by the name of Tom Lathrop, then with Bell Labs, invented the earth-gradient method of fault locating. Using a transmitter, an earth contact frame, and a receiver, transmitted voltage differences are detected and shield-to-earth or conductor-to-earth faults are exactly pinpointed in buried plant.
Two test sets were manufactured by Western Electric, one for cables, one for drops. And finally, the difficult world of buried fault-locating was made simple. Several manufacturers build similar devices, and they all use the same principle as Tom’s sets. Today’s earth-gradient test sets work on any cable or drop that is faulted to earth. Any field tech that shoots trouble in buried plant should have access to one.
Most field technicians are about 90% successful with these test sets. When a buried fault is pre-localized to an area, for example in a faulted section of buried cable between 2 pedestals, the earth frame will pinpoint the fault so accurately that it can be exposed with a post hole digger.
Just pull out the set and hook up the transmitter. Turn it on and proceed along the cable path with the receiver and earth frame, probing with the earth frame until the receiver needle reverses. Then, turn the earth frame 90 degrees and probe until it reverses again. Dig up the cable and fix the damage. Success rate is about 90%. It’s the remaining 10% failure rate that we’re going after here.
How Does It Work?
The test set applies a voltage to the isolated shield or faulted conductor. A good ground is placed in line with the cable in the direction opposite the fault. When the test set is turned on, current flows along the isolated shield or conductor to earth returning to the transmitter ground. Two gradients are created. One gradient with the most positive voltage is at the fault, and the other negative gradient is at the test set’s earth ground return.
The receiver uses a 2-pronged frame to measure the voltage difference between the 2 probes. A meter on the receiver indicates which probe is detecting the most positive voltage. When the receiver reverses the technician rotates the earth frame 90 degrees allowing the fault to be pinpointed.
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First, use your resistance bridge, open meter and Time Domain Reflectometer (TDR) to prove a shield-to-earth fault to the drop or the cable. The measurements are fairly general, and the exact point below ground cannot be identified unless there’s visual damage in the area such as a brand new mailbox on the cable path or fresh digging.
An improper hookup can easily mask a suspected drop fault. Let’s say that a faulted drop has its trouble 10 feet from the network interface. The drop is short, approximately 60 feet long, and encapsulated directly into the cable. The next-door neighbor’s drop is in common trench with the faulted drop. The neighbor’s protector is 15 feet away from the fault.
If we try to connect the sheath fault locator "by the book", we’re not going to have much luck. First, we cannot properly place the ground in line with the drop path away from the fault.
The next best method appears to be running the ground along the side of the house at an angle. With this, an absolute electronic mess is created when the test set transmitter is turned on. The negative gradient from the ground rod overlaps the positive sheath-fault on the drop.
If it’s not possible to disconnect the neighbor’s ground, place the test set ground as close as possible to the neighbor’s ground. As the neighbor’s ground appears as positive to the test set, and the set’s ground is negative, the 2 gradients should cancel each other enough to allow the positive earth-fault to be pinpointed with the frame.
To solve this problem, use the cross-connect wire connected to the ground rod and place the ground rod a good 200 feet from the immediate area of the fault. Next, remove the ground from the neighbor’s protector (without interrupting service). This cleans the area electronically and the fault gradient should be distinct and easy to locate.
When locating a shield to earth fault in any air-core PIC cable, test the shield to earth for DC volts. If DC voltage is present, there is water in the cable and the earth frame will not work. There is no shield to earth fault. Use your TDR to measure the start of water from each end of the cable. Replace the cable as soon as possible to avoid losing customers.
If there are no shield to earth faults especially in filled cable sections and the root cause of cable failure is water in a splice, connect the transmitter to faulted ring or tip conductors even if there is crossed-battery on the conductor. High measured crossed-battery on a conductor above -40VDC does not have a path to earth and cannot be located with the earth frame. Low measured crossed-battery below -20VDC does have a path to earth and is locatable.
If you picture what’s going on down there and realize that the size of the gradients can be controlled, the earth-gradient test set can be as accurate and easy in congested plant as it is in electronically clean areas. Your success ratio with the earth-gradient device should increase.
Signing Off
Wishing you wonderful holidays! Contact me with ideas for 2019 columns: [email protected] or 831.818.3930.
