Dual bus fusing

[lmxr]

I'm in the process of designing a dual bus lithium installation (à la Nordkyn). I'm having some difficulties determining proper fusing.

The main issue concerns the interrupt rating. Most fuses (MEGA, ANL) are only designed to interrupt 2000 A, at least that's what's usually in the datasheet. Our LFP battery is 840 Ah, 12 V. I don't expect that it will have any difficulty to supply over 2000 A for several minutes.

This raises a few questions
- ABYC demands Class-T fuses. I understand this is because of the interrupt rating. Class-T fuses (and their fuse holders) appear difficult to procure (especially > 600 A). Can I replace this with other fuses, of course those that have sufficiently high interrupt ratings? Was thinking of NH fuses. While I'm in the EU, I assume there may be some good reasons why ABYC settled on Class-T fuses...

- We want to avoid that the system gets in a state where there is no battery connected (aka with charge bus connected to load bus). In case there is a single battery main fuse, then this will happen when this fuse blows. So, we want other fuses (downstream) to blow first except for the most exceptional cases.

- If we use one high-interrupt rating fuse for the battery main fuse and smaller, non-high-interrupt rating fuses on the two buses, then in case of a short-circuit downstream, the smaller fuse is not guaranteed to interrupt (because current above interrupt rating). Thus the main fuse will blow, which results in the undesired no-battery-connected. Also considering that Class-T fuses appear to be quite fast acting.

- Thus, we should use three high-interrupt rating fuses: one main on the battery, one for the charge bus and one for the load bus? Of course, with the two bus fuses a far lower rating than the main one, such that they blow first.

- To reduce complexity, can we leave the main battery one out on the condition that the bus fuses are installed very close to the battery?





There were a few previous discussions but they don't appear to discuss the dual bus aspects:
https://www.cruisersforum.com/forums...e-28258-2.html
https://www.cruisersforum.com/forums...65069-154.html


Perhaps I'm overthinking the issue, seeing imaginary problems. Looking forward to your thoughts.

[Frankly]

When you try to get your mind around interrupt scenarios with multiple series fuses typically have to take a little leap of faith (there are sometimes actual test data but not often). Also keep in mind that if you are careful about wiring arrangements (+ - separation and double insulation are your friends) then you should never face an actual short circuit or ground fault.

I am not in the lithium battery business but a large bank of golf cart batteries. I just installed a class T in the main bus bar feed and then a bunch of ANLs for the individual high amp loads. Under a really high amperage fault condition probably get that class T but my wiring arrangement pretty much precludes that ever happening. Also remember those really high current numbers are at the battery terminals, once you move downstream circuit resistance will significantly reduce the avaliable fault current.

While the available fault current might exceed the lower rated fuses just don't think it will ever be a problem (-particularly at 12V). Higher probability of being hit by falling space debris.


Frankly