Melting Fuse Blocks

Generally, if a fuse HOLDER melts but the fuse is not clearing, that is the result of a really bad fuse clip connection to the fuse ferrules, in other words your clips or the springs in them are too weak to maintain good contact with the fuse itself. That sets up a high resistance in that connection and even though the current to the load (through the fuse) is low, the heat created in the clip gets transferred to the fuse base and it melts. If the 20A fuses are not clearing, no way the 60A breaker is going to see it!

Are you sure the fuse holders are designed for Class J fuses? Just because you can jam them in there doesn't mean you should! What is the part number of the disconnect? If someone replaced the fuse holder once before, was it with the right one for that fuse or a replacement of the wrong one?

Sorry for all the fuss - this is solved. I trusted my plant electrician a bit too much and fired off this question before I surveyed the site myself. I checked the machine and the builders tag clearly states 46A draw at 480VAC. The elctrician was using the info in a general manual for the line of welders and not reading the machine tag.

What is happening is we are seeing up to 46A across the fuse very sporadically. Actually, we are probably only tapping the low 30A range occasionally, due to the nature of manual tig welding, with much down time in between. The slow blow feature is allowing it heat the fuse up enough to do in the plastic view port, but not take out the fuse immediately.

Yes the clip is sized corrctly - I specified this myself. My first thought was poor connection to the clip, but the hole out the back of the fuse added to the above stated facts convinced me otherwise.

Yeah, probably over time the welders learned how they could "tease" the system just enough to keep operating without blowing the fuses. Still, I would think the fuses design should be such that you can't do that.