Sel SEL-787 - Figure 4.52 Schematic Diagram of a Traditional Latching Device; Figure 4.53 Logic Diagram of a Latch Switch

4.71
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Logic Settings (SET L Command)
Latch Bits
Latch control switches (latch bits are the outputs of these switches) replace
traditional latching devices. Traditional latching devices maintain output
contact state. The SEL-787 latch control switch also retains state even when
power to the device is lost. If the latch control switch is set to a programmable
output contact and power to the device is lost, the state of the latch control
switch is stored in nonvolatile memory, but the device de-energizes the output
contact. When power to the device is restored, the programmable output
contact will go back to the state of the latch control switch after device
initialization. Traditional latching device output contact states are changed by
pulsing the latching device inputs (see Figure 4.52). Pulse the set input to
close (set) the latching device output contact. Pulse the reset input to open
(reset) the latching device output contact. The external contacts wired to the
latching device inputs are often from remote control equipment (e.g., SCADA,
RTU).
Figure 4.52 Schematic Diagram of a Traditional Latching Device
Thirty-two latch control switches in the SEL-787 provide latching device
functionality. Figure 4.53 shows the logic diagram of a latch switch. The
output of the latch control switch is a Relay Word bit LTn (n = 01–32), called
a latch bit.
Figure 4.53 Logic Diagram of a Latch Switch
If setting SETn asserts to logical 1, latch bit LTn asserts to logical 1. If setting
RSTn asserts to logical 1, latch bit LTn deasserts to logical 0. If both settings
SETn and RSTn assert to logical 1, setting RSTn has priority and latch bit LTn
deasserts to logical 0. You can use these latch bits in SEL
OGIC control
equations to create custom logic for your application.
The SEL-787 includes 32 latches. Table 4.28 shows the SET and RESET
default settings for Latch 1 through Latch 4. The remaining latches are all set
to NA.
Set
Input
Reset
Input
Output
Contact
Traditional
Latching
Relay
(+)
(–)
LTn
SETn
RSTn
(Set)
(Reset)
(n = 01 through 32)
Relay
Word
Bits
SELOGIC
Setting