> The one that went bad is the pullstring.

That works for short runs, but for long runs, the only way to pull the cable without breaking it is to use wire pulling lubricant.

And there's no guarantee that the cable you are pulling is undamaged. Rats don't improve pull strength, and even an electrically good cable can have the tensile strength cord severed.

If you are in that situation, pulling the old cable with dried lube on it may snap it in two, especially if it's pulling the new strand.

That's why leaving a pulling tape in each conduit is a good idea.