S&C Instruction Sheet 662-510
19
Switching with Uni-Rupter
®
Interrupters
CAUTION
Ferroresonant overvoltages may occur when switching
unloaded or lightly loaded three-phase transformers
(or banks of three single-phase transformers) or single-
phase transformers connected phase-to-phase. The
excessive voltages caused by ferroresonance can damage
transformers, cables, and secondary low-voltage loads.
Ferroresonant overvoltages can also result in an external
arc across the Uni-Rupter Interrupter when it is switched
open, followed by a high-current fault with its associated
arc-flash hazard.
To minimize the possibility that ferroresonant overvoltages
will occur, only switch three-phase loads with the Uni-Rupter
Interrupter where each phase is carrying 5% or more of the
rated connected load current. Do not switch unloaded three-
phase transformers with the Uni-Rupter Interrupter.
System Voltage
Minimum Load Current to Prevent Ferroresonant Overvoltages, Amperes
Total Connected kVA
Up to 600 kVA
750 kVA through 1250 kVA
1500 kVA through 2500 kVA
12 through 15
2
4
8
17 through 27
1
3
5
Figure 22. Closing the fuse with a swift, unhesitating stroke. The upper left inset shows a close-up of a Grappler tool in the
preferred position to close the fuse. The upper right inset shows a close-up of a Grappler tool in an acceptable position to close
the fuse. The lower right inset shows a close-up of a Uni-Rupter Interrupter with the fuse fully closed.
Uni-Rupter
Preferred
Acceptable
A Note on Single-Pole Switching
