Quantcast Types of grounding systems

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
CHAPTER 8
GROUNDING
and functioning properly. Static ground systems are
8-1. Ground maintenance.
generally not called upon to conduct much current
The term grounding implies an intentional electri-
at any given frequency. Smaller gauge, bare conduc-
cal connection to a reference conducting body, which
tors, or brushes with metallic or conductive bristles
may be earth (hence the term ground), but more
make up most parts of the static ground system.
generally consists of a specific array of intercon-
b. Equipment grounds. An equipment ground
nected electrical conductors. The resulting circuit is
pertains to the interconnection and connection to
often referred to by several terms, such as: ground
earth of all normally non-current carrying metal
plane, ground grid, mat or ground system. Ground-
parts. This is done so the metal parts with which a
ing systems should be serviced as needed to ensure
person might come into contact are always at or
continued compliance with electrical and safety
near zero volts with respect to ground thereby pro-
codes, and to maintain overall reliability of the fa-
tecting personnel from electric shock hazards.
cility electrical system. Action must be initiated and
Equipment grounding consists of grounding all
continued to remove, or reduce to a minimum, the
noncurrent-carrying metal frames, supports and en-
causes of recurrent problem areas. When possible,
closures of equipment. All these metallic parts must
maintenance inspections should be performed at
be interconnected and grounded by a conductor in
times which have the least affect on user activities.
such a way as to ensure a path of lowest impedance
The complexity of ground systems and the degree of
for the flow of ground fault current from any line to
performance expected from such systems is growing
ground fault point to the terminal at the system's
source. An equipment grounding conductor nor-
all the time. Maintenance or shop personnel are
mally carries no current unless there is an insula-
encouraged to become familiar with Article 250 of
the National Electrical Code (NEC), which deals
tion failure. In this case the fault current will flow
back to the system source through the equipment
with grounding requirements and practices.
grounding conductors to protect personnel from
8-2. Types of grounding systems.
electrical shock. The equipment grounding conduc-
tor must never be connected to any other hot lines.
Six (6) types of grounding systems will be described.
Equipment grounding systems must be capable of
They are static grounds, equipment grounds, sys-
carrying the maximum ground fault current ex-
tem grounds, lightning grounds, electronic (includ-
pected without overheating or posing an explosion
ing computer) grounds and maintenance safety
hazard. Equipment grounds may be called upon to
grounds. All of these systems are installed similarly.
conduct hundreds to thousands of amperes at the
However, their purposes are quite different. Some of
line frequency during abnormal conditions. The sys-
the systems carry little or no current with no freed
tem must be sized and designed to keep the equip-
frequency. Others carry small to moderate currents
ment surface voltages, developed during such ab-
at 50 or 60 Hz. Still others must be able to carry
normal conditions, very low. An example of this
currents over a very broad range of frequencies in
system is the bare copper wire (green conductor)
order to be considered effective. Most grounding
connected to the frames of electric motors, breaker
system troubles are caused by one of two prob-
panels, outlet boxes, etc., see figure 6-1 for typical
lems: 1) loss of effectiveness due to poor mainte-
equipment grounding. Electrical supporting struc-
nance and, 2) inadequate ground system for the
tures such as metal conduit, metal cable trays or
degree of performance expected.
metal enclosures should be electrically continuous
a. Static grounds. A static ground is a connection
made between a piece of equipment and the earth
and bonded to the protective grounding scheme.
for the purpose of draining off static electricity
Continuous grounding conductors such as a metallic
charges before a spark-over potential is reached.
raceway or conduit or designated ground wires
The ground is applied for more than just the com-
should always be in from the ground grid system to
fort of the equipment operator. The possibility of an
downstream distribution switchboards to ensure
explosion ignited by an electrical spark must be
adequate grounding throughout the electrical distri-
considered. Dry materials handling equipment,
bution system. A typical grounding system for a
flammable liquids pumps and delivery equipment,
building containing significant electrical equipment
plastic piping systems, and explosives storage areas
and related apparatus is shown in figure 8-2, The
all need static ground protection systems installed
illustration shown depicts three most commonly en-
8-1





 


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