38
11.6.2 Grounding configuration without input neutral
When the neutral line cannot be fed to the bypass input from the input source which is wye-connected,
the neutral terminal in the UPS cabinet must be connected to the ground bus in order to stabilize the UPS
potential as shown in Figure 11.10. Thus, the jumper cable should stay on. See Figure 11.9 for the
jumper cable location.
However, there will be circulating current between the UPS and the bypass, whenever the UPS transfers
to/from the bypass. And the current will flow through the ground path instead of neutral line like that in
Figure 11.8.
Figure 11.10 - Grounding configuration without input neutral.
If a ground fault interrupter is installed, the system is likely to trip the interrupter during transferring
to/from the bypass circuit. It is recommended to put 0.5 sec or longer delay on the ground fault
interrupter.
11.6.3 Recommended configuration with delta-input
If the input source has a delta-connection (3-phase 3-wire) without ground, the neutral terminal in the
UPS cabinet must be connected to the ground bus as shown in Figure 11.11.
So, the jumper cable should stay on. See Figure 11.9 for the jumper cable location.
Figure 11.11 - Grounding configuration with non-grounded delta-input.
N
G
3
φ
3
φ
3
φ
N
G
< UPS Cabinet >
N
3
φ
3
φ
G
Input Source
Keep the
Cable on
N
G
3
φ
3
φ
3
φ
N
G
< UPS Cabinet >
3
φ
3
φ
G
Input Source
Keep the
Cable on
G
N
3
φ
To the
Load
G
N
3
φ
To the
Load
