As a fuse manufacturer, I have read all the e-mail on this subject with great interest. Unfortunately, many responses emphasize common misunderstandings.
In the fuse industry, there is no such thing as a "Bad" fuse. The chain of e-mails clearly indicate why we avoid this word due to its various meanings. Similarly, we try to avoid the word 'failure'. A "bad" or 'failed' fuse can mean any number of things, but the most common is that the fuse is open, (it operated). Thus, the fuse did what it was supposed to do - operate - and yet it is deemed 'bad' or a 'failure'. There are numerous occasions when a customer returns a 'bad' fuse. The fuse was 'bad' because it operated properly. In many instances, the choice of fuse type or amperage rating was flawed. For being such a 'simple' device, there are many items that influence the correct choice. I will address some of these by picking quotes out of the various e-mail on the subject. Among these are: * Voltage rating - (This was addressed in Mr. Richmond's e-mail - I've heard of a "bad" fuse, in this case, a fuse not adequate to protect the circuit and user. A 30 volt fuse can't be counted on to interrupt a 408 volt circuit. This is absolutely correct. Choosing a voltage rating below your application can be very dangerous. * Continuous Current rating - This was addressed in Mr. Beckwith's e-mail - Fuse operation when there is no fault can also sometimes be caused by incorrect specification at the design stage, i.e. the fuse is rated too low for the actual current consumed. Very true. One must look at surges mentioned below and temperature. * Temporary Surges that you want the fuse to pass - A time delay fuse is typically chosen for motor or transformer applications. This is addressed a little in Mr. Beckwith's e-mail - We have all seen cases where a fuse blows when there is no apparent fault in the equipment, hence the maintainer's comment "bad fuse". In my experience, this is usually caused by a temporary overload condition or a power surge, not a "bad" fuse. * Temporary Surges that you do not want the fuse to pass. * Ambient temperature rating - This was addressed in Mr. Hughes e-mail below. * Interrupting Rating - This was addressed in Mr. Hughes e-mail - A couple of other common design drop-offs I would like to mention include failure to select a fuse with an adequate prospective fault current (i.e. selecting a 5x20 mm glass fuse - rated at Isc=35A max and using it in a mains circuit where the prospective fault is in the order of 1kA) and 'forgetting' to de-rate the fuse for use in a high ambient temperature (although this has already be mentioned re. pico-fuses). * Shock/Vibration - Most fuses do not claim to be suitable for such applications. From Mr. Schlentz - I have seen one instance where a fuse opened during a vibration test. I think that was a BAD FUSE. I don't know if the fuse in question was supposed to be suitable or not. If chosen incorrectly, and 'nuisance' opening occurs, as Mr. Powell mentions - Often the answer is, increase the fuse size to prevent nuisance trips. The risk, of course, is that for every incremental increase in fuse value, you increase the risk of fire proportionally. In doing this, you may be defeating the purpose of having the protection there in the first place. If a fuse nuisance opens, it is likely that one of the above bulleted items was overlooked. Additionally, as Mr. Richmond points out - Many fuses ARE mechanically fragile; it's simple physics that says a low-current fuse will be a fragile thread. Think of them as light bulbs. We do not call a light bulb unreliable if it fails after being dropped on the floor; we call it broken. This is particularly true for low amperage fuses. As Mr. Beckwith points out - AIUI, there is a specified time it takes the fuse to operate at 200% of the rated current, so for example a 1A normal blow fuse requires 2A for approximately 10 seconds to go open circuit. This time reduces exponentially as the current increases. There are different opening time requirements depending on the type of fuse, (Time Delay versus Fast Acting), and its physical size. The 135%, 200% and/or 500% opening times are documented in the tri-national, harmonized 248 Series of fuse Standards, (248.1 through 248.16 - The standards are UL, CSA, and ANCE). If anyone has any specific questions, or would like more information, send me a separate e-mail and I'll see what I can do. Dan Manager, Industry Standards & Certifications Cooper Bussmann
