A quality multi-functional test set is an absolute necessity for today’s field technician. You may have seen a series of articles I wrote earlier this year about a few quality test sets.
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 electronic 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 all safety precautions are required. 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 will be 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 than 0VDC is acceptable. If that crossed battery fault is not located and fixed that tip conductor will go 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 will affect bandwidth?
Customer equipment will show a short from the tip to ring ranging from 1 Meg ohm to more than 3 Meg ohms. This will 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 will affect bandwidth. Any tip or ring ground more solid will affect 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 will give you a percentage — it should be greater than 98%.
If the capacitive balance is less than 98% it will fail 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.
Will a split cable pair affect the longitudinal balance?
A split cable pair will fail the longitudinal balance, and no bandwidth circuits such as xDSL, HDSL, or T1 will work. Once the split pairs are identified the Time Domain Reflectometer (TDR) will locate the distance to the split cable pairs using the cross-talk feature.
How much series resistance will affect bandwidth?
As little as 5 ohms difference between the tip and ring conductor will affect 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.
As an example: 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
If you want more info on test sets, look at earlier issues of OSP® magazine at www.ospmag.com, and you will find my review of a few sets. If you want to discuss these or any sets (I didn’t review all of them but I am aware of and have used all.), call, text, or email. If you have questions and thoughts that you think others would enjoy hearing about, send them to me at [email protected] or call 831.818.3930.
