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.
TJ, while there are always potential problems on the maintenance end where there can be a substitution, I don't believe that this is the manufacturer's fault. Back when we had fuses at home (sometime in the third quarter of the last century of the last millenium) we made sure we had a full set of the correct fuses on hand. We also made sure we put the correct fuse in as a replacement. Now, my father worked in an place where precision was paramount, so it was in his nature that we did so.
Now, the situation you describe is probably all too common these days. I think it becomes a management issue to ensure that you are properly equipped with spares.
I agree with all that you say, Naperlou, but I also believe that short-sighted management decisions lead to litigation for assigning blame when something goes wrong.
I've watched the quality of maintenance departments decline over the years - pick your reason why: lower pay offered, staff cuts leading to overwork, running equipment longer without commesurate maintenance. All reasons point to maximizing profit.
When a manufacturer names three similar but functionally very different items with very similar part numbers, ANYONE can make a mistake.
ATQR
ATDR
ATMR
The varying character should either be first, or last, plain and simple.
I've seen a number of maintenance departments run on a shoe-string budget (penny-wise, pound foolish) and am amazed at what the technicians can do with almost nothing. That environment is ripe for misusing a same-sized but differently rated fuse.
One alternative is to use a resettable polymer PTC (PolySwitch Device) instead of a fuse. That way, after the fault condition is cleared and the power is removed, it will reset itself to the low resistance condition, and the sytsem will work again. This eliminates the possibility of someone useing the wrong fuse. I should point out that these devices are not designed for large power systems, but in lower votlage applicatoins and circuit board level applications, they are very useful.
I perfer ciruit breakers in the applications I'm involved. And the same or similar information applies.
Some of my motorcycle ciruits were over loaded. The solder holding the fuse element melted but the element and fuse was still "good".
We had a coffee vender that had a problem with a 50 amp CB kicking out. Well it was very hot inside the coffe shack and the thermal CB was only working as design. You have to get the heat out of the environment not replace the CB.
We were challenged for 60 Hz ciruit protection for a flying test bed that used much GPTE intended for ground use. The only manufacturer Cultler - Hammer with some slight derating. Typical 400 Hz CBs do not open wide enought to quelch the 60 Hz arc.
bbrents, polyfuses are in general not rated above 60V. Class CC fuses are rated for 600V and are branch circuit rated. That was the focus of this article.
I agree wholeheartedly with your statement, "That environment is ripe for misusing a same-sized but differently rated fuse." Maintenance personnel are also not necessarily conversant with subtle nuances like time-delay versus fast blowing fuses...I had a friend that used to work on control boxes for door openers and I would shudder to see what he would pull out of a fuse holder in place of the correct fuse - tin foil, screws and pennies were popular choices.
The nature of electronics demands that folks may need to substitute parts but to do so without proper cross-referencing them is to me, very shabby work. Unforeseen problems may or may not develop with parts that are not OEM or are comparable but specs aren't exactly the same and these problems are hard to predict.
While I see your concern and the reality is, people will often make assumptions without further verification, I think having the responsibility of replacing a part should require the person doing it to make sure the part is adequate for the task. So if we are speaking about the ideal - the fuses would always be verified. But companies have to deal with reality. A smart manager would make sure there is a ready supply of the correct fuses available!
At some point the owners of equipment must assume responsibility for not behaving in a stupid manner! This is particularly applicable to fuses, and the replacement of fuses. When we mark a fuse holder with a particular fuse type and rating that is what should be used as a replacement. It is the users responsibility to install the correct fuse type and size. An individual who does not understand that is fundaamentally unqualified to do the job. It is that simple. Unfortunately the courts are choosing to reward stupidity at the expense of the rest of us.
William K, I'm sure you're happy to hear that you'd make a bad lawyer. You're thinking too sensibly.
I agree with you, and I bet most readers do too. That doesn't change the fact that improper fuse use happens, doesn't change the fact that product misuse happens all the time. As the link I gave stated, a screwdriver used to open a paint can is a misuse of the tool which should be expected.
The woman who initially won her civil suit against McDonalds for their too-hot coffee should have been sensible enough to expect a burn with hot liquid near her lap. A jury disagreed and gave her a substantial award for her utter stupidity.
Luckily, an appeals judge had more sense and drastically slashed the award.
What you said is what SHOULD happen, but is not what DOES happen.
Just once, I'd like to see a news story about a judge who told the plaintiff "Are you a complete idiot? Are you unable to pour water from a boot even when the instructions are printed on the heel? Case dismissed for utter lack of common sense".
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