Electrical Overload, Part 3

Yesterday in part 2 of this series on electrical overload issues you could see that fuses that are continually stressed to near or even above their nominal trip specification will actually begin to generate a significant amount of heat. In the video be...

20th August 2010.
By Ed Sherman

Yesterday in part 2 of this series on electrical overload issues you could see that fuses that are continually stressed to near or even above their nominal trip specification will actually begin to generate a significant amount of heat. In the video below, you can see just how much heat we were able to generate in our test set-up over the course of just 10 minutes at a high load. It was enough to surpass the thermal rating for the wire we were using….

The issue here is excessive heat being generated. Keep in mind that the fuse will not blow under these circumstances, but enough heat can be generated to cause damage to the fuse holder. so, several things come in to play here.

First, use of high quality fuse holders is a must. They are not all made of the same material, so some are definately better than others, and unfortunately you really can’t tell much just by looking at them. I’m obviously a bit biased, but do know for sure that my friends at Blue Sea check for these sorts of issues. I’m reasonably certain that the folks at BEP and others probably do as well, but understand that testing may actually have to go beyond some of the traditional tests for thermal rise as prescribed by UL and SAE, to name several engineering Standards entities that help to define test criteria for such things.

OK, so fuses and their holders can get really hot, maybe hot enough to actually melt a fuse holder if undersized wiring is used or the fuse is being run near it’s maximum capacity on a continuous basis.

Next, let’s talk about the issue I alluded to in part 1 of this series.

Today, we as boaters are being offered some really high tech batteries. Companies like Mastervolt, Odyssey and others are really pushing the envelope with battery technology. This is of course a good thing, but opens the door to some new potential problems that frankly haven’t been seriously considered by most people in the industry. As I mentioned in part 1 of this series the ABYC has historically based its AIC rating (read part 1) on the CCA rating for the battery or batteries connected to the circuit. The question is, is that still a valid approach today? Is that good enough? Well both Wayne Kelsoe and I know for sure that the available short-circuit current at a battery’s posts is much higher than the CCA rating for the battery, but we really weren’t sure by how much exactly. So in the video below, we subjected a Maxi-fuse to some super abuse and unleashed four golf-cart sized batteries on it to see what would happen. While we were doing that, we were monitoring the current flow. Of course we wanted to see what would happen to the fuse and its holder as well. Keep in mind that the fuse has a 1000 Amp AIC rating, so if exposed to more than that, all bets are off as to what’s going to happen.  

In the video below, our measured peak amperage ended up being approcimately 10,000 amps. Let’s see what happens:


 

Well, in this case the fuse did its job. It effectively opened the circuit shutting off power and saving the day. But, not without a little damage to the fuse holder as shown in the photos below. Personally I would not re-use this fuse holder. Others might, but I personally don’t think its worth the risk, as some of the plastic has clearly been carbonized during this event. Carbon in a moist environment is actually a good conductor of electricity. I don’t want a fuse holder on my boat that could actually contribute to a low level current leak (which will only get worse over time).

So, at this point, all I can say is stay tuned. We are looking at some new fuse types that may be quite viable to deal with this extremely high short circuit current issue and we are considering asking the BCI (Battery Council International) to expand its labeling requirements so that end users can have better information. We still have a way to go here, but if anything at this point I’m sure that some of the earlier assumptions that “there really is no problem” are wrong. Stay tuned.

 


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Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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