3.7
Date Code 20110408 Instruction Manual SEL-551 Relay
Relay Elements and Logic
Remote Control Switches
Local Bit States
Retained When Power
Is Lost or Settings
Changed
Power Loss
The states of the local bits (Relay Word bits LB1–LB8) are retained if power
is lost to the relay and then it is restored. If a local control switch is in the ON
position (corresponding local bit is asserted to logical 1) when power is lost, it
will come back in the ON position (corresponding local bit is still asserted to
logical 1) when power is restored. If a local control switch is in the OFF
position (corresponding local bit is deasserted to logical 0) when power is lost,
it will come back in the OFF position (corresponding local bit is still deasserted
to logical 0) when power is restored. This is akin to a traditional installation
with front-panel control switches. If power is lost to the panel, the front-panel
control switches remain in position.
Settings Change
If relay settings are changed, the states of the local bits (Relay Word bits LB1–
LB8) are retained, much like in the Power Loss explanation. The exception is
if a new local control switch is configured as an OFF/MOMENTARY switch.
Then, the corresponding local bit is forced to start at logical 0 after the settings
change, regardless of the local bit state before the settings change.
If the local control switch is made inoperable because of a settings change, the
corresponding local bit is fixed at logical 0, regardless of the local bit state
before the settings change.
Remote Control Switches
Remote control switches are operated via the serial communications port only
(see CON Command (Control) on page 5.22).
The output of the remote control switch in Figure 3.3 is a Relay Word bit
(remote bit RBn, n = 1–8). These remote bits are used in SEL
OGIC control
equations.
Any given remote control switch can be put in one of the following three
positions:
➤ ON (logical 1)
➤ OFF (logical 0)
➤ MOMENTARY (logical 1 for one processing interval)
With SEL
OGIC control equations, the remote bits can be used in applications
similar to those that local bits are used in (see Local Control Switches on
page 3.5).
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