As you read the following sections on ungrounded
physical parts, but are an inherent part of the design of
electrical equipment and cable.
systems, look at figure 1-2.
Several factors determine the value of the
A Perfect Ungrounded System
capacitance generated between the conductor and
A perfect ungrounded system (fig. 1-2, view A)
ground: the radius of the conductor, the distance
exists under the following conditions:
between the conductor and the bulkhead, the dielectric
The insulation is perfect on all cables,
constant of the material between the two, and the length
of the cable. Similar capacitance exists between the
switchboards, circuit breakers, generators, and
generator winding and ground and between various
load equipment.
load equipment and ground.
There aren't any filter capacitors connected
between ground and the conductors.
Ideally, capacitors have an infinite impedance to
direct current; therefore, their presence can't be detected
The system equipment or cables don't have any
by a Megger or insulation resistance test. In addition to
inherent capacitance to ground.
the nonvisible system capacitance, typical shipboard
If these conditions are met, there would be no path
electrical systems contain radio frequency interference
for electrical current to flow from any of the system
(RFI) filters that contain capacitors connected from the
conductors to ground.
conductors to ground. These falters may be apart of the
load equipment, or they may mount separately. To
Look at figure 1-2, view A. Here you can see that
reduce interference to communications equipment,
if a person touches a live conductor while standing on
filters are used.
the deck, no completed path exists for current to flow
from the conductor through the person's body. No
Look at figure 1-2, view C. If physical contact is
electric shock would occur.
made between cable B and ground current will flow
from the generator through the person's body to ground
However, shipboard electrical power distribution
and back through the system resistances and
systems don't and can't meet the definition of a
capacitances to cable A. This current flow completes
PERFECT ungrounded system.
the electrical circuit back to the generator and presents
Real Ungrounded Systems
a serious shock hazard.
In a shipboard real ungrounded system (fig. 1-2,
Suppose you are using a Megger to check for ground
view B) additional factors (resistance [R] and
in this system, and you get a reading of 50,000 ohms
capacitance [C]) must be considered. Some of these are
resistance. You can conclude that no low-resistance
not visible.
ground exists. However, don't assume that the system
When combined in parallel, the resistances form the
is a perfect ungrounded system without checking the
insulation resistance of the system that is periodically
circuit further. Don't forget the system capacitance that
measured with a 500-volt dc Megger. Look at figure
exists in parallel with the resistance.
1-2, view B. Here, you can see that there's a generator
insulation resistance, an electric cable insulation
Remember, never touch a live conductor of any
resistance, and a load insulation resistance. The
electrical system, grounded or ungrounded. Make
resistors cannot be seen as physical parts, but represent
insulation resistance tests to ensure the system will
small current paths through equipment and cable
operate properly, not to make the system safe. High
electrical insulation. The higher the resists.rw, the
insulation readings in a Megger test do not make the
better the system is insulated; therefore, less current will
system safe-nothing does.
flow between the conductor and ground.
Representative values of a large operating system can
SHOCK-MOUNTED EQUIPMENT
vary widely, depending on the size of the ship and the
Normally on steel-hulled vessels, grounds are
number of electrical circuits connected.
provided because the metal cases or frames of the
Figure 1-2, view B, also shows the capacitance of
equipment are in contact with one another and the
the generator to ground, the capacitance of the
vessel's hull. In some installations grounds are not
distribution cable to ground, and the capacitance of the
provided by the mounting arrangements, such as
load equipment to ground. As before, these
insulated shock mounts. In this case, a suitable ground
capacitances cannot be seen, since they are not actually
connection must be provided.
1-8
