The Rebbe's Letter on the Need for an Expert Rav When Establishing an Eruv, 3 Tammuz 5722
The Rebbe directed Rabbi Pinchas Weberman to bring the question to a Rav who was expert in hilchos eruvin.
Particular expertise in hilchos eruvin
City eruvin combine reshus harabim, walls, breaches, tzuras hapesach, communal acquisition, sechiras reshus, and continuing supervision. Each area has technical halachos, and the conclusion depends on their interaction.
The Rebbe therefore directed Rabbi Pinchas Weberman to bring the question to a Rav with special expertise in the subject. General rabbinic stature does not remove the value of a posek who has worked through these questions in practice.
Expertise must meet the actual city
The Rav must know or verify the streets, walls, crossings, population, ownership, and route to which the psak will be applied. A correct teshuva about assumed facts cannot permit an eruv when the real city is different.
Site visits, maps, measurements, legal documents, transportation facts, and discussion with local authorities can all be part of that investigation.
The expert accepts responsibility for public use
A private opinion about one feature is not the same as authorizing a public system. The Rav who permits the eruv must accept the complete design and the conditions under which people will carry.
He must also direct inspection and repairs and decide when the public must be told that the eruv cannot be used.