- Lee Ould
Blown Away by Picking the Wrong Fuse
Updated: Jul 17, 2021
When selecting a fuse for overcurrent protection in products, it is natural to focus on the operating voltage and the current rating, but one of the often overlooked ratings is the Current Interrupt Capacity or AIC -- also known as the breaking capacity or short circuit rating. Overcurrent devices are designed to clear a certain amount of current from the source, and typically the higher the voltage, the more difficult the clearing becomes. This rating has only been fairly recently added to the UL Online Directories as an openly published item for each fuse type. The European fuse standard EN 60127-2 created "recipes" or "sheets" that fuses must meet, with Sheets 1 and 5 as the high interrupting types with an H, such as 4AH or T4AH on the ferrule, for fast acting and time-lag types respectively. Such fuses must be able to clear 1500A short circuit, which is the max expected to be seen on a standard branch circuit. You will have to look closely at the fuse manufacturers' application notes to see that they recommend only the H versions be used on the AC mains, despite the other sheets (non-H) being rated 250V, as well. These H types typically have a ceramic white powder to absorb the energy of the rupturing element. Did you realize that this much current could be available for a short duration on your 20A branch when shorted? The video below show what happens when a fuse that is rated for a lower interrupt capacity is overwhelmed by a circuit with more short circuit current, and shows the dramatic difference between the same test at 120V, and then as 208V.