Ask the Rov: How can I ensure the eruv in my camp or bungalow colony is kosher?
By Rabbi Chaim Hillel Raskin – Rov of Anash in Petach Tikvah
Min haTorah, one may not carry from an open public domain (reshus harabim) into an enclosed private domain (reshus hayachid) or more than four amos in a reshus harabim. A reshus hayachid is enclosed with walls or an area raised off the ground (though not every reshus hayachid allows carrying). A reshus harabim is a street that is 16 amos wide and, according to some opinions, is part of a domain for 600,000 people.1
An unenclosed area that isn’t widely traveled (karmelis) — such as a small street or an open field — is neither of the above and is permissible for carrying min haTorah.2 However, Chazal prohibited it since it can be confused with a reshus harabim.3 If walls are placed around the karmelis, it can become permissible.
Halacha says that a doorway (tzuras hapesach) is like a wall, and one could make an entire wall of “doorways” around a karmelis (but not around a reshus harabim). The doorways may be constructed with just poles and a string, so long as they are formed correctly.4
Thus, the string must be above the pole and not to the side of it5 (or through it6), the string should remain relatively level (on a hill, the height of the poles should be adjusted accordingly),7 and it should run in a straight line between the poles and not be pulled out from between the poles (or loose enough that it can sway in the wind).8 The poles should be sturdy like a wall9 and straight up so the string is above them.
How wide can each “doorway” be?
While some opinions hold that each “doorway” can stretch on for miles, the Rambam rules that each “doorway” must be less than ten amos. Still, if most of the wall is solid (omed merubeh al haparutz), then the remaining “doorways” can be more than ten amos. Some acharonim say that this majority is of the whole circumference (even if some walls are mostly “doorways”), while some hold that each side must be mostly solid (or two complete parallel walls).10 Another advantage in making each “doorway” less than ten amos is that if one of them breaks, it can be counted as an entrance, and not invalidate the entire eruv.11
Of course, the eruv must be checked each week to verify its continuous kosher status.
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From The Weekly Farbrengen by Merkaz Anash