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Leakage Currents
 

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insulation from scraps of electric cables; how-
and cable, and cannot be eliminated by practical
technical means.
ever, do this only in an emergency situation.
Many persons believe it is safe to touch one
Do not wear a wristwatch, rings, other metal
conductor since no electric current would flow. This
objects, or loose clothing that could become
is not true. It is NEVER safe to touch one conductor
caught in live circuits or metal parts.
of the ungrounded shipboard system. This is because
Wear dry shoes and clothing, and ALWAYS
each conductor and the electrical equipment
wear a face shield.
connected to the system have an effective capacitance
to ground, which provides an electric current path
Tighten the connections of removable test leads
between the conductors and the ship's hull. The higher
on portable meters. When checking live cir-
the capacitance, the greater the current flow will be for
cuits, NEVER allow the adjacent end of an
your fixed body resistance. This situation occurs when
energized test lead to become unplugged from
one conductor of the ungrounded system is touched
the meter.
while your body is in contact with the ship's hull or
Be sure a person qualified to administer mouth-
other metal enclosures. When your body resistance is
to-mouth ventilation and cardiac massage for
low due to wet or sweaty hands, for example, the
electrical shock is in the immediate area.
inherent capacitance is sufficient to cause a FATAL
electric current to pass through your body.
Be sure a person who is knowledgeable of the
system is standing by to de-energize the equip-
A perfect ungrounded system (fig. 1-12, view A)
ment.
exists when the insulation is perfect on all cables,
switchboards, circuit breakers, generators, and load
Be sure a nonconducting safety line is tied
equipment; no filter capacitors are connected between
around the worker's waist to pull the person
ground and the conductors; and none of the system
free if he/she comes in contact with a live cir-
equipment or cables have any inherent capacitance to
cuit.
ground. If all of these conditions were met, there
Where work permits, wear rubber gloves on
would be no path for electrical current to flow from
both hands. If this is not possible, work with
any of the system conductors to ground.
one hand and wear a rubber glove on the other
As shown in figure 1-12, view A, if a person
hand.
touches a live conductor while standing on the deck,
there would be no completed path for current to flow
from the conductor through the person's body, and no
LEAKAGE CURRENTS
electrical shock would occur. However, shipboard
electrical power distribution systems DO NOT and
The electrical distribution systems found on Navy
CANNOT meet the above definition of a PERFECT
ships are ungrounded. The reason for using an
ungrounded system.
ungrounded system is to achieve maximum system
In a shipboard real ungrounded system (fig. 1-12,
reliability and continuity of electrical power under
view B) additional factors (resistances, R, and
combat conditions. If one line of the distribution
capacitances, C) must be considered, some of which
system is grounded, due to battle damage or
are not visible.
deterioration of the system insulation resistance, the
circuit protective devices (circuit breakers, fuses, and
The resistances, when combined in parallel, form
so on) will not de-energize the circuit having the
the insulation resistance of the system, which is
ground, and electrical power will continue to be
periodically measured with a 500-volt dc Megger. In
delivered to vital load equipment without further
figure 1-12, view B, there is a generator insulation
damage to the system.
resistance, an electric cable insulation resistance, and
a load insulation resistance. The resistors cannot be
Shipboard ungrounded electrical systems are
seen as physical components, but are representative of
capacitively grounded to the extent that lethal currents
small current paths through equipment and cable
can flow through a person's body if a live conductor
electrical insulation. The higher the resistance, the
is touched while in contact with ship's ground. The
capacitance that causes this electrical ground leakage
better the system is insulated; therefore, less current
current to flow is inherent in the design of equipment
will flow between the conductor and ground.

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