Johan
Off-grid energy systems enthusiast.
Problem
It's very annoying when yet another expensive and hard to source class T fuse blows. So how can we visualize why they blow?
The graph below may explain why Class T fuses blow due to the inrush current from a (for example) LFP battery to an inverter. It may also explain why taking a fuse with the next higher current rating (e.g. 60A instead of 30A in the example below) does not always work. So the protective Class T fuse may need protection itself by a precharge circuit that limits the inrush current. Or mabe replace that fuse by a DC-rated breaker (with high enough AIR) that "ignores" the inrush current? Thoughts?
Notes
It's very annoying when yet another expensive and hard to source class T fuse blows. So how can we visualize why they blow?
The graph below may explain why Class T fuses blow due to the inrush current from a (for example) LFP battery to an inverter. It may also explain why taking a fuse with the next higher current rating (e.g. 60A instead of 30A in the example below) does not always work. So the protective Class T fuse may need protection itself by a precharge circuit that limits the inrush current. Or mabe replace that fuse by a DC-rated breaker (with high enough AIR) that "ignores" the inrush current? Thoughts?
Notes
- The Littelfuse Class T fuse graphs are way more tricky to extrapolate, that's why I am using Eaton/Bussmann Class T fuses here
- Inductance is neglected, so the model may be a bit pessimistic.
- Example inverter capacitance (C) values: Victron Multiplus-II 48/3000 120V: 4.42mF; Victron Quattro 48/5000 120V: 39mF.
- Bussmann Class T fuse time current curve: https://www.eaton.com/us/en-us/catalog/fuses-and-fuse-holders/bussmann-series-class-t-fuses.html
- RC network model: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html
- Slope of log-log plot: https://en.m.wikipedia.org/wiki/Log–log_plot
- Fuse theory: See Preece, Onderdonk, etc: https://en.wikipedia.org/wiki/American_wire_gauge
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