Custom Search
|
|
|
||
 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
Figure 8-2. Typical grounding system for a building and its apparatus.
(d) Where a grounded service conductor is
tided by the number of ohms of resistance used. The
uninsulated in accordance with the NEC Exceptions
grounding resistances are rated in terms of current
to Sections 230-22, 230-30 and 230-41.
and its duration for different voltage classes.
(3) Resistance grounded system. Limiting the
(a) Low resistance grounding. Low resistance
available ground fault current by resistance
grounding refers to a system in which the neutral is
grounding (fig 6-5) is an excellent way to reduce
grounded through a small resistance that limits
damage to equipment during ground fault condi-
ground fault current magnitudes. The size of the
tions, and to eliminate personal hazards and electri-
grounding resistor is selected to detect and clear the
cal fire dangers. It also limits transient overvoltages
faulted circuit. Low resistance grounding is not rec-
during ground fault conditions. The resistor can
ommended on low-voltage systems. This is primar-
limit the ground fault current to a desired level
ily because the limited available ground fault cur-
based on relaying needs. At the occurrence of a
rent is insufficient to positively operate series trip
line-to-ground fault on a resistance grounded sys-
units and fuses. These trip units and fuses would be
tem, a voltage appears across the resistor which
dependent upon both phase-to-phase and phase-to-
nearly equals the normal line-to-neutral voltage of
ground fault protection on some or all of the distri-
the system. The resistor current is essentially equal
bution circuits. Low resistance grounding normally
to the current in the fault. Therefore, the current is
limits the ground fault currents to approximately
practically equal to the line-to-neutral voltage di-
100-600A. The amount of current necessary for se-
8-4
|
 
|
|
|
||
