A cable that moves is a cable that will fail

Strain relief, service loops, and where to clamp.

A cable that moves is a cable that will fail because a wire's insulation and the fine copper strands inside it can survive being bent once without complaint, but repeated bending at the same fixed point slowly fatigues the material there in a way that standing still, however long that standing still lasts, simply never does.

Why standing still is what a cable is actually built for

A length of cable coiled up and left completely undisturbed can sit for decades with no meaningful change to its electrical performance, since ordinary insulation and copper are chemically and mechanically stable across a long, static life. What actually uses up a cable's working life is motion, specifically motion repeated at the same location over and over, because each flex at that one point works the insulation and the strands there slightly, and slight, repeated working is exactly the mechanism behind ordinary metal fatigue, a crack starting at some microscopic flaw and growing a little further with every cycle until enough of the material's cross-section has failed that the rest gives way all at once. A cable that never moves never accumulates that damage no matter how many years pass, and a cable that moves constantly at one fixed spot can fail within months even though the cable as a whole, measured by total length or total time in service, still looks practically new everywhere except that one point.

The hairdryer-cord comparison

A hairdryer's cord almost never frays somewhere in the middle of its length; it frays right where the cord exits the handle, the one point that bends every single time the hairdryer is picked up, aimed, and put back down. The rest of the cord, coiled loosely in a drawer between uses, barely moves at all across the appliance's entire working life, while that one exit point absorbs every flex cycle the cord will ever experience, thousands of them accumulated a few degrees of bend at a time. Nobody designs a hairdryer intending the cord to fail there specifically, but the geometry guarantees it anyway: the cord is rigid enough to resist bending everywhere else and forced to bend only at the one place where it transitions from the stiff handle to the flexible cord itself, concentrating every cycle of use onto a single, small length of material.

Why the damage always collects at the same single point

The reason cable damage concentrates rather than spreading evenly is that the cable itself is rarely the most rigid thing in the system; whatever the cable connects to, a housing, an enclosure, a bracket, usually is. That difference in stiffness means bending has to happen somewhere, and it will happen at whichever point offers the least resistance, which is almost always the exact spot where a flexible cable meets a rigid fixed object. Without any deliberate feature added to spread that bending out, all of it collects at one small transition, and the fatigue clock for the entire cable is really only running at that one location rather than anywhere else along its length. This is precisely the mechanism strain relief is built to interrupt, and it is why guidance on vibration-resistant power and signal connectors treats the cable exit as a feature to be engineered rather than left to whatever shape the moulding happened to produce: a moulded or added feature at the cable's exit point that widens gradually rather than transitioning abruptly, spreading the bend over a longer length of cable rather than concentrating it at a single sharp edge, and reducing the sharpest curvature any single strand inside the cable ever has to survive.

Why the clamp has to hold the jacket rather than the wires

A cable clamp's entire job is to decide exactly where a cable is allowed to move freely and where it is not, and it can only do that job correctly if it grips something strong enough to actually resist the pulling and flexing forces the cable will meet in service. The outer jacket of a cable is built for exactly that kind of mechanical load, thick and tough enough to be gripped, bent, and pulled without damage, while the fine wires and any soldered or crimped connection inside the cable are built to carry current, not to carry mechanical strain. A clamp placed so it grips only the jacket, with the wires inside left free to flex gently within it, keeps every pulling force at the point where the cable is designed to take it. A clamp placed too far from a connector, or missing entirely, lets any tug on the cable travel straight through to the connection itself, and a joint that was never meant to bear mechanical load will eventually crack, loosen, or pull free entirely under stress a properly placed clamp would have absorbed completely on its own.

The number that matters here

A cable flexed repeatedly at one fixed point can fail after a number of cycles measured in the low thousands, even though the identical cable left completely undisturbed for the same span of calendar time would show no measurable wear at all, since it is the counted flex cycles at that one location, not the passage of time, that use up a moving cable's working life.

Where this stops being true

Strain relief alone only solves the problem of concentrated bending at a fixed exit point; it does nothing for a cable that has to physically travel back and forth as part of a machine's normal motion, following a moving arm or a sliding carriage rather than staying put and simply flexing in place. That kind of motion needs a service loop instead, a deliberate extra length of cable coiled or slack-routed so the cable itself never has to stretch taut as the moving part reaches its furthest travel, and so the actual bending during that travel happens gradually across the loop's whole length rather than sharply at either end of it. The loop also needs to be clamped correctly at both of its fixed ends, gripping the cable's own outer jacket rather than relying on the internal wires or a soldered joint to take any of the pulling force, since a clamp that grips only the jacket lets the cable move as designed while a clamp, or the absence of one, that lets force reach the connections inside turns an ordinary service loop into a slow way of pulling a joint apart from the inside.