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 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. An ungrounded generator
neutral lead is acceptable in the applications in which
definite measures have been taken 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 and treated
as a system. The owner should engage a certified
and registered consultant if a new distribution system
is being developed. The owner should also engage a
certified and 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 applications, 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 wiring codes.
Multiple Units
Operation of multiple generators in parallel, having all
neutrals grounded, may result in the circulating
current through the neutral connections. In order to
eliminate the possibility of circulating currents,
ground the neutral 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 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 secondary of the step-down transformer in the
utility system is also a Wye connection, the following
may happen. The grounding of both Wye neutrals
may 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 and registered
consultant who is familiar with generator systems.
The study will determine which grounding method
should be used.
i01172156
Voltage Connections
SMCS Code: 4450
Three-Phase Voltage Connections
The Wye Configuration for a 480 V generator and the
Delta Configuration 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 and is required with 590 frame size
generators and above. If the generator winding pitch
is not two-thirds, third harmonics will cause a high
neutral current. This current can damage the wiring
and cause a fire hazard. A floating neutral does not
require a two-thirds pitch winding.
SEBU7125-13
27
Product Information Section
Voltage Connections
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