Circuit Protection Beyond the Numbers

In my previous blog, we examined the numbers involved in sizing the overcurrent protection for a control transformer. With help from our Littelfuse manufactuter's rep, we decided that a 5A KTDR fuse from Littelfuse would be satisfactory to protect a 1,500VA transformer from Allen Bradley.

We checked the time/current curves for the 5A fuse and determined that it would handle the inrush current of the transformer. This matched the transformer manufacturer's recommended fuse size of 5A, so we felt confident of the choice.

The manufacturer's representative knew a time-delay fuse, not a fast-acting one, should be used in the transformer application. And yet they have exactly the same size and shape and very similar part numbers: KTDR for the time-delay fuse and KTKR for the fast-acting one. Littelfuse also has a fuse in that particular family with a dual element -- CCMR.

Littelfuse is not alone in offering similar-looking part numbers in the class CC fuse size:

In doing the numbers last time, we discovered that a Littelfuse KTKR 5A fast-acting fuse would melt at 26A in 0.05 seconds, and a KTDR 5A time-delay fuse would melt at 48A in the same amount of time. We picked a time-delay fuse for our transformer application in order to ride through the inrush; had we selected a fast-acting fuse, we would have had nuisance trips in our hypothetical machine.

What if our circuit protection needed a fast-acting fuse? What if the circuit did not have a large inrush, where a fast-acting fuse makes sense? We'd go through the same exercise for sizing to get the proper fuse. But what happens if (when) that fast-acting fuse blows? It needs to be replaced by the same type (KTKR high speed).

Can you trust your customer to replace the fuse? If the machine is critical to its process, you can hope the customer has proper spares, or that a local supplier has them in stock. But you know what happens when the proper replacement isn't available, don't you? A customer will use anything handy if the equipment is down and needs to be running.