This fuse is thermally acting and is connected in close contact to the primary winding of transformer. This fuse will only blow, when the temperature here extremely rises. So, this kind of fuse is ideal, because only longer lasting and extreme overheating overloadings will blow this fuse. Once blown, though, it's gone forever.

You should analyze, why this fuse is blowing at all! Ususally this happens, when a transformer is too weak for an application, causing him to become abnormally hot. Then, just use a somewhat stronger transformer.

If, on the other hand, short circuits occur, eventually in the cable from transformer to your board, than just add a fuse at the output of supply, which limits the current to a considerably lower level than what the transformer is able to deliver!

An example: Assume the polyfuse RXEF030 is used, which provides a specified hold current of 300mA and a specified trip current of 600mA, then it will typically last 1000sec to make the fuse trip at a current of 500mA! Or, it will need 10sec of a current of 850mA to make it trip. So, your transformer must be such strong, that it can deliver this trip current/times, without becoming so hot that its thermal fuse is blowing!!

So, your mistake is just to use a too weak transformer for your subsequent polyfuse, or vice versa, to use a too strong polyfuse for your transformer. Your transformer/polyfuse pair does not match.

The other side of this analysis is: If a so much stronger transformer is needed to make this darn polyfuse thing work, should you really add such a polyfuse, or better think "let the thermal fuse do its job"? Or by other words, adding a polyfuse can turn out to be over engineering...

Kai