Field technicians are dispatched on a daily basis to install or repair service on paired copper conductor circuits. For the experienced field technician this is a slam dunk; however, for the new technician, it can be intimidating. To help sort out a good approach, I am providing a sequence of tasks requiring that you first test the cable pair and then qualify the working circuit.

Following is the step-by-step process that I use in the field to test a vacant cable pair. Look for:

AC Volts
Test the pair tip to ground and ring to ground for any AC voltage on the pair. Expect to see AC voltage from .2VAC to 10VAC. When testing circuits longer than 18,000 feet (5,550 meters), expect to see greater than 10VAC. Any AC voltage higher than 50VAC is a safety hazard, and proper safety procedures are in order.

Longitudinal Balance
A longitudinal balance test will show the ability to transport AC signals along the cable pair and not carry AC signals from the tip conductor to the ring conductor or from the ring conductor to the tip conductor. On a narrow band circuit this difference in signal is referred to as circuit noise. In the bandwidth business this difference will cause modems and set-top boxes to dump, freezing, and pixilation, and a myriad of other problems. Acceptable longitudinal balance should be at least 60dB.

Crossed Battery
Crossed battery will cause static on POTS circuits, fail the longitudinal balance test, and cause bandwidth problems. When testing a vacant cable pair DC volts tip to ground and ring to ground. Nothing other than 0 VDC is acceptable -- and anything other than 0 VDC will cause a repeat report.

A Short, Tip Ground, and/or a Ring Ground
A ring ground or a tip ground will affect the longitudinal balance, and both POTS and bandwidth services will be affected. Any ring or tip ground that is more solid than 20 Megohms (20,000,000 ohms), will affect all services in the Triple Play business. These type faults as well as crossed battery are easy to identify, find, and fix. Any shorted cable pair that tests from 1 to 3 Megohms may be the end user’s equipment. When testing inside wire, remove the associated end user’s equipment. Remove the end user’s equipment and retest. Any short more solid than 20 Megohms should be identified and repaired.

Capacitive Balance
When the cable pair is free of DC type faults, test the capacitive balance tip to ground and ring to ground using the open meter function on your test set. They should measure the same length. If there is a difference in length, divide the smaller number by the larger number -- this will give you a percentage. For the Triple Play feature, the capacitive balance should be greater than 98% in order to pass the longitudinal balance test. Less than 98% will indicate that the tip or ring conductor of the cable pair, whichever is the largest, is crossed with a non-working conductor; open one side on a lateral; or open one side beyond the customer’s terminal or pedestal, and it will fail the longitudinal balance test.

Series Resistance
Any series resistance will affect the bandwidth services and may not affect the POTS portion unless the contact is making and breaking and then it will cause static on the POTS service. If the series resistance is close to the far end of the cable pair the pair could pass the longitudinal balance test when the test should have failed. There must be at least 1,000 feet (305 meters) beyond the series fault to get a proper test.

To avoid this problem place a grounded short at the far end. This is the physical center of the cable pair. If the electrical center is in the same place it will pass the longitudinal balance test. If there is any series resistance (as little as 5 ohms) on either the ring or tip side of the cable pair, the longitudinal balance test will fail.

To identify which conductor has series resistance, measure tip to ring ohms, then tip to ground, and ring to ground ohms. Tip and ring to ground ohms should measure equal. If the conductors measure a difference in ohms the conductor with the larger ohms measurement is the one with series resistance.

Series resistance is hard to see with a Time Domain Reflectometer with distance. To get the series fault down to a reasonable distance to where the fault occurs use the divide-and-conquer method. Go half way to a pedestal or terminal and bridge the test set on to the cable pair. Be sure to leave the grounded short at the far end. Test again. If the longitudinal balance test fails: the series fault is still toward the grounded short. If the longitudinal balance test passes: you just went by it.

The above simple procedure will assure the field technician that the cable pair is quality and can be used for any service as long as it is within the reach of that service.

Signing off
Thank you, loyal readers. If this article is helpful, let me know. If it’s not helpful, let me know what’s important to you. You can reach me at 831.818.3930 or dmccarty@mccartyinc.com. Plan to attend OSP EXPO 2011, the leader in OSP education. Tell your boss this is a must-attend event: September 12-14 in Cincinnati, Ohio. I’ll be presenting and you can challenge me with any questions -- I love being stumped!