Page 262 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 245

contact cannot be carried out by obstacles or by placing live parts out
of reach.
An effective protection against indirect contact can be obtained
by disconnection of supply carried out by residual-current protective
devices with a residual rating not exceeding 30 mA (Fig. 15.1). In
this arrangement, a protective conductor (PE) connects the dockside
earthingelectrodetotheboat’smetalhull,theunderwatergear,and/or
the d.c. negative bus, allowing the RCD to trip upon both boat- and
dock-originated ground faults.
A drawback of this arrangement is the energization of the hull un-
der fault conditions because of its equipotentialization with the boat’s
ECPs. Consequently, across hull and land a potential difference arises,
which causes part of the fault current to flow through the parallel
path eventually constituted by the seawater. This potential difference
is present during the RCD’s clearing time and may be dangerous for
swimmers. The residual device, in fact, is designed to operate in stan-
dard conditions (i.e., dry skin), and may not act fast enough if the
person’s body impedance is lowered due to wet conditions. In addi-
tion, if the boat is moored in fresh water, which is a poor conductor,
nearly no stray current would circulate, unless swimmers, by entering
the water, increase its conductivity and become themselves a return
path to the source.
Other negative aspects concerning the presence of the PE between
berth and boat is the possibility to trigger the electrolytic corrosion of
the hull. The dockside earth electrode and the hull are, in fact, unlike
materials immersed in electrolytes (i.e., seawater and earth). The two
metals electrically connected by the protective conductor in the 3-core
flexible cable, as shown in Fig. 15.1, constitute a galvanic cell, which
causes circulation of direct current. If the hull is anodic to the earth
electrode, corrosion will occur at its expenses. The same phenomenon
may occur to vessels docked alongside one another when they plug
in at the same pedestal. Their respective protective conductors elec-
trically link the boats’ hulls to each other and, if they are made of
dissimilar metals, the less noble metal will corrode.
To prevent corrosion, protection against indirect contact can be
carried out by electrical separation (Fig. 15.2).
The isolating transformer separates the shipboard electrical sys-
tem from the shore supply and can be either shore-mounted or in-
stalled onboard. The effectiveness of the electrical separation is en-
sured by not connecting the protective conductor serving the vessel’s
loads to the grounding system ashore. As a consequence, since there
is no longer a metal connection between hull and earthing electrode,
corrosion cannot occur.
In addition, even if the hull becomes energized, there will be no
circulation of fault current through the seawater due to the galvanic
decoupling with evident benefit for the swimmers safety.