These small barrier strips are not of the highest quality. The screws are ok but the plastic used for the body is a thermoplastic. This means that if they heat up a little, they can melt. The good ones I know of are made out of a thermosetting plastic like Bakelite, which is relatively impervious to heat. However, the small tab on the transparent lids is a nice touch. In general they are fine for most applications, just try not to run too much current through them.