917. The Eiruv's Accessibility During Twilight

Eiruvin 1:21

Neither an eiruv nor a shittuf may be made on Shabbos. Rather, they must be completed before Shabbos begins, though one may make an eiruv or a shittuf during the twilight period (bein hashmashos) even though there is a doubt as to whether that time is part of the day or part of the night. The eiruv and the shittuf must remain accessible so that it may be eaten throughout the twilight period. Therefore, if a ruin collapsed on the food, or if it was lost or burned, or if it was terumah and if became impure – if any of these things happened before nightfall, it is not considered an eiruv. If the mishap occurred after nightfall, the eiruv remains valid. If there is a doubt as to when the mishap occurred, the eiruv remains valid because whenever there is a doubt as to the validity of an eiruv, we rule leniently.

Eiruvin 1:22

If an eiruv or a shittuf was placed in a tower, which was then locked and the key became lost before nightfall, then if it is impossible to remove the eiruv during twilight without performing a proscribed act of labor, it is considered lost and invalid because it cannot be eaten. If one separated terumas maaser or terumah from produce and stipulated that sanctity not apply to these portions until nightfall, they may not be used for an eiruv because during the twilight period the food is still untithed (tevel, which may not be eaten).