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.
If you want to get parametric about it (an approach that makes more sense to me than these arbitrary fast/slow ratings), your typical fuse has two parameters of importance: The current that will raise it to its melting temperature (Imax) and the energy required to melt it (usually expressed in current and time, I²t). Using the second requires evaluating an integral, but it can be approximated pretty easily.
One of the most common errors I see in circuit protection is when wires are sized according to the load, instead of the protection. You see it all the time, you get a lamp fed by these little tiny wires. If you ask about it, the answer always is "the lamp will never draw that much." True, but if you short the lamp, the wire explodes. Wires should be sized according to the available current, not the expected current. If you want to use smaller wires, the available current should be limited first.
Would it be OK to use a BUSSMANN fuse instead of the Littelfuse in your applications?
On the serious side ...... A GREAT short article to explain the importance of selecting the proper circuit protection device rating. It SHOULD be required archiving for EVERY engineer/designer who has any involvement with circuit protection.
On that note, here's an interesting story from my annals.....
We had built an extensive temperature-control system for a large home appliance manufacturer. They had recently converted a critical component of an appliance from a metal unit to an injection-molded piece. The plant supply was basically 480/ 3-phase Delta service. The heaters are rated at 240. Our equipment was designed for the typical 240 service, BUT we also had available an accessory kit for those installations where the mains were 440-480.
When the unit arrived, it was connected by the plant electricians, and set to go. However, it was NOT energized initially because other equipment was not yet installed. When the day finally arrived to power up, the primary side fuses blew immediately (with MUCH gusto, I was told!) When the plant electrician did some preliminary investigation, he discovered that SOMEONE (in our plant) had supplied a set of SUPER FAST-BLOW, CLASS H style fuses in the main disconnect, instead of the RK5 fuses. Obviously, THIS error was the problem. The electrician called our company to inquire, and we apologized for the error, and sent a couple of sets of correct fuses as a token of our appreciation. Once that error was corrected, the temperature controller worked as advertised for years WITHOUT further problems.
Thanks for the article. Engineers often overlook circuit protection and because it's not as straightforward as it appears. To make matters even more complicated, as you pointed out, there are many products available with similar ratings but very different performance. Fuse selection is also dependent on tests that may be performed for agency approval. I once had to increase the size of a fuse a very small appliance because the MOV in the power supply was pulling a great deal of current during Fast Transient Burst testing. The appliance only needed fractional current, but in order to pass FTB I had to increase the fuse size to an Amp or more.
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