Electrical-Three-Phase Systems

Three-Phase Systems
Outputs from the three sets of conductors in the alternator are delivered to three separate bus-bars in the switchboard. This is necessary because of the voltage and current disparity between them at any instant.
Three-phase, four systems use a single return wire which is connected to the neutral point of the star windings. Current in the turn wire is the sum of currents in the individual phases. If loads on each phase are balanced with voltages equal and at .1 20 apart, the three currents will sum to zero and the return wire will carry no current The fourth (return) wire will carry a small current it there is imbalance.
Three-phase, three-wire systems have no return wire. This is acceptable for snips where, direct from the main switchboard, three-phase motors make up much of the load and unless there is a fault they take current equally from the phases. Also some imbalance is acceptable with a three-phase, three-wire system provided load is connected in delta Supplies for lighting, heating, single-phase motors and other loads are taken through delta-star or delta-delta transformers.
The Neutral Point
The majority of British ships use three-phase, three-wire distribution with the neutral points of alternators insulated (Fig. Very little current will flow through an earth fault on one phase, because there is no easy path for it back to the electrical system. With such a system, an essential electric motor with an earth fault can be kept running until stoppage for repair is convenient This would be as soon as possible to avoid a full-phase fault that would result if an earth occurred on another phase as well.
Although fault current is negligible with an insulated/unearthed neutral point, over voltages are high. The transient likely is 2.5 X line voltage. Such a voltage surge is within the capability of the main insulation of marine electrical equipment which is tested to 2 X line voltage + 1000 volts.

A few British vessels have electrical distribution systems with an earthed neutral . This is a connection of the system, via the neutral point of the alternator, to the hull steel. The result of not isolating the electrical system from the hull is that current flow from an earth fault on any phase has a path through the hull steel and earthed neutral back to the system. The availability of the path encourages higher fault current flow than is the case where the neutral is insulated or connected by resistance. Equipment with an earth fault, where-the system is earthed, must be disconnected immediately if a fault develops. This can be effected automatically with an earthed neutral system because the level of fault current is ugh enough to operate a trip.
Earth fault current is high with earthed neutral systems, but over- voltages due specifically to earth faults are lower. The earthed system is chosen to limit over voltages and to give automatic earth fault location and disconnection.
Over voltages due to switching are not affected by choice of earthing or insulating the neutral. These high surges, and the possibility of others from failure of the voltage regulator, means that the same standard of equipment insulation is required for both arrangements.