A ship shore-supply is required so that the ship’s generators and their prime-movers can be shut down for major overhaul during a dry-docking period.
There must be a suitable connection box conveniently located to accept the shore supply cable.
The connection box is often located at the entrance to the accommodation or in the emergency generator room.
The connection box for ship shore supply must have suitable terminals to accept the shore supply cable, including an earthing terminal to earth the ship’s hull to the shore earth.
The connection box must have a circuit breaker or an isolator switch and fuses to protect the cable linking the connection box to the main switchboard, with a data plate giving details of the ship’s electrical system (voltage and frequency) and showing the method for connecting the shore supply cable.
A voltmeter is fitted to indicate polarity of a d. c. shore supply.
For an a. c. shore supply a phase-sequence indicator is fitted to indicate correct supply phase sequence.
This indicator may be arranged as two lamps connected as an unbalanced load across the three phases via resistors and capacitors. The sequence is “right” (or correct) when the light side lamp is bright and the other is dark.
An alternative P.S.I. indicator is a rotary pointer driven by u small 3-phase induction motor.
At the main switchboard an indicator is provided, usually a lamp, to indicate that the shore supply is available for connection to the bus-bars via a connecting switch or circuit-breaker.
It is not normally possible to parallel the shore supply with the ship’s generators.
The ship’s generators must, therefore, be disconnected before the shore supply can be connected to the main switchboard. Normally, the shore supply switch on the main switchboard is interlocked with the generator circuit breakers so that it cannot be closed if the generators are still connected.
The shore supply may have a different frequency and/or voltage to that of the ship’s system.
A higher frequency will cause motors to run faster, be overloaded and overheat.
A higher voltage will generally cause equipment to take excess current and overheat.
It will also cause motors to accelerate more rapidly and this may over stress the driven loads.
A lower voltage is generally not so serious but may cause motors to run slower and overheat, and may cause motors to stall.
If the shore supply frequency differs from the ship’s normal frequency then, ideally, the shore supply voltage should differ in the same proportion.