We have an obscure and rarely understood term in connection with the fuses and circuit breakers on our boats. It is ‘ampere interrupting capacity’, or AIC for short. It is becoming increasingly important as we install more and more electrical equipment, both DC and AC, and especially as we increasingly move to the installation of lithium-ion batteries.
Fuses and Circuit Breakers
The function of a fuse or circuit breaker is to shut down a circuit if some fault condition results in excessive current (amperage) flowing through the circuit. If the circuit is not shut down, the excessive current will heat up wiring, melt insulation, and potentially set the boat on fire. A fuse or circuit breaker has an amp rating. If the amperage goes significantly above this the fuse blows or the circuit breaker trips.
The amp flow in a circuit is governed by the resistance of the device at the end of the conductors. For example, a 12-volt bilge pump which is rated at 10 amps. When running, its internal resistance limits the current flow to 10 amps. So long as the circuit is not compromised, and the bilge pump does not have an internal fault we will not see more than 10 amps flowing through the circuit.
Short Circuits
The worst case for excessive current flow is typically something known as a ‘dead short’ – a condition in which a direct connection occurs in the wiring to a device at some point before the device. For example, poorly supported positive and negative conductors to the bilge pump chafe through until bare copper is exposed and then the two conductors touch each other. The only resistance in the circuit is now the resistance of the copper wiring in the circuit, which is very low. There will be an almost instantaneous rise in the amps to whatever level the power source – in this case a battery – can supply.
In a dead short even a relatively small lead-acid battery is capable of momentarily putting out thousands of amps. AGM batteries have published short circuit currents as high as 5,000 amps. These extraordinarily high current levels can cause a fuse or circuit breaker (collectively known as ‘overcurrent protection devices’ or OCPD) to arc over at which point the OCPD continues to be conductive even though a fuse has blown, or a circuit breaker has tripped: the circuit is not broken, creating an immediate fire hazard on the boat. The maximum current an OCPD can safely break before this arcing over occurs is described by its AIC rating.
Main OCPD AIC Ratings
There are two places in boat electrical circuits where it is essential to have an OCPD with an AIC rating in the thousands of amps. One is the first fuse connected to the positive side of circuits coming off a battery, and the other is the circuit breaker immediately after the AC shorepower inlet. In both cases, these OCPDs are referred to as a ‘main’ OCPD. The main OCPD takes care of the worst-case short circuit currents that can be delivered by the power source (the battery or shorepower). Once we get beyond the main OCPD we have ‘branch’ OCPDs. These can have a lower AIC rating.
In a DC system, the main OCPD AIC rating requirement varies depending on the battery type and size. For lead-acid batteries, the main fuse typically requires an AIC rating as high as 5000 amps. Two types of fuse, MRBF and ANL, both of which have AIC ratings up to 10,000 amps at 12-volts, 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.
There are a couple of words above that are really important and which are frequently disregarded! They are “at 12 volts”. If you double the voltage on a system, the AIC rating is cut approximately in half. An ANL fuse with an AIC rating of 10,000 amps at 12 volts will have an AIC rating of 5,000 amps at 24 volts (barely adequate for AGM batteries and certainly not adequate for lithium-ion) and 2,500 amps at 48v (not high enough for any substantial battery bank, either lead-acid or lithium-ion). It is important to pay attention to the small print!
In AC systems, typically the main OCPD requires an AIC rating of 5,000 amps. Suitable circuit breakers are readily available.
Conclusion
In summary, when choosing a main OCPD, 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!
