Date Code 20000421 Specifications 2-21
SEL-251, -2, -3 Instruction Manual
Table 2.3: Setting Group Selection Input Truth Table
SS3 SS2 SS1
0 0 0 GROUP Command Selection
0 0 1 Group 1
0 1 0 Group 2
0 1 1 Group 3
1 0 0 Group 4
1 0 1 Group 5
1 1 0 Group 6
1 1 1 GROUP Command Selection
If SS1, SS2, or SS3 is not assigned to an input, it defaults to 0. If no inputs are assigned as set-
ting group selection inputs the GROUP command entry controls group selection. With only SS1
assigned, GROUP command selection determines settings to use if the input assigned to SS1 is
not asserted. If the input is asserted, setting Group 1 is used.
For example, to switch between Group 1 and Group 5, program input IN1 to SS1 and use the
GROUP command to select Group 5. With IN1 asserted, the relay uses Group 1. With IN1
deasserted, it uses Group 5.
When the status of any assigned setting group selection input changes, the relay waits a settable
time period (TGR) for inputs to stabilize before changing the active setting group. Thus, if a set-
ting group selection input status changes and reverts to its previous state before TGR expires, the
relay does not change the active setting group. The TGR setting is one of several global settings
and is entered with the SET G command.
Active setting group changes (via setting group selection inputs or GROUP command) disable
the relay for less than 0.5 seconds to allow loading of new active settings. The ALARM output
contacts close during this time and all timers and relay elements reset.
The DEMR setting allows you to specify whether or not demand values for current, MW, and
MVAR are reset when the active setting group changes. The relay resets demand values as it
would for METER RD execution. The following example illustrates a situation when you should
reset demand values.
You might want to change the active setting group for distribution feeder switching where sig-
nificant load is removed from the feeder. If the new active setting group has lower demand
current thresholds (PDEM, QDEM, and NDEM settings) than the previous active setting group,
the corresponding PDEM, QDEM, and NDEM demand ammeter threshold bits could assert.
This is because the respective demand ammeters have not yet adjusted to the lower loading level,
as dictated by the relatively long demand ammeter time constant (setting DATC = 5 - 60 min-
utes). If PDEM, QDEM, and NDEM are assigned to programmable output contacts (A1 - A4),
a false alarm would result. To overcome this problem, set DEMR = Y. With this setting, the
relay resets demand values to zero when the active setting group changes.
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