Isolated Receptacle Circuits

winding and ground and between various load

SHOCK-MOUNTED EQUIPMENT

equipment and ground. Since capacitors ideally have

Normally, on steel-hulled vessels, grounds are

an infinite impedance to direct current, their presence

provided because the metal cases or frames of the

cannot be detected by a Megger or insulation

equipment are in contact with one another and the

resistance test. In addition to the nonvisible system

vessel's hull. In some installations, grounds are not

capacitance, typical shipboard electrical systems

provided by the mounting arrangements, such as

contain radio frequency interference (RFI) filters that

insulated shock mounts. In this case, a suitable ground

contain capacitors connected from the conductors to

connection must be provided.

ground. These filters may be a part of the load

equipment, or they maybe mounted separately. Filters

are used to reduce interference to communications

CAUTION

equipment.

Before disconnecting a ground strap on

If physical contact is made between cable B and

ground (fig. 1-12, view C), current will flow from the

equipment supported by shock mounts, be

generator through the person's body to ground and

sure the equipment is de-energized and a

back through the system resistances and capacitances

danger/red tag is installed.

to cable A, thus completing the electrical circuit back

When the grounding strap is broken

to the generator. This presents a serious shock hazard.

and the equipment cannot be de-energized,

Suppose you check the system of figure 1-12, view C,

use a voltmeter from the equipment to

for grounds with a Megger and get a reading of 50,000

ground to ensure that no voltage is present.

ohms resistance. You can conclude that no low-

resistance grounds exist. Don't assume that the system

is a perfect ungrounded system without checking the

Maintenance of grounding cables or straps

circuit further. Do not forget the system capacitance

consists of the following preventive procedures:

that exists in parallel with the resistance.

Clean all strap-and-clamp connectors peri-

It should be clear to you why you should NEVER

odically to ensure that all direct metal-to-metal

touch a live conductor of an electrical system,

contacts are free from foreign matter.

grounded or ungrounded. Insulation resistance tests

Replace any faulty, rusted, or otherwise unfit

are made to ensure the system will operate properly,

grounding straps, clamps, connections, or com-

not to make the system safe. High insulation readings

ponents between the equipment and the ship's

in a Megger test do not make the system safe--nothing

hull.

does.

When replacing a grounding strap, clean the

metallic contact surfaces and establish electri-

ISOLATED RECEPTACLE CIRCUITS

cal continuity between the equipment and the

ship's hull. Check continuity with an ohmmeter

Isolated receptacle circuits are installed on all new

(the reading must be according to PMS).

construction ships. These receptacle circuits help

Recheck to be sure the connection is securely

reduce the inherent hazard of leakage currents where

fastened with the correct mounting hardware.

portable tools and appliances are plugged in and out,

which is when personnel are more likely to get an

If a voltage is present, and the equipment can-

electrical shock. These circuits are individually

not be de-energized, wear electrical rubber

isolated from the main power distribution system by

gloves and use a rubber mat while replacing the

isolation transformers. Each circuit is limited to 1500

grounding strap.

feet in length to reduce the capacitance to an

acceptable level. This design is intended to limit

SWITCHBOARDS, SWITCHGEARS,

ground leakage currents to 10 milliamperes, which

AND ENCLOSED EQUIPMENT

would produce a nonlethal shock. To maintain a safe

level of leakage currents, the isolated receptacle

Switchboards and switchgears must have safety

circuits must be free of all resistance grounds.

precautions, operating instructions, wiring diagrams,

1-14