Overcurrent protection in shipboard electrical systems

AS SHIPBOARD POWER DEMANDS have increased, it has been necessary to install more and larger generators aboard ship. It has been found that the most reliable and flexible system can be attained by arranging the bus ties in such a manner that they form a closed loop if all bus tie circuit breakers were closed. This arrangement can result in potential fault currents well over the maximum interrupting capacity of the available circuit breakers if all generators were operated in parallel. Therefore, it is necessary that the ship´s service power plant be split in such a manner at all times so that the fault current will be within the capacity of the circuit breakers. Interrupting capacity of the U. S. Navy´s largest circuit breakers has been increased to 100,000 amperes. This rating determines maximum numbers of generators that can be operated in parallel. On ships with eight 1,500-kw generator sets, only three generators may be operated in parallel at one time. With three generators operating in parallel, the rms fault current for the first half cycle in the phase carrying the maximum asymmetrical current is 86,000 amperes at the generator distribution switchboard and 62,000 amperes at the load center switchboards.