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Configuring Your Neutral and Ground Connections
16
6.3 Preferred Grounding Configuration With Isolated Bypass
Another configuration in this power range is the Multi-Module System with 480 or 600 VAC input,
208 VAC output, a Bypass Isolation Transformer and a connected load consisting of multiple distribu-
tion panelboards or switchboards. Figure 6 shows a typical installation.
The Bypass Transformer provides isolation and may step down the voltage to the bypass input. The
Bypass Transformer and the SCC together constitute a separately derived system, since there is no
direct electrical connection between the input (service entrance) circuit conductors and the output cir-
cuit conductors.
The bonding of the neutral to the grounding conductor can theoretically be done at either the SCC or
the Bypass Transformer. However, we recommend bonding at the Bypass Transformer because the
UPS module will sometimes be powered down for maintenance and its output transformer will be out
of the circuit. The neutral should be bonded to ground and a local grounding electrode should be
installed at the Bypass Transformer, per NEC 250-30.
Figure 6 Preferred grounding configuration with isolated bypass
Features of this configuration include:
• The UPS receives its bypass neutral from the Bypass Transformer
• The output is isolated from the input circuit conductors, and
• Some amount of common-mode noise attenuation can be obtained for sensitive loads if the
UPS module and Bypass Transformer are located close to sensitive loads.
NOTE
Figure 6 shows a wye-connected source, but the same grounding scheme would apply for a
delta source at the service entrance.