Quantcast Grounding - mo9120010

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TM 5-685/NAVFAC MO-912
longer insulation life of generators, motors, trans-
given load; higher voltage, unity power factor, low
formers, and other system components by suppress-
resistance/impedance, and lower frequency gener-
ally result in lower distribution losses. Use of equip-
ing transient and sustained overvoltages associated
----
ment to change or regulate voltage, frequency or
with certain fault conditions. In addition, system
phase introduces resistance, hysteresis and me-
grounding improves protective relaying by provid-
ing fast, selective isolation of ground faults.
chanical losses.
b. A lagging power factor due to inductive loads
b. Equipment grounding, in contrast to system
grounding,  relates to the manner in which
(especially under-loaded induction motors) results
noncurrent-carrying metal parts of the wiring sys-
in resistive losses
because greater current is
tem or apparatus, which either enclose energized
required for a given power level. This may be cor-
conductors or are adjacent thereto, are to be inter-
rected by the use of capacitors at the station bus or
connected and grounded. The objectives of equip-
by "run" capacitors at induction motors to have the
ment grounding are:
generator "see" a near-unity but yet lagging power
factor.
(1) To ensure freedom from dangerous electric
c. Overcorrection, resulting in a leading (capaci-
shock-voltage exposure to persons.
tive) power factor must be avoided. This condition
(2) To provide current-carrying capability dur-
ing faults without creating a fire or explosive haz-
results in severe switching problems and arcing at
contacts. Switching transients (voltage spikes, har-
ard.
monic transients) will be very damaging to insula-
(3) To contribute to superior performance of the
tion, controls and equipment. The electronics in ra-
electric system.
dio, word and data processing, and computer arrays
c. Many personal injuries are caused by electric
are especially sensitive to switching and lighting
shock as a result of making contact with metallic
transients, over/under voltage and frequency
members that are normally not energized and nor-
mally can be expected to remain non-energized. To
changes.
d. The distribution system must include sensing
minimize the voltage potential between noncurrent-
devices, breakers, and isolation and transfer feed
carrying parts of the installation and earth to a safe
switches to protect equipment and personnel.
value under all systems operations (normal and ab-
normal), an installation grounding plan is required.
2-7. Frequency.
d. System grounding. There are many methods of
The frequency required by almost all electrical
system grounding used in industrial and commer-
cial power systems (refer to fig 2-2), the major ones
loads is the standard 50 or 60 Hz. Most electrical
equipment can operate satisfactorily when the fre-
being:
quency varies plus or minus ten percent
(1) Ungrounded.
(2) Solidly grounded.
Steady state frequency tolerance (required for
frequency-sensitive electronic equipment) should
(3) Resistance grounding: low-resistance, high-
not exceed plus or minus 0.5 percent of design fre-
resistance.
quency. Since some equipment are sensitive to fre-
(4) Reactance grounding.
quency changes, operators must closely monitor fre-
e. Technically, there is no generally accepted use
quency meters and regulate frequency when
of any one particular method. Each type of system
grounding has advantages and disadvantages. Fac-
necessary.
tors which influence the choice of selection include:
2-8. Grounding.
(1) Voltage level of the power system.
Grounding implies an intentional electrical connec-
(2) Transient overvoltage possibilities.
tion to a reference conducting plane, which may be
(3) Type of equipment on the system.
earth (hence the term ground) but more generally
(4) Cost of equipment.
consists of a specific array of interconnected electri-
(5) Required continuity of service.
cal conductors referred to as grounding conductors.
(6) Quality of system operating personnel.
The term "grounding" as used in electric power sys-
(7) Safety considerations, including fire hazard
tems indicates both system grounding and equip-
and others
ment grounding, which are different in their objec-
f. An ungrounded system is a system in which
there is no intentional connection between the neu-
tives.
a. System grounding relates to a connection from
tral or any phase and ground. "Ungrounded system"
literally implies that the system is capacitively
the electric power system conductors to ground for
coupled to ground.
the purpose of securing superior performance quali-
ties in the electric system. There are several meth-
(1) The neutral potential of an ungrounded sys-
ods of system grounding. System grounding ensures
tern under reasonably balanced load conditions will
2-4





 


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