Page 282 - Introduction to Marine Engineering

256 Electrical equipment.

distribution board feeds minor supplies such as lighting and may itself
be fed from the main board or a section board. The distribution system
is shown in Figure 14.2.
A two wire system is usual to provide a supply and return to each item
of equipment. An earth lead would be the only electrical connection
between any item of equipment and the ship's structure. With
compound wound generators a third bus-bar would be introduced as the
equalising connection between machines.
A fuse is a type of switch which isolates a circuit if an excessive current
flows. To reconnect the circuit, after discovering the cause of the
overload, the fuse must be rewired or replaced. The fuse is in effect a
weak link in the circuit designed to break and protect equipment from
damaging high currents. A semi-enclosed or rewirable fuse will have
provision for a wire to be replaced after it has burnt out. The correct
rating of fuse wire should be replaced within the holder to reinstate the
circuit. A cartridge fuse has the wire enclosed within a ceramic body and
it is not rewirable. A 'blown' cartridge fuse must be replaced by a new
one, The cartridge fuse is to be preferred since the fusing current value
is more reliable than for a rewirable type.
A circuit breaker is an isolating switch which also functions as a fuse. It
has two designed ratings: one of the normal safe working current, the
other the overload current. The breaker is closed against the action of a
spring to make the circuit and supply the section board or auxiliary. A
trip mechanism opens the breaker, a fast opening being ensured by the
spring. When desired the breaker is tripped or opened manually. It will
also open if the overload current rating is exceeded for a period of time.

Bus bars
Circuit
breaker

Cargo Section
Auxiliaries board

Section
board
Distribution
board
.Minor
Navigation supplies supplies
Minor
supplies
Figure 14.2 D.C. distribution system