Testing Copper Infrastructure

0

Every time someone says copper is dead, it isn’t and there is often a resurgence as new technology increases copper bandwidth and reliability. One day, which is still in the rather distant future, we can expect wholesale FTTP but not yet. Therefore, how can you take care of your copper today so that you are profitable and you can keep your customers happy?

A quality multi-functional test set is an absolute necessity for today’s field technician. I have tested and used all of the leading test sets for many years: EXFO’s MaxTester 635, Tempo Greenlee’s Sidekick Plus, TesCom’s 990 CopperPro Series II, and VIAVI’s OneExpert test set. All do a great job performing physical layer tests and bandwidth tests. In addition to these basic tests, each test set offers many excellent advanced functions. Think about what you want your test to do beyond the basics and then shop all of the sets.

So, let’s look at actually using the test sets and, in the process, developing an understanding of how they do what they do.

A working example
Yesterday a bandwidth customer who found me on Google called and was at wit’s end with an Internet problem. His complaints were twofold: his Internet kept dropping, and when it was up he could get only 3 Megabits down.

Three technicians were dispatched, and they finally told him that he had an electrical problem in his home that was causing the modem to drop. At the customer’s insistence a 4th technician was dispatched, and when testing the cable pair, the tech identified and removed 250 feet of bridged tap. The customer is up and running at 6 Megabits. So, let’s review the testing procedure.

The multi-functional test set, properly used and interpreted, identifies problems with the copper infrastructure and where these problems occur. Interpretation can be difficult for the novice field technician or even the experienced field technician, especially if that field technician does not have an electronics background.

Following are questions from field technicians about functions of the test set when testing a cable pair:

How much is too much AC voltage?
The test set has a lock out circuit that opens if the AC voltage exceeds a certain value so that the test set does not burn up. If you see any AC voltage greater than 50VAC, back away and implement safety precautions — the circuit that you are testing may be crossed up with phase power.

When testing a cable pair for Longitudinal balance, what is acceptable?
When testing a cable pair, greater than 60dB is acceptable and less than 60dB is unacceptable. The root causes of a longitudinal balance failure of a copper cable pair are:
• Crossed battery tip and/or ring to ground from another working circuit
• Any resistance tip and/or ring to ground
• Any capacitive balance less than 98% capacitively balanced
• Series resistance
• Split cable pairs

When testing a vacant cable pair, what is the least amount of indicated DC voltage tip and ring to ground?
Indicated crossed battery is less than applied when measuring a vacant cable pair tip and/or ring to ground if there is any resistance other than 0 ohms at the fault where the crossed battery occurs. That resistance is in series with the voltmeter’s ohm/volt network, so less than applied voltage is indicated on the DC voltmeter.

For example, a tip conductor is crossed with a ring conductor with an applied voltage of -51VDC. The path through water from that ring conductor to the tested tip conductor is 270,000 ohms. When measuring the voltage tip to ground those 270,000 ohms are added to the voltmeter’s ohm/volt network and -33VDC is indicated on the voltmeter, not the applied -51VDC.

When testing a vacant cable pair tip and/or ring to ground, nothing other than 0VDC is acceptable. If that crossed battery fault is not located and fixed, that tip conductor goes open.

Multi-functional test sets have the capability of identifying a tip to ring short or a tip and/or ring ground up to 999 Meg ohms. What is the minimum resistance that affects bandwidth?
Customer equipment shows a short from the tip to ring ranging from 1 Meg ohm to more than 3 Meg ohms: this does not affect bandwidth. There should not be any path tip and/or ring to ground and the test tip or ring to ground should test greater than 999 Meg ohms. Any resistance greater than 20 Meg ohms tip and/or ring to ground rarely affects bandwidth. Any tip or ring ground more solid affects bandwidth.

As an example, ringing voltage ionizes a 20 Meg ohm ring ground. At that point the resistance becomes 0 ohms and the modem or set-top box dumps. Those faults that are more solid than 20 Meg ohms can be located with the resistance bridge Resistance Fault Locate (RFL) mode on the multi-functional test set.

How much difference in the capacitive unbalance fails the longitudinal balance test?
When testing the capacitive balance measure tip to ground and ring to ground with the open meter function on the multi-functional test set. If tip and ring are of equal value, the capacitive balance is 100%. If the tip to ground and ring to ground values are different, divide the small number by the large number and that gives you a percentage — it should be greater than 98%.

If the capacitive balance is less than 98%, it fails the longitudinal balance test. The root cause of the failure is that pair is open on one side beyond the customer or open on one side on a lateral or crossed up with some amount of a non-working conductor.

Does a split cable pair affect the longitudinal balance?
A split cable pair fails the longitudinal balance, and no bandwidth circuits such as xDSL, HDSL, or T1 work. Not only is that pair affected, all other pairs on a DSLAM are affected. Once the split pairs are identified, the Time Domain Reflectometer (TDR) locates the distance to the split cable pairs using the cross-talk feature.

How much series resistance affects bandwidth?
As little as 5 ohms difference between the tip and ring conductor affects bandwidth. When all else tests acceptable short and ground the cable pair on one end and run the longitudinal balance test. If the test fails, measure tip to ring ohms and then tip and ring to ground ohms. Any difference indicates a low resistance series resistance fault.

For instance, a 100-ohm short should show 50 ohms tip to ground and 50 ohms ring to ground. If a 100-ohm short shows 60 ohms tip to ground and 40 ohms ring to ground the tip conductor has 20 ohms in series somewhere along the cable pair.

Use the TDR to locate the fault.
If none of the above faults exist on the cable pair and it passes the longitudinal balance test it is qualified for bandwidth within the reach of that bandwidth service.

Signing off
Investing in test sets is expensive, and learning to use them can be time-consuming so choose carefully. Talk with the reps and, if you want, give me a call. I have used all sets during my training. If you have ideas about what you want to see in future columns, give me a call at 831.818.3930 or send an email to dmccarty@mccartyinc.com.

About Author

Don McCarty is the OSP EXPERT columnist for ISE magazine, discussing the issues around provisioning, testing, and maintaining copper for all services from POTs to IPTV. Don is also president of and the lead trainer for McCarty Products, a technical training and products company training field technicians, cable maintenance, installation repair, and Central Office technicians and managers. For more information, email dmccarty@mccartyinc.com or visit www.mccartyinc.com.

Comments are closed.