What is the reason for this difference between the distribution systems of power companies and the systems implemented at ships?
On low voltage power systems (<1000V), ships use insulated neutrals to protect the integrity of the power system.
On land, a grounded neutral is used. Any ground fault (hard or instantaneous) will cause breakers to trip or fuses to open. At worst, you are in the dark. The offending breaker would be easy to find.
But at sea, a power outage can cause the ship to be at risk while coming through a channel, narrows or docking. So to protect the integrity of the power system, an insulated neutral is used.
A single ground fault will not activate circuit protection. If a single ground fault occurs, the hull is energized. The crew is not directly at risk, unless they make contact with another power line. For this reason, ground faults must be cleared as soon as possible. All voltage bus-bars must have ground-fault detection.
This is much like the uninsulated power lines used on power poles on land. Birds can land on them and unless they make contact with the grounded pole, bypassing the pole insulator, they are safe. Their claws are at the same potential.
For a power outage, two ground faults on two different power lines are required to activate fuses or circuit breakers. Ideally, selectivity of the power system would isolate two ground faults close to the loads but the ship could be blacked out.
No maintenance work is ever scheduled while maneuvering and periodic maintenance is delayed until ground faults are cleared. The two ground faults can occur on any power line, including transformers and single-phase lines, so all power lines must be protected by breakers or fuses.
On high voltage installations (>1000V), each generator neutral is grounded to the hull via a resistance or impedance to limit short-circuit current. Any ground fault may cause a ship blackout but it is just too dangerous for high voltage systems (6.6kV) to have an uninsulated neutral.
The first ground fault (Red L1) would not affect the operation of the ship. The motor would operate normally. A second ground fault on the same line would have no effect. A second ground fault on the same line would make clearing the ground faults slightly harder to troubleshoot.
It is only when a second ground fault (Green L2) occurs on a second power line that a short circuit occurs and circuit protection devices are activated.