More on the Resistance Bridge

In last month’s column, Tim, a skilled field tech, stated:

Maybe I missed it, but I’ve never seen a good column on the resistance bridge feature. It is the best time-saver there is when used properly, especially when using it in the field by yourself. It can help you get a good idea where the trouble is as opposed to having to be dead wrong when digging up a street or someone’s front yard. Any tricks and tips would be appreciated.
Thank you,
Tim Segada

In the June column I explained the basics of electronic reference, and Tim stated that he understands what I wrote, but he had further comments:

I get that but I’m just trying to get a good idea of which terminal or pedestal a single pair trouble is located in as opposed to having to be dead on when I am digging up a street or someone’s yard. That might have been lost in your translation. Other than that, your article is exactly what I was talking about when I am chasing a single pair fault when there is trouble on one conductor of the pair. I can then use the single pair hookup on the resistance bridge.

To be honest, if I have a good pair to strap to for using a separate good pair hookup, my company would want me to transfer my customer to it, close it out and pick up another job.

Another thing I’ve been trying is when I go to the far end of the strap, I look at it with the TDR. Then when I get the far end strap measurement, I play with temperature and gauge to get in the ballpark of that measurement. Again, we’re not digging up the street here. We’re just trying to know what pole to climb to clear the trouble. In this scenario, there is always gauge changes. From the 26-gauge cable at the cross-box to a few sections of 24-gauge cable and then 26-gauge cable and possibly some 22-gauge cable, you get the idea. If you have some more suggestions on how to use the resistance bridge, I’m all ears.

Tim, you are dead on with your method of ball parking the area of the fault. I am going to cover the how and why of your analysis.

In my first example at the cross-box with the pair open, we have identified a 5,000-ohm ring ground and the tip conductor tests good. First, an open measurement is required. The tip side may test good because it is open 300 feet away. In my example, the tip side shows the tip conductor open at 4,100 feet which is approximately the distance to the customer’s terminal. Next, we strap the pair tip to ring at the cross-box and proceed to the customer’s terminal.

At the customer’s terminal we prove the fault into the cable and we use the Time Domain Reflectometer (TDR) to identify the strap is good at the cross-box and the distance to shorted cable pair measures 3,900 feet with the TDR.

After running the resistance bridge with a single pair hookup, we measure resistance to strap (RTS) 102 ohms, resistance to fault (RTF) 14 ohms, and resistance strap to fault (RSTF) 88 ohms. With 24-gauge cable in the customer’s terminal, we set 24-gauge into the resistance bridge to 24-gauge setting a reasonable temperature and measure distance to strap (DTS) 4,250 feet, distance to fault (DTF) 560 feet, and strap to fault (STF) 3,690 feet.

Because the resistance bridge, the TDR, and the open meter, all show about 4,100 feet of cable, we can assume that the cable is all 24-gauge and there are no laterals (bridged tap) and the DTF measurement would be good.

Had the resistance bridge shown a DTS around 4,100 feet, and the TDR around 4,100 feet, and the open meter had shown around 5,000 feet, then we would have suspected a lateral (bridged tap). Had the TDR shown a lateral at our DTF reading, the fault could be anywhere down the lateral.

Had the resistance bridge shown a DTS around 3,500 feet, the TDR around 4,100 feet, and the open meter had shown around 4,100 feet, we would have suspected a gauge change. Less distance when measuring DTS in 24-gauge would indicate that the other gauge would be 22-gauge or 19-gauge cable. More distance when measuring DTS in 24-gauge would indicate that the other gauge would be 26-gauge.

In the example, even if there is a gauge change, more than half of the cable is 24-gauge, and the DTF is fairly accurate, and we could look for a terminal or pedestal around the 560-foot measurement.

These techniques do work if the field technician is given the time to pursue the root cause of single pair cable troubles. When multiple faults are proven into a section of buried cable, the measurements must be more precise in order to find and fix the faults by digging 1 or 2 holes rather than unnecessarily replacing the section. (That’s the topic of next month’s column.)

Signing off
Thank you, Tim, for both your original comment and for following up. When I have real-world problems to discuss, it’s tremendously helpful to the readers. So, readers, send in your tough cases, and we can discuss them both through emails and in the columns. Contact me at [email protected] or 831.818.3930.