
30 SEBU8088-06
Product Information Section
Product Identification Information
Grounding the Frame
In any generator set installation, the frame of the
generator must be positively connected to an earth
ground or to the hull of a vessel. This connection is
the first connection that is made at the installation.
This connection is the last connection that should be
removed. If the generator set is on flexible pads or on
resilient pads, the ground connection must be flexible
in order to avoid possible breakage in later operation.
Ground connection cable or ground connection straps
should have at least the current carrying capacity of
the largest line lead to the connected load. Joints in
cables or in straps must be clean, free of electrical
resistance, and protected from possible oxidation.
Bolted ground connection joints eventually oxidize.
The joints are frequent sources of radio frequency
interference (RFI). Silver soldered joints and bolted
joints are electrically and mechanically sound.
Neutral Connections
The generators with Wye Configuration usually
have the neutral ground when the generator is
installed. However, there a re some cases when
definite measures should be taken in order to
prevent grounds on the load side. The purpose of
the grounding of the neutral is to prevent load side
equipment damage.
If the neutral wire is grounded and one of the phase
leads becomes grounded, the excessive current will
open a load circuit breaker. The excessive current
will also collapse the generator voltage. The result
depends on the following items: particular generator
electrical characteristics, type of fault, and trip rating
of the circuit breaker. An undervoltage device may be
required in order to provide an adequate short circuit
protection.
There are some instances in which it is undesirable
to ground the neutral wire. In other applications, it is
acceptable to have an ungrounded generator neutral
lead. Definite measures should be taken in such
applications in order to prevent grounds to the phase
leads. An example of such measures are ground fault
protective circuits. Ground fault protection requires
that the entire group of distribution circuits should
be studied. The entire group of distribution circuits
should be treated as a system. The owner should
engage a certified, registered consultant if a new
distribution system is being developed. The owner
should also engage a certified, registered consultant
if an existing system should be modified for the
ground fault protection.
Single Units
In a three-phase, four-wire system, the neutral wire
should be grounded according to local wiring codes.
In application
s in which definite measures are taken
in order to prevent grounds to the load leads, an
ungrounded neutral can be used. Be sure to check
your local wir
ing codes.
Multiple Unit
s
Operation of multiple generators in parallel that have
all neutrals
grounded, may result in the circulating
current through the neutral connections. In order
to eliminate the possibility of circulating currents,
ground the ne
utral of only one generator. If multiple
generators are alternated on line, a switch should
be installed in the neutral ground circuit of each
generator.
In this case, all neutral ground circuits
except one circuit can be opened. Be sure that one
of the neutral ground circuits is closed.
Parallel to Utility
When a Wye connected generator is going to operate
in parallel with a utility system (infinite bus) and
when the se
condary of the step-down transformer
in the utility system is also a Wye connection, the
following may happen. The grounding of both Wye
neutrals m
ay result in circulating currents through
the neutrals. Also, the coordination of ground fault
protection requires an entire system study. This study
should be
done by a certified, registered consultant
who is familiar with generator systems. The study will
determine which grounding method should be used.
i01553204
Voltage C onnections
SMCS Code: 4450
Three-Phase Voltage Connections
The Wye Configuration for a 480 V generator and the
Delta Configuration for a 240 V generator are given
in the following diagrams.
The terminals must be connected securely. The
terminals must also be insulated with a good quality
electrical tape.
If a Delta Configuration with grounded neutral is being
used, a two-thirds winding pitch is recommended. If
the generator winding pitch is not two-thirds, third
harmonics will cause a high neutral current. This
current can damage the wiring, which may cause
a fire h azard. A floating neutral does not require a
two-thirds pitch winding.