Back in the February BICSI NEWS, I wrote an article concerning bending radius in copper patch cords. Now, it's time to give fiber-optic patch cords the same attention.
Let's look at the typical fiber-optic patch cords specified in the TIA/EIA-568-A and ISO/IEC 11801 used in premises wiring systems today, the 62.5/125 micron, multimode. What do the standards say about the bending of fiber patch cords? Nothing.
"What?" you say to yourself, "You must kidding! Everyone knows if you have a tight bend in a fiber patch cord, you're in for a heap of problems. After all, it's glass."
Let's look first at the TIA/EIA-568-A standard. What does it say about patch cords? Section 12.4.5 addresses the bend radius of fiber-optic cable in the outlet/connector box, not the patch cords. Even in the outlet connector box, it is addressed at 1.18" radius or 2.36" diameter. But, is this even close to being realistic for patch cords, I think not. I suggest that you not take my word on this, but go to your desk and open your top left draw and pull out your handy dandy compass. Yes, the same one you used back in '76 in your high school trig class. Draw a circle of 2.36" diameter on a piece of cardboard. Cut it out and fold it in half, then go over to your patch cords and start "ameasurin'."
Now that you know that fiber in the real world gets bent, quite a bit, let's take a look at how this can affect the patch cord. It is time to introduce the term frequently used in the fiber industry, "statistical anomaly." What does this mean in laymen's terms? For example, you're at a BICSI golf tournament on the 18th tee, 6 strokes in the lead, on a beautiful sunny day. You pull out your Big Bertha driver, tee up, and hit the ball: but, on the follow-through, a lighting bolt strikes your club.
Let's look at the following diagram and chart of "statistical anomaly," and use what you have measured your fiber-optic patch cord's bend radius at, apply this to your fiber life cycle and low and behold, that is your answer.
Although smaller bends in a patch cord may increase attenuation minimally, fiber in a premises is typically bandwidth restrictive not attenuation restrictive.
So, where does this lead us in the standard process for the future TIA/EIA-568 and ISO/IEC 11801 documents. The same place we are with copper patch cords, looking at a real life approach to defining guidelines and rules that can be followed rather than ignored.