Siman 394

A doubt regarding an eruv, such as one who placed his eruv and it was consumed, and it is a doubt whether it was consumed while it was yet day—and it is not an eruv, or a doubt whether it was after dark—and it is an eruv; and similarly, one who placed terumah and it became impure, and it is a doubt whether it became impure while it was yet day—and it is not an eruv, or a doubt whether it was after dark—and it is an eruv; and so too anything similar to this—we say a doubt regarding an eruv is valid. And specifically in cases such as these, where the eruv had a presumption of validity—we establish it upon its presumption. But when it did not have a presumption of validity, such as a doubt whether he placed it or a doubt whether he did not place it, or if he placed terumah that was a doubt whether it was impure or a doubt whether it was pure, or a doubt whether it was terumah or a doubt whether it was tevel and anything similar to this—we do not say the doubt is to be lenient. And all the more so when there is a presumption to the contrary, such as one who established an eruv with non-sacred produce from which terumah and ma'aser were separated, but it is a doubt whether they were separated while it was yet day or whether after dark, for we establish it upon its presumption of tevel. (Gemara 36)

And the reason for the matter is that it is not applicable to say "a doubt regarding a rabbinic law is judged leniently" except when the action was performed but a doubt arose regarding it. However, if there is a doubt whether the action was performed at all, it is not applicable to say "a doubt regarding a rabbinic law is judged leniently." And that which is stated, that if one placed it during bein hashemashot it is kosher, is because there as well the action was performed, but there is a doubt regarding the time. However, when there is a doubt if it was placed at all, there is nothing upon which to say "a doubtful eruv is kosher." And similarly, a doubt whether he placed it with impure terumah or with pure terumah is like a doubt if it was placed, for impure is like it was not placed. This is not the case regarding a doubt if it became impure while it was yet day or after it grew dark, or a doubt if it was eaten while it was yet day, for the eruv certainly existed—we establish it upon its chazakah, for while it was yet day it was certainly fit, and for an eruv we require a meal that is fit while it is yet day. (See Magen Avraham subsection 1, and this is his intention; see there carefully)

The עירוב must be at bein hashemashot in a place where it is fit to be taken. Therefore, if a mound of debris fell upon it and he cannot take it without a shovel or a pickaxe, which is a Torah prohibition—it is not an עירוב. But when he can take it by way of a rabbinic prohibition—it is an עירוב, for regarding anything that is a matter of שבות—they did not decree against it during bein hashemashot in a case of some mitzvah or great need, as has been written in siman 342. And therefore, if he placed it in a cabinet and locked it, and the key was lost before it grew dark, if it is impossible for him to take the עירוב unless he performs a Torah labor during bein hashemashot—this is as if it were lost and is not an עירוב, for it is impossible to eat it. And if he can remove it by way of a rabbinic prohibition—it is permitted, for the reason that has been explained. And the Tur holds that regarding the dismantling of a tent—even with a rabbinic prohibition it is not an עירוב, for the dismantling of a tent appears like a Torah prohibition, and it requires further study. (See Taz and Magen Avraham, subsection 2) And if the key was found on Shabbat in a place where he could have brought it during bein hashemashot without a Torah labor—it is an עירוב, even though it was not in his hand during bein hashemashot. Since it is common that he would find it—it is as if it were in his hand during bein hashemashot. (Tur)

The eruv only needs to exist during bein hashemashot. And even though an eruv is for the purpose of a dwelling, and the dwelling is for the entire day of Shabbat—nevertheless, the primary dwelling must be during bein hashemashot at the time the eruv is acquired. And we hold that the beginning of the day of Shabbat acquires the eruv, and therefore when it is certainly dark—one may eat the eruv. Our teacher, the Rema, wrote that one should break bread upon it on Shabbat morning, for since a mitzvah of eruv was performed with it—one should perform another mitzvah with it, to break bread upon it during the Shabbat meal and recite the blessing of Hamotzi over it. And this was according to their custom, as they would eat the evening meal while it was still day, but for us—it is an obligation to break bread upon it during the night meal. And all this is if one makes an eruv every erev Shabbat, but it has already been explained in siman 368 that it is better to make an eruv for the entire year; see there.