Quantcast High resistance grounding

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TM 5-683/NAVFAC M-116/AFJMAN 32-1083
plant operators to locate and isolate a ground fault
lective relaying determines the value of resistance
is of vital importance. To avoid high transient
to be used.
overvoltages, suppress harmonics and allow ad-
(b) High resistance grounding. High resis-
equate relaying, the grounding transformer and re-
tance grounding refers to a system in which the
neutral is grounded through a predominantly resis-
sistor combination is selected to allow current to
flow that equals or is greater than the capacitive
tive impedance whose resistance is selected to allow
a ground fault current through the resistor equal to
changing current.
d. Lightning grounds. Lightning grounds are de-
or slightly more than the capacitive charging cur-
signed to safely dissipate lightning strokes into the
rent of the system. Because grounding through a
earth. They are part of a lightning protection sys-
high resistance entails having a physically large
tem which usually consists of air terminals (light-
resistance that is both bulky and costly, high resis-
ning rods), down conductors, arresters and other
tance grounding is not practical and is not recom-
connectors of fittings required for a complete sys-
mended. However, high resistance grounding
tem. A lightning protection system's sole purpose is
through a grounding transformer is cost effective
to protect a building, its occupants and contents
and accomplishes the same objective. High resis-
from the thermal, mechanical and electrical effects
tance grounding accomplishes the advantage of un-
of lightning. Effective grounding for lightning
grounded and solidly grounded systems and elimi-
strokes is sometimes difficult to achieve because it
nates the disadvantages. It limits transient
is nearly impossible to predict the maximum dis-
overvoltages resulting from a single phase-to-
charge current. Currents from direct strikes can
ground fault by limiting ground fault current to
reach magnitudes of 100,000 amperes or more with
approximately 8A This amount of ground fault cur-
frequencies of tens to hundreds of kilohertz. Fortu-
rent is not enough to activate series overcurrent
nately, the event is very short, thus allowing most
protective devices, hence no loss of power to down-
properly sized and maintained systems to survive
stream loads will occur during ground fault condi-
the "hit".
tions. Special relaying must be used on a high resis-
tance grounded system in order to sense that a
(1) Requirements. Main lightning protection re-
quirement is dependent upon the height of the
ground fault has occurred. The fault should then be
building. According to NFPA 78-1986, there are two
located and removed as soon as possible so that if
classifications for a building. Class I is a building
another ground fault occurs on either of the two
with less than 75 feet height. The Class II building
unfaulted phases, high magnitude ground fault cur-
is higher than 75 feet or has a steel frame with any
rents and resulting equipment damage will not oc-
height. For further information about the lightning
cur. High resistance grounding is normally applied
protection code see NFPA 78-1986 which contains
in situations where it is essential to prevent un-
more detail.
planned system power outages, or previously the
e. Electronic and computer grounds. Grounding
system has been operated ungrounded and no
for all electronic systems, including computers and
ground relaying has been installed. Once the
computer networks, is a special case of the equip-
ground point has been established through the re-
ment ground and the system ground carefully ap-
sistor, it is easier to apply protective relays. The
plied. In fact, grounding systems for electronic
user may decide to add a ground overcurrent relay
equipment are generally the same as for system
ANSI/IEEE device 50/51G. The relay maybe either
ground with an additional requirement: the degree
current actuated using a current transformer or
voltage actuated using a potential transformer. De-
of performance required. Electronic equipment
pending on the priority of need, high resistance
grounding systems must not only provide a means
grounding can be designed to alarm only or provide
of stabilizing input power system voltage levels, but
direct tripping of generators off line in order to
also act as the zero voltage reference point. How-
prevent fault escalation prior to fault locating and
ever, the need to do so is not restricted to a low
removal. High resistance grounding (arranged to
frequency of a few hundred hertz. Grounding sys-
alarm only) has proven to be a viable grounding
tems for modem electronic installations must be
mode for 600V systems with an inherent total sys-
able to provide effective grounding and bonding
tem charging current to ground (31CO) of about
functions well into the high frequency megahertz
5.5A or less, resulting in a ground fault current of
range. Effective grounding at 50-60 Hz may not be
about 8A or less. This, however, should not be con-
effective at alI for frequencies above 100 kilohertz.
strued to mean that ground faults of a magnitude
(1) Requirements. There are several aspects to
below this level will always allow the successful
the requirement for good grounding performance for
location and isolation before escalation occurs.
electronic equipment; all of which are due to electri-
Here, the quality and the responsiveness of the
cal circuit behavior. Digital systems operate at high
8-7





 


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