A mechitzah teluyah is a partition that is raised three tefachim (24 cm) or more above the ground. Since a goat can pass beneath it, the partition is not considered a valid enclosure, and one does not apply to it the rule of gud achis (“extend and lower”).
Eruvin are generally constructed from both tzuras hapesach and from partitions, fences, buildings, etc. To determine the validity of the partitions, we must ensure that all fences are either flush with the ground or raised less than three tefachim (24 cm) above it. Therefore, one should not evaluate fences from a distance with a rough estimate, but must examine the bottom of the fence up close. This problem is very common in eruvin. Even in areas with security fences or military bases, suspended partitions have been found due to animals or erosion.
Additionally, there are cases in which a partition invalidates a tzuras hapesach, and in order to validate the eruv, one may intentionally invalidate the partition by cutting the bottom of the fence so that it becomes a mechitzah teluyah, instead of dismantling the entire structure.
