Siman 394
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א סְפֵק עֵרוּב, כְּגוֹן סָפֵק אִם הָיָה קַיָּם בֵּין הַשְּׁמָשׁוֹת אִם לָאו, מֻתָּר; וְהוּא שֶׁהָיָה לוֹ חֶזְקַת כַּשְׁרוּת, כְּגוֹן שֶׁהִנִּיחוֹ שָׁם וְאֵרַע בּוֹ סָפֵק, אֲבָל אִם לֹא הָיָה לוֹ חֶזְקַת כַּשְׁרוּת, כְּגוֹן סָפֵק אִם הֻנַּח שָׁם אִם לָאו, לֹא.
1 *Law regarding a doubtful erev. Containing 3 paragraphs.* A case of doubt regarding an eruv, for example, if there is doubt whether it existed during twilight or not, is permitted, provided it had a presumption of validity. This means it was placed there and then doubt arose about it. However, if there was no presumption of validity, for example, if there is doubt whether it was placed there at all, it is not permitted.
ב צָרִיךְ שֶׁיְּהֵא הָעֵרוּב בֵּין הַשְּׁמָשׁוֹת בְּמָקוֹם שֶׁרָאוּי לִטְּלוֹ, הִלְכָּךְ אִם נָפַל עָלָיו גַּל וְאֵינוֹ יָכוֹל לִטְּלוֹ בְּלֹא מָרָא וַחֲצִינָא, אָסוּר. הַגָּה: וְהָעֵרוּב אֵינוֹ צָרִיךְ לִהְיוֹת קַיָּם רַק בֵּין הַשְּׁמָשׁוֹת. (טוּר). וְיוּכַל לְאָכְלוֹ כְּשֶׁוַּדַּאי חֲשֵׁכָה (הַגָּהוֹת אֲשֵׁרִי וּמָרְדְּכַי פֶּרֶק בַּכֹּל מְעָרְבִין). וְיֵשׁ לִבְצֹעַ עָלָיו בְּשַׁחֲרִית בְּשַׁבָּת (מִנְהָגִים). וְדַוְקָא בְּמָקוֹם שֶׁנּוֹהֲגִים לְעָרֵב כָּל עֶרֶב שַׁבָּת, אֲבָל עָדִיף טְפֵי לְעָרֵב עַל כָּל הַשָּׁנָה בְּפַעַם אַחַת כְּדִלְעֵיל סי' שס"ח (בֵּית יוֹסֵף).
2 The eruv must be in a place where it is possible to retrieve it during twilight. Therefore, if debris fell upon it and one cannot retrieve it without great effort and tools, it is forbidden. Rema: The eruv need only remain intact during twilight, and may be eaten once it is certainly dark. One should recite a blessing over it on Shabbat morning, but specifically in a place where the practice is to make an eruv every Friday. However, it is even better to make an eruv for the entire year at once, as discussed above in section 368.
ג נְתָנוֹ בְּמִגְדָּל וְנָעַל בְּפָנָיו וְאָבַד הַמַּפְתֵּחַ קֹדֶם שֶׁחָשְׁכָה, אִם אִי אֶפְשָׁר לְהוֹצִיא הָעֵרוּב אֶלָּא אִם כֵּן עָשָׂה מְלָאכָה גְּמוּרָה בֵּין הַשְּׁמָשׁוֹת, הֲרֵי זֶה כְּמִי שֶׁאָבַד וְאֵינוֹ עֵרוּב שֶׁהֲרֵי אִי אֶפְשָׁר לְאָכְלוֹ.
3 If one placed the eruv in a tower and locked it, and the key was lost before nightfall, if it is impossible to remove the eruv unless one performs a complete forbidden labor during twilight, this is equivalent to the eruv being lost, and it is not valid, since it is impossible to eat it.