968. An Inaccessible Eiruv

Eiruvin 6:9

If a person intends to spend Shabbos in a private or a public domain, and he puts his eiruv in a carmelis (the quasi-public domain), or he intends to spend Shabbos in a carmelis and he puts his eiruv in a private or a public domain, then the eiruv is valid. This is because during the twilight hour, when the eiruv takes effect, it is permitted to transfer between a public or a private domain and a carmelis for purposes of a mitzvah. Shabbos prohibitions of rabbinic origin were not instituted during twilight in cases of a mitzvah or great need.

Eiruvin 6:10

Let’s say that a person put his eiruv in a cabinet, locked it and subsequently lost the key. If he can access the eiruv without performing any form of labor that is proscribed on Shabbos, then it is still valid. If one placed his eiruv on top of a reed or stalk that grows from the ground, it is not valid out of concern that one may come to break off the plant (which would be prohibited on Shabbos). If the reed or stalk was already detached and then just stuck into the ground, then the eiruv is valid.