Many people have contacted me regarding my column “Trace Currents and Temperatures: How Hard Can We Drive ‘Em?” in the May, 1998, issue. In that column I talked about the possibilities and the problems associated with developing a set of equations for the current/temperature curves we all have occasion to reference. But many of you posed a different question. It typically went something like this: “I need the trace to carry 5 Amps for only about .5 seconds before…“ something catastrophic happens. “Then I don’t care if the trace melts or not. What size trace do I need?” My first reaction was that this is a fuse question, something I’ve never seen discussed. When enough of you asked me about it I began digging for information. What I found was there isn’t much to be found! But thanks to some direction from a few experts in the field (Note 1) I discovered that there is some very interesting theory to draw from. CAUTION: The information that follows is based on theory and, to my knowledge, has never been tested on printed circuit boards. Designs based on this theoretical discussion should be significantly derated and/or tested before being committed to production.