Difference between Neutral isolated/Neutral earthed

Firstly, we should clearly understand two schools of thought when it comes to design in the 3-phase electrical distribution system onboard ships.

1. When the priority requirement onboard is to maintain continuity of the electrical supply to equipment in the event of a single earth fault occurring, this is generally employed in low voltage ships 
2. When the priority is to safeguard the electrical equipment and by isolating the electrical equipment if a single earth fault occurs on the live line thus preventing further damage.this is generally employed in high voltage ships, at homes and shore factories, etc.

Outline of three-phase system 

The electrical output from the 3 sets of the conductor in an alternative system is delivered to three separate busbars in the switchboard and is feed by the three-phase current that is displaced in time by 120 degrees.

• Three-phase and four-wire uses a single wire connected to the neutral point in the star winding
• Three-phase and three-wire has no return wire and provided, load are connected in delta winding 

Neutral isolated system(NIS)

The majority of the low voltage ship uses a three-phase and three-wire with the neutral of the alternator insulated, priority is to maintain the continuity of the electrical supply in the event of a single earth fault.

In this system, the supply is not interrupted but raises an alarm in the earth detection system giving the engineers enough time to troubleshoot and clear the fault at a convenient time. It requires two earth faults in the system to cause an earth fault current to flow and trip the equipment.

• Very little transient current will flow if earth fault occurs in one line as there is no easy path back to the electrical system
• Although fault current is negligible the overvoltage is high, transient voltage is likely 2.5 times the line voltage
• There is no loss of power to critical equipment hence improving the safety of the vessel.
• Tracing and clearing of earth fault can be tricky and requires experience and time

Neutral earth system(NES)

The majority of the high voltage ship uses a Three-phase and four-wire, in which the system is earth via a resistor connecting the generator neutral to the hull, the availability of path encourages a higher transient fault current which may lead to the isolation of equipment. ohmic value of the resistor is chosen so as to limit the maximum earth fault to not more than the generator full load current, this is monitored by the earth fault relay to create alarm and trip function

• Earth fault current is high but overvoltage due to earth fault is lower
• loss of power may lead to a hazardous situation especially of critical equipment like steering gear.
• A special 3-phase transformer is connected to HV system busbar to initiate alarm/trip to a connection protection relay