Patch-cord Bend Radius Field Analysis
This white paper contribution on bend radius analysis of field-installed patch cords was submitted for a presentation at the March 2nd 2000, EIA/TIA, TR 42.1 committee meeting in Orlando, Florida. The field analysis of bend radius along with the testing contribution of bend radius provided the support for the TR 42.1 committee to provide a two-thirds vote to adopt a one times (1x) the cable diameter bend radius for UTP patch cords in the future 568-B.1 standard. This new standard is anticipated to be ratified by the end of this year.
ABSTRACT
The Nov 98' 568-B.1 document (clause - 10.2.1 - minimum bend radius) has proposed a stringent requirement of four times the outer cable diameter for UTP patch cords (this equates to approximately 2" diameter, 1" radius). This requirement has not existed in the previous drafts of the 568-B.1 or in the 568-A standard. As a result of this newly proposed requirement, field site analysis of standard installation practices of bend radius for patch cords when installed in the field was presentated.
Field Site Analysis of Patch-Cord Conditions
PerfectSite conducted its field examination in the metropolitan, Washington, DC area at twenty sites during an eight-month period. To qualify for the research, a site had to have at least 100 UTP patch cords attached to a single or adjacent racks or cabinets. The sites included:
Figure 1 - Breakdown of Sites

Figure 2 - Research Totals

The analysis consisted of random selection and examination of 1624 patch cords, 10% of the total, for a statistical accuracy level of +-3%. The patch cords examined were divided into four categories based upon their condition at the cross-connect. 1) Acceptable Patch-Cord Slack - The inspectors applied the "three-finger" guideline to determine acceptable slack in the patch cord. That is, the inspector placed three fingers at the point where the patch cord entered the horizontal manager and was routed to the data-equipment port. Acceptable slack of 5 inches or less resulted when the cord could be looped around three fingers at that point patch-cords surveyed, 6% were found to have acceptable slack (see figure 3). 2) Unacceptable Slack- Any length of patch cord over the "three-finger limit" was deemed unacceptable. The research found that 93% of all patch cords examined were too long. The average overage per patch cord was 20 inches. Although this may not seem like much, consider that, multiplied by 187, the average number of patch cords per cross-connect, a total of 312 feet of patch cord, longer than a football field, sits unused at each cross-connect. This accumulated slack, hangs in disarray, affecting manageability, aesthetics, bend radius, and bend-radius stability (see figure 3). 3) Kinked - This condition occurred when a patch cord was collapsed on itself, bent 180 degrees, virtually without bend radius. In every case, kinked patch cords, accounting for 2% of the total examined, also had excessive slack (see figure 3). 4) Banjo - This condition is found between the equipment port and the horizontal patch-panel port when a patch cord that is too short is installed without benefit of a vertical management system. One percent of the patch cords examined had this problem (see figure 3).
Figure 3 - Conditions of a Patch-cord

Inspectors found three types of horizontal patch-cord managers in use, each of which affected bend radius differently. Distribution rings were the most common way of directing the patch cords from the patch panel to the vertical management system. Routing clips were used at some sites. Smaller than distribution rings, they could be more densely populated on the horizontal manager, allowing a straighter patch. The third type of horizontal manager was the channel or duct (see figure 4).
Figure 4 - Typical Horizontal Managers

Figure 5 - Distribution Ring and Routing Clip Distances Impact Patch-cord Bend Radius

If a strain relief or boot was present, the bend might be sharpened even more when directing the patch cord into, say, a 1-inch routing clip. Patch panels from some manufacturers also extended 0.5-inch out from their horizontal plane, further reducing the distance between the distribution ring and patch panel. A vertical patch-cord management system usually consisted of either brackets or channels (see figure 6). The brackets came in many size and shapes, but often comparable in size to their horizontal counterparts, even though required to handle many more cables. Patch-cord bend radii was typically affected by the "entangling effect" in the vertical managers. This phenomenon occurs when someone relocates a patch cord and, instead of pulling it out completely and starting over, he or she simply unplugs the patch cord and sticks it into the new port atop the other cables (see figure 6a).
Figure 6 - Typical Vertical Managers
Figure 6a - Entangling Effect

In typical patch-cord management systems excessive slack and lack of horizontal management at the concentrator (due to height of concentrator) results in patch cords being compressed upon another causing tight bend radius at the connector interface (see figure 7)
Figure 7 - Compressed Patch-cord Radius

Field Bend-Radius Analysis of Patch Cords
Inspectors evaluated the bend radius of each patch cord by inserting a radius gauge at the points of the tightest bends. Seventy-one percent of the 1624 patch cords evaluated had bend radius at " or less (see figure 8).
Figure 8 - Patch-cord Bend Radius

Another issue in this area is bend-radius stability. Unlike the situation in horizontal cable runs, where the cable is stable once installed, patch cords may be moved constantly. A patch occur measured on one day may have a different bend radius, or even be kinked, the next day. This situation obviously does not apply to the back of the patch panel, where horizontal cabling can be secured from movement (see figure 9).

Termination Spaces

  • Cable Secured
  • Special Tools Required
  • Trained Personal
 

Patch-cords

  • Movement Must Occur
  • No Tools
  • Plug/Play
BACK
Same Patch Panels
 
FRONT
Figure 9 - Bend-radius Stability

 

Uncontrolled
Bend Radius

(No bend radius stability)

 

 

Larger Managers

Figure 10 - How a Patch-cord is routed

Insertion and Removal
Insertion and removal practices of patch-cords will result in repetitive bending (small bend radius) at the patch-cord's plug as indicated in figure 11.
Figure 11 - Standard Patch-cord Practice

Previous TIA Contributions in the UTP systems working group TR 42.7

  • #414 - Patch-cord return loss by AMP
  • #460 - Effects of service loops on CAT 6 link performance by Microtest

Coiling of Patch-cord slack can have a negative effect on return loss.

Figure 12 - Slack Contributes to Performance Degradation

This research did not include the tight bending that occurs in work area cords. This area is typically confined and supports the use of tight bend radius (see figure 13).
Figure 13 - Work area cords

 

Conclusion

The 87 cross-connect fields examined had adequate vertical and horizontal wire managers, but did not eliminate or minimize patch cord slack which contributed to poor management and un-controlled bend radius.

A one-inch bend radius (2-inch diameter) of patch cords was achieved only 7% of the time in a management field, as where inch or less was measured 71% of the time. The standard installation practice dictates that one-inch is an unachievable and unrealistic requirement that cannot be met by patch cords in the field.

Field Recommendations for Patch Cords

Eliminate slack in each patch cord after routed in a management system. This will provide the following benefits to patch cord bend radius:

  • Control bend radius (bend-radius stability)
  • Reduce compressed patch cords at the connector interface
  • Provides better functional performance.

TR42.1 Committee Reaction

The committee was posed with an open floor to any questions of the field analysis provided on bend radius. No committee members responded.


Authored by: Dennis W. Mazaris, RCDD
Submitted: TIA/EIA TR-42.7 contribution #448, March 2000
Submitted: BICSI Standard Committee, August 1999
 
White paper - TIA/EIA TR-42.7 contribution - Testing
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