There is a need for additional electrical power generation and distribution capacity on the next generation of Navy combatant ships. Because of limitations in the continuous and short circuit current ratings of low voltage air circuit breakers (LVACBs), some of these platforms (like DDG Flight Ⅲ) are considering a move to medium voltage generation and distribution. But the perceived limitations in LVACBs can be overcome with the use of existing commercial circuit breakers connected in parallel. Circuit breakers with large continuous current ratings encounter certain electromagnetic phenomena known as "skin effect" and "proximity effect" which lead to very high ohmic losses in large cross-sectional area conductors. For this reason, modern power circuit breakers with ratings over about 3200 A are built almost exclusively using two circuit breaker "modules", joined side-by-side and operated by a common mechanism and pole shaft. This type of modular construction is now used by most circuit breaker manufacturers and has resulted in ACB ratings up to 6300 A (IEC) continuous current rating (de-rated to ～ 4000-5000 A rating in a navy ACB). Such modular ACBs can have short time current ratings up to 130 kA and an interrupting rating of up to 200kA at 508 vac. Although it is technically possible to develop large circuit breakers with higher ratings (using more modules), the lack of a commercial demand makes this unlikely. If ratings above this level are needed (as they are on DDG Flight Ⅲ), the simplest way to achieve this is to electrically parallel two or more complete circuit breakers and open and close them synchronously using external control. This parallel circuit breaker approach has been commonly used in medium voltage applications where the circuit breakers are used with external control relays, but it has never seen widespread commercial application in the low voltage arena. But with appropriate controls, parallel circuit breakers are safe and practical for ratings beyond the range of existing circuit breakers. The use of a low-voltage distribution system at available fault currents up to 200 kA also requires the use of molded case circuit breakers with similar interruption ratings, preferably without the need for fuses. Although not discussed in this paper, such MCCBs are now available commercially and can also be adapted for use on Navy combatant ships.
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