Many forms of electrical protection on ship are available which are designed to protect the distribution system when a fault occurs.
Protection relays are used to monitor overcurrent, over / under voltage, over / under frequency, earth leakage, unbalanced loading, over-temperature, reverse power (for generators) etc.
The ship HV power system typical protective with relay functions.
No matter how well designed and operated, there is always the possibility of faults developing on electrical equipment.
Faults can develop due to natural wear and tear, incorrect operation, accidental damage and by neglect.
The breakdown of essential equipment may endanger the ship, but probably the most serious hazard is FIRE.
Overcurrent (I2R resistive heating effect) in cables and equipment will cause overheating and possibly fire.
The size of conductor used in cables and equipment is such that with rated full load current flowing, the heat developed does not raise the temperature beyond about 80’C (i.e. 35’C rise above an ambient of 45’C).
A copper conductor can withstand very high temperatures (melts at 1083″C), but its insulation (generally organic materials such as cotton or plastic compounds) cannot withstand temperatures much in excess of 100’C.
At higher temperatures the insulation suffers irreversible chemical changes, loses its insulation properties and becomes burnt out.
Short-circuit and overload currents must, therefore, be detected and rapidly cleared before damage occurs.
Three reasons why protection equipment is essential in an electrical distribution system.
To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system
To prevent damage to equipment from the thermal and magnetic forces that occur during short circuit and overload faults
To protect personnel from electric shock
The protection scheme consists of circuit breakers, fuses, contactors, overcurrent and under voltage relays.
A circuit-breaker, fuse or contactor interrupts the fault current.
An overcurrent relay detects the fault current and initiates the trip action.
The circuit-breaker or fuse must be capable of safely and rapidly interrupting a short-circuit current.
They must be mechanically strong enough to withstand the thermal and magnetic forces produced by the fault current.
The size (strength) of the circuit-breaker or fuse is specified by its breaking capacity which is the maximum fault current it can safely interrupt.
