Question: I was reading an old technical article recently and the author was talking about a fuse’s AIC rating and how important it is to ensure that the fuse will actually blow in the event of a catastrophic short circuit. Can you explain?
Answer: The AIC rating of a fuse or circuit breaker is an obscure but important specification. The rating describes how much amperage a fuse or breaker can be subjected to without literally welding the contacts closed (in the case of a circuit breaker) or turning a fuse into a solid mass of metal. In either case the device will not “open” the circuit, stopping electrical current flow. This is potentially a dangerous situation that can cause an electrical fire.
This issue has become an increasing concern as battery banks grow and have more capacity (amperage) waiting to unleash itself. All of this has caused the ABYC to contemplate changes to its long-standing recommendations regarding over-current protection ratings.
Historically the overcurrent protection ratings for AIC have been based on the connected battery capacity in cold cranking amps (CCA) within ABYC E-11 Standard. In today’s world this is woefully inadequate; we need to consider short circuit current potential.
Comparison testing in the lab have shown that the actual short circuit current potential of a conventional lead acid battery is somewhere in the range of 4 to 4.5 times its CCA rating. An ordinary car or small truck battery might have a CCA rating of about 700 amps, but its actual short-circuit current rating is over 3000 amps! Now take two or three of them and connect them in parallel, as we often do on boats. In parallel configurations the amperage is cumulative, so it’s easy to end up with battery banks on boats that have 5,000-10,000 amps of available short circuit current. This is a bit scary when you consider that the ABYC reference table shows a maximum of 5000 amps.
So, what’s the solution? The only economically viable solution we have today is called the class T fuse.
The class T has an AIC rating of 20,000 amps at 32 volts. Keep in mind that as system voltage decreases AIC ratings go up; so, at 12 volts based on graphed models, the theoretical AIC for a class T fuse is about 100,000 amps. That should be more than enough to cover a pretty large battery bank even by today’s standards. If the Class T is installed in the main battery cable at the point where the batteries are parallel-connected (usually right after the main shut off switch), then all the fuses and breakers downstream from it will be protected.