970. A Doubt Regarding an Eiruv

Eiruvin 6:13

Let’s say that two people tell a messenger to go and make eiruvei techumin for them and that he placed one eiruv before Shabbos began and the other during twilight. The eiruv that was placed before Shabbos was eaten during twilight, while the one that was placed during twilight was eaten after nightfall. In such a case, both eiruvs are valid because the status of twilight (i.e., whether it is day or night) is in of doubt and, when there is a doubt about the validity of an eiruv, we rule leniently. In any event, if there is a question as to whether or not it is after nightfall, one should not make an eiruv. If he did place an eiruv at this time, it is valid.

Eiruvin 6:14

Even if an avalanche falls on an eiruv before Shabbos begins, it remains effective so long as it can be removed without performing a form of labor prohibited on Shabbos because it is permitted to remove it during twilight, which is when an eiruv takes effect. If an avalanche fell on an eiruv after Shabbos began, it remains valid, even if it cannot be removed without performing a Shabbos-prohibited form of labor. If there is a doubt as to whether the avalanche fell before or after Shabbos began, it is valid. Again, this is because, when there is a doubt as to the validity of an eiruv, we rule leniently.