6.4. System protection
System protection
Ship power systems are protected by circuit breakers which interrupt the current in case the threshold value of the current is exceeded. They trip – i.e. they open the contacts and interrupt the circuit leaving it open.
Circuit breakers can interrupt the current several times. Some of them can be used just once and then require replacement just like fuses. Fuses protect distribution boards, and the equipment connected to them as they can operate in quite a narrow range of current and voltage. A thin strip in a fuse melts in case of overheating due to overcurrent or overvoltage and interrupts the circuit.
A separate type of circuit breakers is high voltage (HV) circuit breakers. These are often large and are used to quench an electric arc which always occurs if a HV cable is disconnected. The most typical HV circuit breakers are vacuum type (the arc is extinguished in ceramic bottles, when the contacts disconnect inside them) or sulphur hexafluoride type (SF6) (when the arc is quenched in a container with contactor mechanism with coil which will make the arc rotate and filled with the special gas having a quality of increasing dielectric strength when its temperature increases).
Motors also have overload, overcurrent protection and simple contactors which also serve as motor starters are used for this purpose.
A contactor has a closing mechanism which is operated by a coil pulling an iron core and thus closing the contacts. The motor will be started or stopped depending on the position of the contactor contacts.
Reference:
Borstlap, R., Ten Katen, H. (2021) Ship Electrical systems. Vlissingen.