April 7

Understanding Marine Electrics: Ampere Interrupting Capacity

Welcome to our latest blog post where we'll be discussing an often overlooked aspect of overcurrent protection - the ampere interrupting capacity or AIC for short. Join us as we delve deeper into the importance of AIC ratings in keeping your boat safe from electrical fires and other hazards.

What is AIC and Why is it Important

AIC is the capability of a fuse or circuit breaker to handle the high currents that may be seen in a short circuit. For example, in a dead short even a relatively small lead-acid battery is capable of momentarily putting out thousands of amps. These extraordinarily high current levels can cause a fuse or circuit breaker to arc over and continue to be conductive: the circuit is not broken, creating a fire hazard on the boat. The maximum current a fuse or circuit breaker can safely break is described by its AIC rating.

To prevent the possibility of arc over, for the main fuse or circuit breaker in both DC and AC systems, especially in DC systems, it is essential to choose a fuse or circuit breaker with a high AIC rating.

Battery AIC Ratings

In a DC system, the AIC rating requirement varies depending on the battery size and type. For lead-acid batteries, the main fuse requires an AIC rating of at least 5000 amps and for larger battery banks it should be up to 10,000 amps. MRBF and ANL fuses are commonly used. For lithium-ion batteries, it is recommended to have a fuse with an AIC rating as high as 20,000 amps. Class T fuses are typically the only widely available, compact and affordable fuse.

Conclusion

In summary, when choosing a fuse or circuit breaker, especially a main fuse or circuit breaker, it is essential to consider the AIC rating. This must be high enough to handle the worst-case possible short circuit current if it is to prevent a potential fire hazard on the boat. We explain these things in detail in our courses on Marine Electrical Systems!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.


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