Quantcast Ground fault interrupting methods

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TM 5-683/NAVFAC M-116/AFJMAN 32-1083
maintenance personnel from an inadvertent re-
entrance ground bus. The input power system neu-
energization of the circuit. The ground is removed
tral must also be grounded only at the service en-
tier maintenance operations have been completed.
trance. The isolated ground system stops loop cur-
Application of a maintenance ground is discussed in
rents and common mode electrical noise, but has
more detail in paragraph 12-2d.
several major disadvantages: The long branch
g. Ground system tests. Periodic testing should be
feeder and isolated ground conductors are effective
done to assure grounding system effectiveness. The
only for low data transfer frequencies (fig 8-6c.
following are points that should be addressed dur-
High voltages occur between the conductors during
ing inspection and maintenance:
surges. It is also a very difficult system to inspect
and maintain. Frequency inspections must be made
(1) Inspect and test single point, isolated
ground systems after every electrical system modi-
to ensure the system has not been defeated by inad-
fication. visually inspect outlets and panels for con-
vertent or deliberate installation of a jumper or
ductors forming loops between the equipment
conductor between the two systems. Inspections
ground and the isolated ground.
and tests on this type of grounding must be carried
out after each electrical system modification. It is
(2) Test the ground to neutral voltage at each
power distribution panel included in the particular
best not to use isolated ground systems at all unless
system. The voltage should be taken using a high
forced to by the equipment manufacturers. It is also
impedance AC voltmeter and an accurate record
best to restrict such systems to small areas or only
should be kept. The voltage should be very low; on
one floor of the building.
the order of l0-150 millivolts (0.01-0.150V). Any
(6) Electronic system grounding. Good elec-
sudden changes or increasing trends should be in-
tronic system grounding performance is achieved
vestigated and the cause corrected.
with a properly laid out distribution of multipoint,
well-bonded grounding connections. This system
(3) The made electrode, rod, plate, or selected
ground body contact point should be tested every
can use bare, braided, sheet, or stranded copper
12-24 months. A record should be kept. Any increas-
conductors for grounding or bonding functions. This
ing impedance indicates need for remedial action.
system requires conduit and equipment enclosure
bonding at all junction points. In other words,
8-3. Ground fault interrupting methods.
simple metallic contact between the enclosures, wir-
ing conduits, and power panels is not enough. The
Ground faults result when an electrical components
insulation deteriorates allowing an above normal
multipoint bonding provides low impedance ground-
current leakage to ground. Minute current leakage
ing for the electronic equipment. The low imped-
may normally occur from virtually every electrical
ance between the separate items of electronic equip
device. Ground faults become dangerous when an
ment keeps the noise voltages at or near zero
unintended ground return path becomes estab-
between them and, therefore, provides an
lished. This ground return path could be through
"equipotential plane". This system is much easier to
the normal electrical components and hardware
inspect and test. No special requirements must be
met during modifications or expansion of the elec-
(equipment ground for instance), conductive mate-
rial other than the system ground (metal, water,
trical system. All power panels and all supply trans-
plumbing, pipes, etc.), a person or, any combination
formers feeding an installation with this type of
of the above. Ground fault leakage currents of much
grounding system must be grouped and bonded to-
lower levels than is needed to trip conventional cir-
gether using short lengths of bare, braided, sheet, or
cuit breakers can be hazardous. Therefore, to re-
stranded copper conductors in order to achieve the
duce the possibility of fire, injury, or fatality, the
effective high frequency grounding performance de-
NEC requires additional ground fault protection for
scribed above. As shown in figure 8-6d, a single
certain types of circuits. Ground fault protective
area of power entry with a large equipotential
devices are of two distinct types: ground fault cir-
ground plane and short equipment grounding con-
ductors forms the preferred grounding system for
cuit interrupters and ground fault protectors. It is
large automated data processing (ADP) and com-
extremely important to understand the difference
puter applications.
between them.
f. Maintenance safety grounds. Grounds used for
a. Ground fault circuit interrupters (GFI). A GFI
maintenance work are usually intentional, but tem-
is designed to protect a person from electrocution
porary, connections between equipment power con-
when contact between a live part of the protected
ductors and ground. These connections are always
circuit and ground causes current to flow through a
applied after the power source has been turned off
person's body. A GFI will disconnect the circuit
and the circuit(s) have been tested and are known to
when a current equal to or higher than the calibra-
be de-energized. The ground is intended to protect
tion point (4 to 6 mA) flows from the protected
8-9





 


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