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Continuing on the topic I started Monday where I discussed AIC rating and the phrase “short circuit current”, let’s dig in a bit deeper into the higher voltage DC issue / concerns I mentioned, since we are stating to see higher voltage DC systems more and more on boats, either as part of a hybrid propulsion system or to just simply reduce wire gauge sizes. (Note: use of higher voltage allows the use of smaller wire and cabling to carry the same amperage. In other words, at 12 volts if you need a 4 gauge wire to carry a certain amount of amperage safely, at 24 volts, you could carry the same amperage with a much smaller gauge wire.)

So, regarding the matter of AIC ratings for fuses and circuit breakers, the confusion lies in the fact that the AIC ratings for a given device are tested to specific voltage values, and the actual AIC will vary dramatically based on tested voltages. As an example, the heavy-duty class T fuse shown below has a 20,000 amp AIC rating, but that is at 160 volts DC, not 12 volts DC which is of course more common. In this case the issue is all good news for users. Why? Well because as system voltagedecreases actual AIC ratings increase, quite considerably. At 12 volts, the actual AIC for a class T fuse will be over 100,000 amps!

This fact holds true for all devices but the problem is that the tested voltage values varies. For example a common ANL fuse type is rated at 32 volts DC and has an AIC of 6000 amps. Its rating at 12 volts will be higher, but not five times as in the class T example.

So, with the exception of the very large battery banks mentioned in my Monday post, which can have a huge short circuit current potential, where you may actually need that hidden reserve of the class T fuse at 12 volts, we are probably in pretty good shape with most boats and circuits, simply because 12 volt DC systems have been prevalent for decades.

Now however, we see this movement toward higher voltage DC use on boats and this means that the published AIC ratings for fuses and circuit breakers are going to start becoming more real. A class T fuse on a 48 volt power pack that is created from a Lithium cell with an absolutely off the charts short-circuit current potential may be at risk!

So you ask, well Ed what is the answer to all of this? Ed’s answer at the moment is I’m not sure… The ABYC electrical committee is going to be meeting in two weeks and I’ll be in attendance. This is an issue that myself and some of my colleagues are going to be presenting to the committee. At this juncture, the only safe protection I can envision is a limit to how many batteries or cells can be overcurrent protected by a given protection device, whether fuse or breaker.. Some lithium battery vendors are doing this now, others are not. Should there be an industry standard in place to mandate this? I think yes, but we’ll have to wait and see.

 

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