Fuse Buddy Circuit Testers NAPA Professional Diagnostics' Fuse Buddy Circuit Testers were designed to allow for a trouble-free connection into fuse sockets. The ends are molded into the shape of the fuse itself, and during testing the fuse is removed and replaced into the Fuse Buddy. The circuit being tested remains fuse-protected. Fuse Buddy tests circuits up to 20 amps / 48V.