If one full string of the 8 strings rip off of your Tallis Kattan is it Kosher LChatchila?
If the 8th string is intact for the entire length of the “Gedil” (the lower part of the Tzitzis containing the five sets of knots and the windings in between), then it is Kosher.
Otherwise, it must be replaced.
Source: Siddur.
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