Possible cable compression at duct head

If the duct is long enough, and if a sufficient length of cable is inserted, then the tension will become negative near the duct head. In other words, the propulsive effect of the air in the downstream regions is no longer sufficient to keep the cable in tension near the duct head, and it is necessary to push it from behind. Consequently the cable stuffing device situated just outside the duct head has to provide not only the force due to the pressure gradient pushing the cable back out of the duct, but also that force required to make up for the loss of tension. In such cases the device is ordinarily rapidly overwhelmed, and the cable insertion ceases.