One of my maintenance students sent me the following question on determining the presence of water in a section of Air-Core PIC cable. Here's his question.

Donald,
Having been around a few years and having been taught by you a couple of times, I feel a little slow asking this. Maybe I'm suffering from Old Timer's Disease. I just turned 51.

I was working on a section of wet PIC cable where I suspected that there was water intrusion in the cable core. You told me to record the actual section length, and then measure the length of an open pair with an open meter. Then, if the open meter read long, subtract the actual section length and divide the difference by 1.865. This would show the amount of water in the section.

I realized that I don't know where the 1.865 comes from. When I help the younger technicians out there I should probably have the answer when they ask, rather than saying "I don't know" just because I have always hated that answer myself. So, can you help me without making me feel stupid?
Thanks,
Kevin Pearson

Kevin,
When I was working for Dynatel, prior to the acquisition by 3M, we made the first good open meter called the Dynatel 730 open meter. It was the first open meter that could find open cable pairs in the presence of other trouble, such as a short, ground, crossed battery, or series resistance.

I trained field technicians how to use the meter. One day a field technician called me and said, "Donald, I measured the distance to an open in a buried section that is 500 feet long and your open meter says that the open is at 600 feet. How can that be possible?"

I talked to Bert Stephens, who was the engineer that designed the set, and he concluded that there must be water in the section shorting out the capacitance and making the open meter read long.

This created a dilemma. We needed to know how much water affected the measurements. I went to a second level manager from Pacific Bell and got several pieces of dry PIC cable for research purposes. I then calibrated the open meter to the exact length, then pumped the cables full of water and measured the cables again. The capacitance increased showing more than the actual distance. In each instance the increase was 1.865 over the actual distance. I tested cables from 25-pair to 400-pair.

Here's an example. The actual section of cable is 1,000 feet long and I calibrate the open meter to show exactly 1,000 feet. I then pump the cable full of water and the open meter showed 2,865 feet, which is an increase of 1.865 over air.

Wet Section Cable Replacement
However, it's not just about technical findings. A business case must be provided in order to gain approval for cable replacement. Typically what happens is the field technician approaches his supervisor and tells him, "Boss, I have a section of cable that needs to be replaced." The supervisor says, "What's wrong with the section?" The technician replies, "It has water in it." The supervisor asks, "How much?" The technician replies, "A lot." That is not a good business case to get a section of cable replaced. You need technical facts.

A conceptual picture can be built with a resistance bridge, an open meter, and a Time Domain Reflectometer (TDR).

To measure the amount of water in a section of air-core PIC cable, first establish the actual section length. Next, measure the section length with the open meter, subtract the actual length, and divide the difference by 1.865. For example, a section shows 700 feet. The open meter shows 1,100 feet. The 400 feet difference shows 214 feet of water in the section if it was full of water. (400 divided by 1.865 = 214.4772)

Never start out in a suspected wet section looking for the water pattern with a TDR. First, use the open meter. The increase in footage is the indication that water is present.

Then, use the TDR to see where the water starts from either end. This will show the dry cable. Subtract the dry distance from the actual distance and if it shows 214 feet, then the cable is full of water. If the difference is less than 214 feet, then there is moisture in the section or pockets of water.

The resistance bridge will show that the faults occur throughout the section.

I hope this explanation helps. This formula is in the section analysis portion of my TechEXPERT program. The TechEXPERT program demo is on my web site at www.mccartyinc.com.

You do not have to do the math. Just put in the actual section length, the resistance bridge measurement, the TDR measurement from each end, and the open meter measurement, and the program will do the math for you.

The program also identifies other anomalies in buried sections such as gauge changes, slack loops, butt splices, loading coils, and unknown laterals. When the field technician finishes testing the section the results are recorded showing all problems and a business case can be presented to get cables replaced.

A business case should clearly demonstrate the problems within the bad section allowing an engineer to make a qualified decision as to whether or not to replace the section. While it seems like an unnecessary step, without an acceptable analysis, sections of expensive cable would be replaced unnecessarily. This means less money for those sections that need replacement or for that piece of equipment you want your boss to buy.

Signing Off
Be sure you make it to OSP EXPO™ 2007 in San Jose, California, August 28-30. It's not only informative and can provide you with many tips on how to do your job better, it's also a good time. Please let me know if you have questions that I can bring up to the readership of OSP™ Magazine. You may think your problem isn't important or that you should know the answer, but remember: there are many, many technicians who can benefit from hearing about your problem cases. Contact me at dmccarty@mccartyinc.com or 831.818.3930.

About the Author - Don McCarty

LET`S TALK Check out our website (www.mccartyinc.com). In addition to providing information about our extensive selection of courses, we are adding an equipment review section and a section on troubleshooting, where we will discuss some interesting cases of trouble. If you have an interesting case, send it along (names withheld by request).

As always, I appreciate hearing from you with questions or comments on any topics that you think are of general interest to outside plant technicians and managers.

McCarty Associates
18281 NE Rainbow Lane
Newberg, OR 97132
Telephone: 503.538.2747
dmccarty@mccartyinc.com