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 MIL-HDBK-419A
protection the grounding conductor shall be grounded on the source side of the first service disconnect to the
earth electrode subsystem and also to the ground terminal at the distribution transformer. For 3-phase wye
systems a five-wire service entry cable consisting of one neutral, one grounding, and three phase conductors
shall be employed. In either case, when a single transformer supplies power to a single building, the safety
ground (green wire) shall be grounded to the earth electrode subsystem at the supply side of the first service
disconnect of the facility as well as at the distribution transformer as shown in Figure 4-2. The neutral shall
also be grounded at both locations.
When a single transformer supplies power to more than one C-E building and if noise or hum is encountered in
C-E circuits or equipments, the neutral should be lifted or removed from ground at each service disconnect. In
this case the neutrals from each building are grounded at the distribution transformer only (see Figure 4-3).
To protect personnel from exposure to hazardous voltages, all exposed metallic elements of electrical and
electronic equipment are connected to ground with the green wire. Then, in the event of inadvertent contact
between the hot lead and chassis, frame, or cabinet through human error, insulation failure, or component
failure, a direct fault clearance path is established to quickly remove the hazard.
Grounding of a 3-phase wye power distribution system is accomplished similarly to the single phase system.
The connections for a typical system are shown in Figure 4-3. As in single phase systems, the neutral lead is
bonded to the green wire at the supply side of the first service disconnecting means and grounded to the earth
electrode subsystem as well as to the ground terminal at the distribution transformer. If one transformer
supplies power to more than one C-E building, the neutral is lifted from ground at the service disconnect.
A 3-phase system served by a transformer with a delta connected secondary will require the use of a grounding
transformer to ground the system and establish a neutral. The grounding transformer may be either a "zig-zag"
or "wye-delta" type, both of which have leads which are attached to each of three phases and a fourth lead
which is grounded and serves as the neutral. The typical connections for a grounding transformer are shown in
Figure 4-4.
Figure 4-2. Single-Phase 115/230 Volt AC Power Ground Connections
4-4
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