ABB Relion 650 Series Communication Protocol Manual page 62

Section 6
Glossary
3I
O
3U
O
56
Three times zero-sequence current. Often referred to as the residual or
the earth-fault current
Three times the zero sequence voltage. Often referred to as the
residual voltage or the neutral point voltage
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1MRK 511 243-UEN A
IEC 60870-5-103, 650 series
Communication protocol manual