Richard, Peter, Question: Are all 3A fuses created equal?
Answer: No, Just look at the time-current characteristics. It is oversimplistic to say that a 3A fuse (and nobody has said if it is normal or slow-blow) meeting the requirements of UL 198G will always blow before a 3.15A fuse (perhaps Fast, super-Fast, Medium or Time delay?) per IEC 60127. Firstly, like any electrical component, fuses have a range of operating tolerances which the fuse manufacture will disclose to you if asked. How significant the 5% value difference in Rating is compared to the tolerance range of the fuse type in question is for the manufacturer to determine. If you look at IEC 60127 you will see a series of curves that fuses need to conform to, according to their rating. Equipment will 'pull' a range of currents depending on not only the normal load but also to the type of fault simulated and the supply impedance. It is necessary to ensure that the fuse chosen will always blow whatever fault is simulated, and will never blow whatever the normal current is (for good design). To complicate things some more it is also necessary to repeat the exercise for the range of ambient temperatures expected and the expected tolerance range of the fuse. Call this step 1. If you now look at UL 198G you will find corresponding data and repeat step 2. Call this step 2. Murphy's law says that the graphs for an IEC fuse will not simply be the same as for a UL fuse, but slightly offset. If your discussions with the fuse manufacture confirm this then you need to determine that your chosen fuses protect your equipment for all foreseeable faults, i.e. steps 1 & 2. So far I do not recall a discussion of why the fuse has failed in the first place. If this is due to some foreseeable external event (such as shorting the wires to loudspeakers) then it is understandable why the fuses would be user replaceable. However, if the cause has to be some internal fault then simply replacing the fuse will not fix the problem, so why not make the fuse internal so that it becomes a service person fix only? Lastly it was certainly possible in the past to purchase 5x20 mm fuses that were approved by both N. American and European bodies. Why not fit one of these fuses? Regards, Richard Hughes Safety Answers Limited www.safetyanswers.ltd.uk
