Give your Cable a "Quiet Zone"
A quiet zone is a length of cable as it exits the cable tray where the cable is supported and can keep free from turns and small radius bends. It’s widely understood that when a multi-contours cable is bent in a 180 degree angle, the conductors that happen to be at the inside of the bend radius will be under compression (i.e. getting shorter) and the conductors at the outside of the bend radius will be under expansion (i.e. getting longer). Now consider these forces of compression and expansion in a dynamic environment where they are traveling back and forth down the cable as the cable is under rolling flex. What many don’t realize is that these forces are not as localized at the bend site. These forces are easily capable of traveling several feet. A problem occurs when the forces traveling a certain direction meet with a restriction that stops the natural propagation of the forces and they are then reflected back in the opposite direction. Clamps, connectors, bends can all cause such restrictions. Think about dropping a small pebble in the center of a large pool. A small wave will form and propagate in all directions. If the pebble is small enough and the pool big enough, the waves will have completely dissipated before reaching the sides. Now take that same pebble to a kitchen sink and drop it. The resultant waves will travel to the sides of the sink and be reflected in the opposite direction. The former case is the purpose of the quiet zone. We want the forces to be naturally attenuated by the structure of the cable before reaching a restriction. Now remember that we are not really talking about a single event. In most tracks they are repeatedly cycled back and forth giving the possibility that the forces may be additive at any given point. In such a case, one or more of the conductors, given all these forces, generates excess length. These conductors will take the excess length and try to move outward and the now shorter conductors will then try to become the new neutral axis of the cable. When this happens corkscrewing starts and will get worse with each successive flex cycle.
So how long should a quiet zone be? Here’s where you might get different answers from different people. You can find values of quiet zone length should be at least 15 times the cable OD to 50 times. In truth, there is no good single value that would be optimal for every installation scenario. There are several factors to consider; the big factors we consider are the number of conductors, AWG size, shielding, and length of the cable. In short, the heavier, more complex a cable is, the longer the quiet zone needs to be. Especially for longer tracks, avoid cables with more than 2 cabling layers. A run of three-30 conductor cables is far more installation forgiving than two-45 conductor cables. Conversely, for low conductor count cables, one can gravitate to a shorter quiet zone.
Stay tuned for the next blog entry where we discuss using the recommended specs and how a cable is valuable in a complex system.