Custom Search
|
|
|
||
 MIL-HDBK-419A
2.1.2 Fault Protection. In the event of transformer failure (e.g., disconnect between neutral and ground or
line to ground faults) or any failure between the service conductor(s) and grounded objects in the facility, the
earth electrode subsystem becomes a part of the return path for the fault current. A low resistance assists in
fault clearance; however, it does not guarantee complete personnel protection against hazardous voltage
gradients which are developed in the soil during high current faults. Adequate protection generally requires the
use of ground grids or meshes designed to distribute the flow of current over an area large enough to reduce the
voltage gradients to safe levels. The neutral conductor at the distribution transformer must therefore be
connected to the earth electrode subsystem to ensure that a low resistance is attained for the return path.
(Paragraph 5.1.1.2.5.1 of MIL-STD-188-124A refers.) Ground fault circuit interrupters on 120 volt single phase
15 and 20 ampere circuits will provide personnel protection against power faults and their use is therefore
highly recommended.
2.1.3 Noise Reduction. The earth electrode subsystem is important for the minimization of electromagnetic
noise (primarily lower frequency) within signal circuits caused as a result of stray power currents. For example,
consider a system of two structures located such that separate earth electrode subsystems are needed as shown
in Figure 2-1. If stray currents (such as may be caused by an improperly grounded ac system, dielectric
leakage, high resistance faults, improperly returned dc, etc.) are flowing into the earth at either location, then
a voltage differential will likely exist between the grounding networks within each facility.
Currents originating from sources outside the structures can also be the cause of these noise voltages. For
example, high voltage substations are frequent sources of large power currents in the earth. Such currents arise
from leakage across insulators, through cable insulation, and through the stray capacitance which exists
between power lines and the earth. These currents flowing through the earth between the two sites will
generate a voltage difference between the earth connections of the two sites in the manner illustrated by
Figure 2-2.
Any interconnecting wires or cables will have these voltages applied across the span which will cause currents
to flow in cable shields and other conductors. As shown in Chapter 6, such intersite currents can induce
common-mode noise voltages into interconnected earth electrode subsystems.
2.1.4 Summary of Requirements. Table 2-1 summarizes the purpose, requirements, and resulting design
factors for earth connections of the lightning protection subsystem, the fault protection subsystem, the signal
reference subsystem, and the ac distribution system neutral (grounded) conductor and safety ground (grounding)
conductor. Refer to Article 100 - Definitions of the NEC for additional information on grounding and grounded
conductors (2-2).
2-2
|
 
|
|
|
||
