One interesting difference between UL and IEC standards for fuses is that American UL standards often state a requirement for a fuse to withstand 100% or 110% rated current for a minimum of 4 hours. This means that the fuse is allowed to fail after 4 hours at its rated current.

In effect this means that when selecting a UL certified fuse you need to bear in mind that you should not continuously load it above around 75% capacity to ensure long term reliability. In contrast European IEC standards tend to require a fuse to withstand 100% rated current at room temperature indefinitely. This is one of those historical important points that is often unknown to engineers as they no longer tend to open up a fuse manufacturer catalog and see important information like this in the front, but instead search directly to fuse data sheets on the web where this 100% load issue is typically not made clear.

We benefit hugely from resources on the web so we decided we should try and give back some of our knowledge and resources to the community by opening up many of our company’s internal notes and libraries through mini sites like this. We hope you find the site helpful.
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