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 MIL-HDBK-419A
CHAPTER 4
FAULT PROTECTION SUBSYSTEM
4.1 FAULT PROTECTION. For effective fault protection, a low resistance path must be provided between
the location of the fault and the transformer supplying the faulted line. The resistance of the path must be low
enough to cause ample fault current to flow and rapidly trip breakers or blow fuses. The necessary low
resistance return path inside a building is provided by the grounding (green wire) conductor and the
interconnected facility ground network. An inadvertent contact between energized conductors and any
conducting object connected to the grounding (green wire) conductor will immediately trip breakers or blow
fuses.
In a building containing a properly installed third-wire grounding network, as prescribed by
MIL-STD-188-124A, faults internal to the building are rapidly cleared regardless of the resistance of the earth
connection.
4.1.1 Power System Faults.
A power system fault is either a direct short or an arc (continuous or intermittent) in a power distribution
system or its associated electrical equipment. These faults are hazardous to personnel for several reasons:
Fault currents flowing in the ground system may cause the chassis of grounded equipment to be at a
a.
hazardous potential above ground.
The energy in a fault arc can be sufficient to vaporize copper, aluminum, or steel. The heat can
b.
present a severe burn hazard to personnel.
There is a fire hazard associated with any short circuit or arc.
c.
Burning insulation can be particularly hazardous because of the extremely toxic vapors and smoke
d.
which may be produced.
Some common causes of electrical system faults are:
Rodents getting between ground and phase conductors.
a.
Water infiltration.
b.
Moisture in combination with dirt on insulator surfaces.
c.
Breakdown of insulation caused by thermal cycling produced by overloads.
d.
Environmental contaminants.
e.
Damage during installation.
f.
System age deterioration.
g.
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