Cell, module and pack fuses only do their job if they open in the right order. The aim, usually called selectivity or discrimination, is that a fault confined to one cell trips only that cell’s fuse and leaves the module and pack fuses untouched, so the rest of the pack keeps running.
Getting the order right is a question of energy, not just current rating. Each fuse has a pre-arcing I²t – the thermal energy it absorbs before its element melts and it starts to clear. For a cell-level fuse to reliably clear before a module or pack-level fuse even begins to melt, its total clearing I²t has to sit below the minimum pre-arcing I²t of the fuse tier above it, and that comparison needs to hold across the fault current range the pack can actually see, not just at one test point. As a rule of thumb, published selectivity guidance looks for an I²t ratio of at least 1.5:1 between a downstream fuse and the one above it before treating the pair as reliably selective [1].
In practice, layered fusing on a pack needs:
- A cell fuse whose melting curve sits comfortably inside the pre-arcing curve of the module fuse above it, across the expected fault current range
- A module fuse whose clearing energy at the pack’s maximum fault current stays below the pack fuse’s pre-arcing threshold
- Margin for manufacturing tolerance between fuse batches and for the way ambient and local heating shift each tier’s time-current curve in service
Where a contactor also has to clear above some fault current, the same discrimination logic applies between it and the pack fuse – see the contactor-fuse coordination point above.
References
- Fuse Selective Coordination, Eaton / Cooper Bussmann technical library
- IEC 60269 (Low-voltage fuses) and IEC 60364-5-53 (Selection and erection of electrical equipment – protective devices)
See also: How to select EV Fuses, Contactors: The Basics