Schweitzer Engineering Laboratories SEL-749M - Table 4.8 Neutral Overcurrent Settings

4.11
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
Basic Motor Protection
When the harmonic distortion index exceeds the fixed threshold, which
indicates severe CT saturation, the phase overcurrent elements operate on the
output of the peak detector. When the harmonic distortion index is below the
fixed threshold, the phase overcurrent elements operate on the output of the
cosine filter.
The relay offers two types of ground fault detecting overcurrent elements. The
neutral overcurrent elements (50N1T and 50N2T) operate with current
measured by the IN input. The residual (RES) overcurrent elements (50G1T
and 50G2T) operate with the current derived from the phase currents (see
Figure 4.3).
When a core balance CT is connected to the relay IN input, as in Figure 2.11,
use the neutral overcurrent element to detect motor ground faults. Calculate
the trip and warn level settings based on the available ground fault current and
the core balance CT ratio.
Table 4.8 Neutral Overcurrent Settings
Setting Prompt Setting Range Setting Name := Factory Default
NEUT OC TRIP LVL Off, 0.01–25.00 A
a
a
Setting range is in primary amps.
50N1P := OFF
NEU OC TRIP DLAY 0.00–5.00 sec 50N1D := 0.50
NEUT OC WARN LVL Off, 0.01–25.00 A
a
50N2P := OFF
NEU OC WARN DLAY 0.0–120.0 sec 50N2D := 10.0
NOTE: Neutral OC trip level settings
are in primary amperes.
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