400 A Fuse
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A Bussmann FWH-400A high speed fuse, rated 400 A at 500 V AC and 500 V DC. This is the fuse that goes in the main high voltage path of a battery pack, sized to clear a short circuit before the cells, the cables or the contactor are damaged.
Two things about this fuse decide whether it is the right one for your pack, and neither is obvious from the rating printed on the body.
It is a 500 V part, not a 750 V part. The voltage rating of a DC fuse is not a guideline. Below its rating the fuse breaks the arc and the circuit opens. Above it, the arc can keep burning across the gap and the fuse never clears. At a maximum of 4.2 V per cell, 500 V allows roughly 119 cells in series, so this fuse suits packs up to around 400 V nominal, which covers most Tesla module builds. A 144 cell pack sits above 600 V fully charged and needs a higher rated fuse than this one.
It is an aR fuse, which is semiconductor protection. aR is a partial range class. It is built to clear a hard short circuit extremely quickly, and it is not designed to clear a slow, modest overload. A pack being worked too hard will not necessarily blow it. Overload and over temperature protection has to come from the BMS and the contactor. Treat this fuse as the last line against a dead short, not as the thing that protects the pack from abuse.
Key specifications
| Manufacturer part | Bussmann FWH-400A |
|---|---|
| Rated current | 400 A RMS |
| Rated voltage | 500 V AC and 500 V DC |
| Class | aR, high speed semiconductor protection, partial range |
| Interrupting rating, AC | 200 kA RMS symmetrical at 500 V |
| Interrupting rating, DC | 50 kA at 500 V DC |
| Pre-arc I²t | 19,200 A²s |
| Total clearing I²t at 500 V | 96,700 A²s |
| Power loss at rated current | 65 W |
| Overall length | 110.2 mm, 4.340 in |
| Body diameter | 38.1 mm, 1.500 in |
| Blade width | 25.4 mm, 1.000 in |
| Blade thickness | 6.35 mm, 0.250 in |
| Mounting slots | 10.4 mm wide x 19.1 mm long, suits M10 |
| Approvals | UL Recognized, CSA Component Acceptance |
| Weight | 0.24 kg |
| Documentation | Bussmann datasheet and a STEP model in the Documents tab |
Features
- Genuine Bussmann part, marked 400A 500V ac/dc and carrying the UL and CSA marks
- Bolt on blades with slotted holes, so the mounting centres have some adjustment rather than being fixed
- 200 kA AC interrupting rating, which covers the fault current a large pack can deliver
- Published I²t figures, so the fuse can be coordinated properly against cable and semiconductor withstand
- Full manufacturer datasheet and a STEP model are published here, so the pack can be laid out and the fuse checked before it is ordered
Compatibility
Suits high voltage battery packs and drive systems up to 500 V DC. Check your fully charged pack voltage against that figure, not your nominal voltage.
Budget for the heat. At its full 400 A the fuse dissipates 65 W, which is not trivial inside a sealed enclosure, and the loss falls off steeply at lower current. If the pack normally runs well below the rating, the real dissipation is much lower, but the fuse still needs somewhere for that heat to go.
If 400 A is more than the build needs, the same series is available as a 200 A fuse, which is physically smaller and loses less power.
In the box
- 1 x fuse
Mounting hardware, busbars and the fuse holder are not included.
Downloads
The Bussmann datasheet in the Documents tab above carries the full dimensioned drawing, the arc voltage and power loss curves, and the I²t correction factors for working voltages below 500 V. A STEP model is published alongside it.
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