Page 283 - Introduction to Marine Engineering

Electrical equipment 257

A delay mechanism prevents the breaker opening for short-period
overload currents. The circuit breaker opens or closes both supply and
return leads in the circuit. Where a circuit breaker feeds the generator
supply to the bus-bars a third 'make-or-break' arm will be provided for
the equaliser connection.
Preferential tripping is a means of retaining essential electrical
supplies. In the event that a generator cannot supply all the load then
non-essential loads are disconnected by preferential trips. The intention
is to reduce the generator load while ensuring essential equipment such
as steering gear, navigation lights, etc., retains its electrical supply.
Various circuit faults can occur as a result of either a break in the
conductor (cable) or a break in the insulation. An open-circuit fault
results from a break in the conductor and no current flow will take place,
A short-circuit fault is due to two breaks in the insulation on, for
example, adjacent conductors. The two conductors are connected and a
large current flow takes place. An earth fault occurs when a break in the
insulation permits the conductor to touch an earthed metal enclosure (or
the hull).
Earth faults are usually detected by the use of earth indicating lamps.
Two lamps are used, each rated for the full system voltage, but
connected in series across the system with the mid point earthed (Figure
14.3), If the system is correctly insulated then both lamps will glow at

Bus bars

Lamp Lamp

Figure 14.3 Earth lamp circuit

half brilliance. The lamps are placed close together to enable a
comparison to be made. A direct earth in one pole will short circuit its
lamp, causing the other to shine brighdy. A slight insulation breakdown
would produce a difference in bulb brightness between the two. Where
an earth fault is detected the circuit breakers for each separate circuit
must be opened in turn until the fault location is discovered. The
particular section or distribution box would then have to have its circuits
investigated one by one to locate the fault and enable its correction.