Which SR Blue or black Fuse

The larger fuse is 32 mm, or 1.25".  Get the Blue.  

Just so I understand, these SR fuses work as protective fuses, correct? A SR 4 amp fuse should blow when that current is exceeded correct?  I mean otherwise it's not a designed fuse and it's just a piece of metal.  I'm not so concerned about the UL listing, I just want to make sure they blow when they are supposed to and that there is a performance spec associated with their design.  I ordered a SR blue last week and now you guys have me worried about even using it.

Yes the SR fuses blow as designed! If anything compared to stock fuses they may blow more readily 😀 talk to your supplier about rating or replacement if you have issues with power on surges

Relax, Costco, reversing a fuse never blew anything up. Nobody’s house burned down. Give me a break! You guys are behaving just like Chicken Little. 🐥 The sky is falling! The sky is falling!