Lets know what is Insulated neutral system & Earthed neutral system

Insulated Neutral System: When the neutral of the generator is completely insulated from earth or ship`s hull it is called insulated neutral system. 

It is very advantageous because it does not pose any danger to human life even if there is a single earth fault because the whole ship will be at same potential leading to no damage to machinery and life. N alarm system will detect this earth fault.

Earthed Neutral System: When the generators neutral is connected to the earth or ship`s hull it is called earth neutral system. 

A single fault can cause very large current to flow causing damage to the machinery. But still it is used in high voltage systems because if there is only one earth fault the machinery will trip and protect the equipment.