Are your connectors time bombs?

Master punching down 110 blocks through careful inspections.

Over the years in my work as a consultant involved with the physical layer (the copper and glass glue that holds this whole hi-tech world together) I have run into one problem over and over: IDC punch-down failures. Perhaps I can save others some frustration.

I have never been short of frustration.

My first trip to Great Britain was on behalf of a large Canadian corporation. I was supposed to look over the CAT 5 cable installation in one of its plants. Head office sent a letter of introduction in advance of my trip. The letter said I "was being sent to audit the facility and should be given full cooperation and had camera-on-site privileges." When I arrived, it was like walking into a deep freeze. It took me three days of buying lunches and holding private meetings before they realized that I was sent there by the home office to help, not be the corporate hatchet man.

I really did want to help.

A microscopic look at the metal contact forks on 110 IDC punch-down blocks yields three common types of geometry. Figure 1A shows sharp right-angle corners at the wire entry point. The corners may even have sharp burrs from the stamping/punching process during manufacture. Figure 1B shows a bevelled entry design. Figure 1C shows a rounded wire-entry design.

[Figure 1 ILLUSTRATION OMITTED]

If we now try to punch down a conductor into these IDC forks, we get varying success rates. The forks of Figures 1B and 1C displace insulation and guide the conductor into the contact grooves. Forks from the Figure 1A group pose a problem. Study Figure 3 and note how the sharp edges of the forks "jam" into the copper-conductor cross section.

[Figures 2-3 ILLUSTRATION OMITTED]

There are three possible outcomes of an IDC punch down into a Figure 1A cross-section fork. You can get a good contact as in Figure 2A. Figures 2B and 2C punch downs represent big-time problems. Often, they pass installer visual inspection and--even worse usually make contact so that they pass certification testing. The mode for this connection relies on the retention force of the plastic slots of the wire block against the plastic insulation of the wire on both sides. Both of these connections are ticking time bombs.

The normal IDC punch-down cycle starts with the conductor being located above the slots of the metal fork and being held lightly by the plastic slots of the punch-down block. The punch-down tool then forces the conductor into the slot of the metal fork, and, during that process, it displaces the insulation to make a gas-tight metal-to-metal contact.

Look at the fork with the slot geometry of Figure 1A. When you try to punch down a conductor, the IDC cycle starts with the sharp corners of the forks digging into the copper conductor. At this point, the conductor either breaks and you get a punch down as in Figure 2B, or the conductor jams tightly enough in the slot of the punch-down tool to give you a termination that looks like Figure 2C. A Figure 2C termination can be spotted by careful visual inspection. Unfortunately, the Figure 2B termination looks fine and often passes testing.

This termination can drive you nuts.

After running into a number of Figure 2C terminations in one day, I spent the next day in the lab with a stereo microscope. I had a fairly broad range of products at my disposal to examine. To my surprise, I found a wide variation in the wire lead-in corners of the IDC forks. Some forks actually had sharp edges with protruding burrs. I was also surprised to see that this was more of a quality-control issue than being attributable to one specific manufacturer.

An internal quality-control program that involves testing and inspection of incoming product can yield a large payback. Both spot checks and production lot tracking and testing can pay off. Installer training and supervision is also important. Spending time with the install crew allows you to spot any problems as they develop. Installers are very vocal about things they don't like. Careful visual inspection at every step is also essential and can spot problems that testing can miss.

I have great respect for installers and always end up getting an earful.

Network stability starts at the physical layer, yet many managers target this area for cost reduction or simply ignore it. Certification of the cable plant is a complex issue. Often, managers don't know what they should expect. I have seen companies pass off tests using LED-continuity testers as CAT 5 cable certification ! However, doing tests with a sophisticated cable tester and printing reports is not perfect either. A cable test is only a snapshot of the performance of that cable at that moment.

Product selection, workmanship, thorough visual inspections, and full testing all are required inputs to the "AND gate" of the certification process. You can throw a lighted firecracker into a bear cave and, as long as the fuse is burning, nothing will happen. However, when the firecracker goes off, you will be facing an angry bear looking for a fight.

Networks ate much the same.

Rygalo has been involved over the last 15 years as a consultant in the design and troubleshooting of R.F. communications systems, network TIA/EIA and BICSI physical layer, power quality, and grounding EMI. He can be reached at 780-453-1008.