There are many pitfalls in designing overcurrent protection into a system, but why is it so confusing? Where does a design engineer learn that oversizing is necessary?
Maybe we can answer some of these questions by example.
Let’s assume the role of a newly hired, fresh-from-university electrical engineer. Our mentor wants us to determine the overcurrent protection for a control transformer in an electrical control panel for an industrial machine. Our machine has some signal lights, horns, rotating beacons, and a courtesy outlet that all need 120V AC. The machine will be supplied with 480V AC, so we’ll need a control transformer to make 120V AC. For our discussion, let’s say we need 10A of 120V AC.
If you take a quick look at this guide, 10A times 120V gives us 1,200VA. It gives us choices of 1,000VA or 1,500VA. 1,000VA is too small, so 1,500VA it is (which gives us some margin of error).
We’re building an industrial control panel and our mentor gave us a rule book to follow because municipal codes weren’t taught in university. The book we’re following is UL-508A, Underwriters Laboratories Inc. Standard for Safety Industrial Control Panels. To keep this example simple, we’re using the primary-only protection rules on page 84 of UL-508A, dated February, 2010. What current is the primary side of the transformer going to see? 1,500VA divided by 480V gives 3.125A. There’s a fuse-sizing chart at the bottom of the catalogue page for primary-only protection. The chart says 5A is the maximum. Why is there a difference?
Asking the mentor gets us a gruff “go find out about inrush.” Our mentor also reminds us there is a deadline to the project.
Inrush? Oh, at the top of the catalog page. Inrush. 3-10 TIMES steady-state current. That explains it. OK, so we need a 30A fuse. Then our eyes catch sight of the UL-508A book again. Better check that. Page 84, Primary-only protection. Our transformer draws a little over 3A, which from the chart shown means the maximum overcurrent protection can be 167 percent of the primary current. 3.125 times 1.67 gives 5.22A. How is that going to protect from inrush?
We’re a studious engineer; we’ve been reading Design News since being hired because we saw copies of it on the desks of all the senior engineers. We remember several articles about Littelfuse and their willingness to help with selection. A couple of telephone calls later, and we now have a 5A KLDR fuse from Littelfuse to protect our transformer.
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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