Question

With reference to the provision, of a shore electrical supply to a ship:

(a) Sketch an arrangement for taking A.C. shore supply and checks to be carried out prior taking shore connection.

(b) Describe the method of safely connecting the arrangement sketched in (a) to the shore supply.

A shore supply is required so that the ship generators and their prime mover can be shut down for major overhaul during dry dock period.

There must be 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 must have a circuit breaker or an isolator switch and fuse to protect the cable linking the connection box to the main switch board with data plate giving details of the ships electrical system [voltage and frequency] and showing the method for connecting the shore supply cables.

A voltmeter is fitted to indicate polarity of a DC shore supply.

The connection box must have suitable terminals to accept the shore supply cable including an earthing terminal to earth the ships hull to shore earth.

For an ac shore supply, a phase sequence indicator to be fitted to indicate correct supply phase sequence

Safe connection

The phase sequence indicator may be arranged as two lamps connected as an unbalanced load across the 3 phase is a resistor and capacitor

The sequence is 8 when the right side lamp is bright and the other dark

An alternative PSI is a rotary pointer driven by a small 3phase induction motor

At the main switch board an indicator is provided usually a lamp to indicate that shore supply is available for connection to the bus bar via a connecting switch or circuit breaker.

It is not normally possible to parallel the shore supply with the ships generators.

The ships generators must therefore be disconnected before the shore supply can be connected to the main switch board

Normally, the shore supply switch on the main switch board is interlocked with the generator circuit breaker, so that it cannot be closed if the generators are still connected.