Series Resistance


Its Effect on Bandwidth Circuits on the Copper Infrastructure

In my July 2016 column, “What Is a Good DSL Circuit?”, Vernon May mentioned 66 blocks and DSL.* Later, in a McCarty newsletter, Vernon responded to a reader question concerning 66 blocks in outside plant. In this month’s column, we will again look at the issue of 66 blocks, and the effect of series resistance on copper circuits in general.

When we look at any connection of copper conductors, that connection must be 0 ohms at any frequency. Any series resistance, even as low as 5 ohms in series on paired copper conductors will have a negative effect on any copper circuit. Series resistance can occur in any connection or splice, and it is extremely hard to identify and to locate.

To test for series resistance:
• Place a 0-ohm grounded short on one end of the circuit, preferably at the central office or remote.
• Test the longitudinal balance at the other end, preferably at the customer’s network interface.
• If the longitudinal balance test passes (>60dB), then there is no series resistance in the circuit.
• If the longitudinal balance test fails (<60dB), then determine how much series resistance is there and which conductor it is on.

To do this, first, measure tip to ring ohms and then measure tip to ground ohms and ring to ground ohms. On a resistively balanced circuit tip and ring to ground will measure ½ tip to ring ohms and will be equal. For example, if tip to ring ohms measures 100 ohms then tip and ring to ground ohms should measure 50 ohms. If tip to ring shows 110 ohms and tip to ground measures 60 ohms and ring to ground shows 50 ohms, then there is 10 ohms in series on the tip conductor.

There is a possibility that series resistance will show up on a Time Domain Reflectometer (TDR). Look for an upward anomaly upward from the base line like an open. In long distances divide and conquer.

History of Copper Connections
The first copper connections were soldered. Central Office House cable pairs were soldered at the originating equipment, then, at the horizontal main frame, at the vertical main frame, at the tip splice, the vault splice, at every cable splice in the feeder and distribution plant all the way to the customer’s serving terminal.

Any cold solder joint created series resistance. On a narrow band POTS circuit this series resistance caused static on the line. The root cause of the static was the making and breaking of dial tone. If static was present, then the divide-and-conquer method was used to locate the fault.

TIP: If you find unsoldered twisted splices, then the complete cable should be re-spliced with today’s mechanical splice connector such as a Scotchlok, PICABOND, or equivalent connector.

The expense of soldering increased as labor pay increased. The wire wrap tool became popular in the CO, and construction eliminated the soldered splice. The unsoldered twisted splice created havoc in the field. Countless splices had to be redone because of series resistance.

Salvation came in the early 1960s with the first good mechanical conductor, the .052 or “B” wire connector.
It was commonly referred to as the white bean. Multiple contacts through pulp insulation made a 0-ohm connection across the splices eliminating series resistance. The “B” wire connector worked well on pulp cable splices, but created series resistance on plastic insulated conductor (PIC) splices. If you find “B” wire connector splices in PIC cables, then, again, the complete cable should be re-spliced with today’s mechanical splice connectors.

The first insulation displacement connector (IDC) was the 66 type block, which was introduced in 1962. An IDC is an electrical connector designed to be connected to the conductors of an insulated cable, which forces a sharpened blade through the insulation making a 0-ohm contact across the splice. 66 blocks are designed to terminate 22 through 26 AWG solid copper wire.

IDC or 66 Block Problems
One of the problems with an insulation displacement connector is repeated reconnecting or re-terminating a connector. When properly made, the connector blade cold-welds to the conductor making a reliable gas-tight initial connection, but not on subsequent connections. Also, if the technician does not have a 66 tool available, then he will use needle-nose pliers.

Since the invention of the insulation displacement connector, it has been used in hundreds of applications from cross-connect boxes to phone jacks and anywhere copper splices or connections are made. Use of proper tools and reduction of repeated connections minimize series resistance faults that affect bandwidth.

One more thing. That series resistance failure on that circuit you are testing may not be in the circuit. It could be in the cord on your multi-functional test set. Before attempting to run down a series resistance fault preform a self-test on your test set cords to make sure that there is no series resistance in the cords.

*To read my July 2016 column, please visit

Signing off
As always, thank you for reading my columns. ISE magazine is a valuable source for helping you solve problems and learn new skills. We truly listen to and value your thoughts, and we often base future columns on topics that you feel are important. We also address many interesting topics in our newsletter that support and extend the value of the columns I write for ISE. If you would like to subscribe to our newsletter, send an email to with the subject “newsletter” and include your name and company email. You can always reach out to me at or call 831.818.3930 or check out my site

Vernon May has leveraged his 35+ years of service provider and vendor experience into that of trainer/coach, inventor, systems designer, program manager, product manager, and plant operations expert. He holds 2 US Patents, including #8,027,807 B2 on Remote DSL Circuit Evaluation. If you have questions about your DSL, contact him at 254.979.4749 or email, or visit Vernon May Solutions LLC at

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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 or visit

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