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I find the rules confusing. How strict were the compliance agents on the type and size of fuses you had? What documents were they looking for? What ratings got a pass mark for you?

The rules speak of overload rating and of protecting the electronics. After giving myself a headache literally trying to work out what the resistance rating of my Chevy volt batteries are I clicked and thought ok what fuse was used in the Chevy volt. Turns out it was a 350amp rated fuse with 20ka break rating.

My car is a modest conversion still in progress. The inverter has a modest max input of 120amps and the cables are budgeted to match at 180amps continuous.

I believe (correct me if im wrong) that I should fuse to protect below the amp rating of the motor to inverter wiring. So I am looking fuses in the range of 150 to 175 amps. Between 20ka (original Chevy rating) and 200ka which I guess is superior? All well and good but I am struggling to work out what the 30 to 40 percent overload rating the rules talk about is? Please help. Any Kiwi who knows.
 

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Hey, just saw your post and will be going through this shortly. Yep, the rules aren't quite as detailed as they could be. I suspect they were written by people who don't have a full understanding of the ins-n-outs of electrical hardware. You'd be best to talk to a/your certifier.

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A fuse isn't going to blow at a point below the wires steady-state rating and above the inverters rating, that isn't really how fuses work. There is a time component as well. Those wires will take 20k amps no worries and your batteries and/or capacitors will supply all those amps if allowed. A fuse will take 20k amps too, but ideally for a few ms less than anything else. This is a bit of a nonsense requirement as it is written, a certifier is allowed to interpret poorly worded rules and if necessary put any proposals to TAC
 

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A fuse isn't going to blow at a point below the wires steady-state rating and above the inverters rating, that isn't really how fuses work. There is a time component as well. Those wires will take 20k amps no worries and your batteries and/or capacitors will supply all those amps if allowed. A fuse will take 20k amps too, but ideally for a few ms less than anything else. This is a bit of a nonsense requirement as it is written, a certifier is allowed to interpret poorly worded rules and if necessary put any proposals to TAC
The quoted section says nothing about the inverter, and makes perfect sense. The point is for the fuse to blow in the case of a short circuit, not inverter operation.
 

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Sure, but how do you/they define "current carrying capacity"? It is just a magic number based on some allowable temperature rise when talking about steady-state capacity. Bundle wires together, rating changes. The inverter may do a lot more than the steady-state rating for short periods too. It isn't clear at all, especially when there are many different fuses with the same basic rating and completely different characteristics. Have played this game... A fuse that is too small is just as dangerous as one that is too big.

The intention of the rule is there, just not the technical detail level required to meet it. A common scenario here.
 

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Sure, but how do you/they define "current carrying capacity"? It is just a magic number based on some allowable temperature rise when talking about steady-state capacity. Bundle wires together, rating changes. The inverter may do a lot more than the steady-state rating for short periods too. It isn't clear at all, especially when there are many different fuses with the same basic rating and completely different characteristics. Have played this game... A fuse that is too small is just as dangerous as one that is too big.

The intention of the rule is there, just not the technical detail level required to meet it. A common scenario here.
Yes, the current carrying capacity rating is not trivial, but it's also not magic. The technical detail is in the ratings of components; it would make no sense for a regulator to attempt to reproduce that detail in a rule.

Again, the inverter doesn't matter to this overcurrent protection requirement. You are responsible for determining the current to be used by the inverter and choosing components (wiring, contactors, switches, etc.) adequate for that purpose; the overcurrent protection then follows from your component selections and their installation conditions.
 
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