Fuse/Current Limiter and Circuit Breaker Coordination - Page 1
Forum section: electronics / projects

author: MarginallyStable
author_rank: Regular Contributor
timestamp: April 01, 2025, 03:53:08 pm
content: I working on some early design planning for a battery power system (earthx ETX900).I would like to place a fuse/current limiter (something like an ANL) at the battery for the main lead that will run to a klixon 3tc2 30 amp circuit breaker. I am having some issue getting the coordination figured out. The klixon datasheet only give trip times up to 1000% overload (300amps), and I have to assume that at some overload point the chemical reaction of any fuse will be faster than the mechanical action of the circuit breaker. Am I correct in my thoughts that the only way to guarantee coordination is to somehow limit the available fault current to < 300amps? Another option would be a fusable link, would this be a better choice?Also, I'm not sure what the available fault current would be. The battery data sheet gives a "pulse cranking amps" rating of 840 amps, but also states that the battery has short circuit protection > 1000amps. So I'm guessing 1000amp would be the design value.In the end, I assume that with all the gaps of information in the data sheet that the only way to truly know will be some lab testing with a replication of the intended design. Was just hoping to get the thoughts/insights from the more knowledgeable here on the forums.Thanks

author: jwet
author_rank: Frequent Contributor
timestamp: April 01, 2025, 05:28:53 pm
content: I used to design Military gear mostly for Subs. Fuses, breakers, Ground Fault Detection was some of the most complicated stuff I dealt with and our equipment was pretty sophisticated in itself. Fuses, interrupters and contactors come in an infinite number of varieties and timing curves. You'll likely end up with a fusible link or ATL (these are nearly the same in my mind - they are both quite slow for absolutely gross faults). You'll then have a magnetic breaker and finally a thermal breaker. Your branch circuits can have solutions tailored to their needs.You have to decide what you're trying to protect. In most cases, you're trying to keep wire insulation from reaching combustion or outgassing levels. You want the system to take short surges of 10x, etc for things like motor start without nuisance tripping. Motor contactors are there own thing and are worth looking at if you have motors or compressors. With a battery source, you have an additional peak load issue on the battery. Some Li chemistries can get into a thermal runaway loop after a heavy short that will eventually end in a bad fire. (Boeing 787 Fires of a decade ago)Lay out the problem and then come up with a solution. It will generally involve at least a gross link like an ATL and then one or more Breakers. There are smart circuit breakers these days that are very programmable and smart. You might look at Aircraft or Marine solutions- companies like Eaton, ETA, Phoenix, etc. Don't forget to protect the negative side of the battery as well, DC circuits are unique in that you can have faults that can short this way.There are a bunch of ISO standards that have been developed over the years for Road use. I don't recall numbers- google them or go to Global.comIts a surprisingly complicated topic- good luck.