NCC National Control Centre
NOF Number of grid faults
NUM Numerical module
OCO cycle Open-close-open cycle
OCP Overcurrent protection
OLTC On-load tap changer
OTEV Disturbance data recording initiated by other event than
start/pick-up
OV Overvoltage
Overreach A term used to describe how the relay behaves during a
fault condition. For example, a distance relay is
overreaching when the impedance presented to it is
smaller than the apparent impedance to the fault applied
to the balance point, that is, the set reach. The relay “sees”
the fault but perhaps it should not have seen it.
PCI
Peripheral component interconnect, a local data bus
PCM Pulse code modulation
PCM600 Protection and control IED manager
PC-MIP Mezzanine card standard
PELV circuit Protected Extra-Low Voltage circuit type according to
IEC60255-27
PMC PCI Mezzanine card
POR Permissive overreach
POTT Permissive overreach transfer trip
Process bus Bus or LAN used at the process level, that is, in near
proximity to the measured and/or controlled components
PRP Parallel redundancy protocol
PSM Power supply module
PST Parameter setting tool within PCM600
PTP Precision time protocol
PT ratio Potential transformer or voltage transformer ratio
PUTT Permissive underreach transfer trip
RASC Synchrocheck relay, COMBIFLEX
RCA Relay characteristic angle
RISC Reduced instruction set computer
RMS value Root mean square value
Section 23 1MRK 505 393-UEN B
Glossary
442 Line differential protection RED650 2.2 IEC
Application manual
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