Electrical Overload, Are You Protected?

For several years now I and some of my colleagues have been raising questions about how we have traditionally prescribed applications for fuses and circuit breakers used on boats. The reasoning behind our questions is based on observations we’ve made...

17th August 2010.
By Ed Sherman

For several years now I and some of my colleagues have been raising questions about how we have traditionally prescribed applications for fuses and circuit breakers used on boats. The reasoning behind our questions is based on observations we’ve made in real field applications on real boats. To give you a visual to help you follow along here, the photograph below illustrates what can happen to a fuse holder on a boat under certain conditions. Now the uninitiated might ask: How come the fuse didn’t blow? It’s a great question and one that I’ll answer over the next several installments to Ed’s Boat Tips.

That is not the only question however. The next question is whether or not a fuse or circuit breaker will function in the event of a serious short circuit at or near your boat’s battery bank.

 First, you need to understand one of the basic, but extremely obscure ratings that all fuses and circuit breakers have, known as the AIC or ampere interrupting capacity rating.

 Many folks wrongly think that if they install a 10 amp fuse in an electrical circuit the fuse will blow at a circuit load of anything greater than 10 amps. Well, that’s not actually the case. Most fuses and circuit breakers will actually trip out at somewhere between 125-200% of their nominal rating depending on their specific design. The AIC rating really has nothing to do with this fact.

AIC in simple terms describes how much amperage a fuse or circuit breaker can be exposed to without either turning into a solid ball of metal in the case of fuses, or welding the internal contacts together in the case of circuit breakers. This is obviously important because in the event of an electrical short circuit, we want to open the circuit and shut off the power flow ASAP!

For decades, the ABYC electrical standard has recommended AIC ratings for fuses and circuit breakers in DC applications based on a battery or battery bank’s CCA or cold cranking amp rating; the larger the CCA rating the higher the AIC rating, up to a maximum of 5000 amps.  But, some of us in the industry have begun to really question this thinking because we are seeing increasingly larger battery banks in use, particularly on cruising boats both power and sail. Further, we are seeing more and more new battery technology being employed; AGM and lithium technologies are beginning to become mainstream in more sophisticated applications. We need to start thinking in terms of short circuit potential vs. CCA rating. This problem and the question gets further complicated by the fact that the battery vendors often don’t even provide a CCA rating on any labeling on their batteries and you sure can’t tell much by simply looking at the battery. Suffice to say that my colleagues and I know for sure that the short circuit amperage potential at a modern battery far exceeds the CCA rating in any event. The question becomes by how much? Well, it will certainly depend on the specific battery and its age and general condition for sure, but the value will in virtually every case be significantly higher than the battery’s CCA rating.

So during this week I’ll share with you some of the findings in a recent series of experiments myself and Wayne Kelsoe at Blue Sea Systems in Bellingham, Washington conducted in his lab. By the way, if you want to find out the CCA rating of fuses and circuit breakers you might be using, the Blue Sea web site is a great resource as they are all listed. Check it out at: www.bluesea.com


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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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