But we’re still confused. Why did the sales representative tell us we need a KLDR? Why not a KLKR, which is the same size and shape, with similar words? What about a CCMR? How is that different? Better call the sales rep back.
He tells us that KLDR is a time delay fuse especially for dealing with the inrush of transformers. He says that a KLKR is a fast-acting fuse that has no time delay to limit current, and a CCMR is a dual element fuse, with time delay AND current limiting characteristics. The sales rep points us to time/current curve charts for the three types of fuses: KLDR time-delay fuse time/current chart, CCMR dual element fuse time/current chart, and KLKR fast-acting fuse time/current chart).
We learned inrush happens in the first few cycles, so let’s pick three cycles and see what the charts tell us (three cycles: three divided by 60 cycles per second gives .05 seconds):
5 Amp Fuse, .05 seconds time, the fuse types melt at:
KTDR – 48A
KTKR – 26A
CCMR – 48A
The KTDR 5A fuse can withstand 48A for .05 seconds, more than the 30A (10 times) the inrush draw of the transformer. We’ve learned a lot today.
We’ve got enough information now to pick the right fuse to deal with current inrush and still be within the codes that dictate electrical safety. The non-time delay fuse (KTKR) can’t handle the inrush, even at 167 percent of the steady state current.
What if we didn’t know about inrush? What if, as a new engineer, we had sized the fuse for the steady-state load?
On this chart, a 3A fuse will melt at 30A in .05 seconds. That’s matches the 10x value we’ve been using as a guideline, but do we REALLY want to be right on the edge? Better go back and check the inrush of our 1500VA transformer. The chart at the top of the page shows this transformer could have as much as 15724VA inrush. 15724VA divided by 480V gives 33A inrush current, higher than a 3A fuse can handle.
Circuit protection is not a simple matter. It takes time for a new engineer to learn the nuances of properly sizing a fuse for even a simple application. As simple as they look, fuses are very sophisticated devices.