The Halachic Form of a Tzuras Hapesach
Tzuras hapesach means the form of a doorway. In a city eruv it often uses utility poles or dedicated posts with an overhead wire, but the physical pieces must form the doorway recognized by halacha.
The side posts
The two side posts must serve as the visible sides of a doorway. Dedicated posts are often used, while an existing utility pole may be accepted only when its form and position meet the halachic requirements.
A short lechi can sometimes establish the required post beneath a utility line. It must be fixed in the correct place and reach the ground; an unrelated object standing nearby does not become part of the doorway merely because a wire passes over the area.
The crosspiece above the posts
The overhead line forms the lintel of the doorway. It must pass over the side posts in the required alignment, not attach to the side of a pole or miss the lechi below.
Sag, multiple utility lines, crossarms, and changes in height can complicate what appears simple from street level. At a turn, the end of one doorway and the beginning of the next must meet without leaving the corner open.
Where tzuras hapesach can be used
Tzuras hapesach is a powerful halachic enclosure, but it is not sufficient in every setting. A full reshus harabim may require actual doors or another stronger enclosure under the governing psak.
Large breaches, roads running through the boundary, and other features can also require treatment beyond a routine sequence of poles and wires.
Stability and continuing condition
The doorway must be capable of standing in the manner halacha requires. Wind, leaning poles, sagging lines, construction, and replaced utility equipment can alter a section that was previously accepted.
For that reason the original design and the weekly inspection answer separate questions: the first establishes a valid doorway, and the second confirms that it still exists.