Leakage Currents

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.

1-12