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ISE Columnist Don McCarty, OSP Expert

How come?

April 1, 2022
Field technicians call or send emails or texts to me asking questions about a problem they are encountering when provisioning and maintaining the copper infrastructure. These questions may involve anything […]

Field technicians call or send emails or texts to me asking questions about a problem they are encountering when provisioning and maintaining the copper infrastructure. These questions may involve anything the technician deals with on a daily basis or sometimes that oddball problem that rarely occurs. Following are a few examples.

Example #1. Recently, a field tech told me that when measuring tip-to-ground or ring-to-ground with a multimeter in DC volts or ohms, now and then the numbers roll. How come? 

If the numbers roll when measuring DC volts, unknown voltage from another cable pair is the root cause. If the rolling happens when measuring ohms, the internal battery from the multimeter is the cause. 

Example #2. A tech whose full-time job is locating cable path and depth stated that occasionally, when locating and marking the path of the cable and then turning around to check his work, he finds that his marks are a foot or so off. How come? 

When locating cable path using a higher frequency your body capacitance interferes with the locate. To reduce the interference, use a lower transmitter frequency.

Example #3. A field tech stated that when measuring the distance to a fault using the Resistive Fault Locate (RFL) feature on his multi-function test set shows different footage measurements to the fault even though the trouble is all at one spot such as a wet cable splice. How come? 

When fault locating with a multi-function test set to find the distance to a fault, we measure cable pair footage using the RFL, opens, and the TDR function. The goal is to find the sheath footage to the failure. In the manufacture of PIC cables there are 25 different twists in any 25-pair sub-unit, ranging from 2 inches per twist on the white-blue pair to 4.7 inches per twist on the red-slate pair. All other pair twists in the sub-unit fall somewhere in between. 

Cable pairs are 1% to 3% longer than the cable sheath. To compensate for this, subtract 2%, when converting from ohms to feet. If you are using a multi-function test set, this is done automatically for you.

Example #4. A field tech asked how much temperature impacts the result when converting ohms to feet. How come? 

When someone asks me that question, I always say that it depends on who is digging. As the temperature of a conductor increases, the resistance increases and when the temperature of a conductor decreases the resistance decreases. It is about .018 feet per degree. If you were 10 degrees wrong in calculating temperature in 1,000 feet of cable, then there would be about 18 feet wrong. When converting ohms to feet using your multi-function test set, this is done automatically. 

The question always comes up "How do I know what the temperature is?" Early in my fault locating career wherever I was training, I would go down to the hardware store and buy an indoor/outdoor thermometer. I would find a convenient place in the telco yard and set a pedestal and mount the thermometer on it. I would then use a rod and make a hole inside the pedestal at 30 inches, which is about the depth of most buried distribution cables. I would place the outdoor probe in that hole at 30 inches. Then, any time a field tech needed to know the temperature to input, all he had to do is look in the pedestal. 

If you use the RFL function in your area, I suggest that you do the same as I do. 

For aerial cables, place the indoor thermometer in a black ready access terminal. 

I was surprised that when the outside temperature was 90 degrees in the air the internal cable temperature was 130 degrees. 

Over a long period of time I have installed my makeshift temperature probes into telco cable yards in every state in the union.

Example #5. A field tech stated that he liked the resistance fault locate (RFL) feature on his multi-function test set, but sometimes when he retests he gets a different answer. How come? 

There may be more than one answer to the problem, so I offered a step-by-step process to find the root cause of multiple measurements. Most resistance bridges in the field today use very similar principles to locate trouble. Some steps on the latest sets are done internally by microprocessors, but the result is the same. When operating any resistance bridge use the following procedure.

Easy Procedure for  Using the RFL

  • Identify the faulted conductor.
  • Use a VOM to identify any crossed battery or resistance tip to ground, ring to ground or tip to ring. Any resistance must be more solid than 20 megohms.
  • Test the good pair. The good pair must be free of faults; this is one reason you get different answers when you retest. Any fault on the good pair will cause an accurate fault measurement.
  • Attach the far end strap; use only an approved connector. Do not skin the insulation and twist the bare wires together; that is not a good strap and you will get different answers when you retest. 
  • Set the gauge and temperature, and measure the distance to fault.
  • If you get different measurements, then there is more than 1 fault on that conductor.

Signing off
I really enjoy hearing about any problems you are having. Please don’t be uncomfortable about asking or thinking it’s probably a dumb question. (There is no such thing as a dumb question!) Or maybe it’s such a strange problem that you think it’s something you’ve done wrong. There are some really bizarre cases of trouble that I’ve come across. I like being challenged, so please send in your questions, concerns, or tips, you can share to make your fellow technician’s job easier. Reach out to me at [email protected]. Or you can call or text 831.818.3930.

About the Author

Don McCarty

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 [email protected] or visit