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U.2.14
SEL-421 Relay
User’s Guide
Date Code 20090715
Installation
Plug-In Boards
I/O Interface Board Inputs
The INT1, INT5, and INT6 I/O interface boards have eight independent
control inputs. All independent inputs are isolated from other inputs. These
high-isolation control inputs are direct coupled and hence polarity-sensitive.
You cannot damage these inputs with a reverse polarity connection; though,
the relay will not detect input changes with a reverse-polarity input.
The INT3 and INT4 I/O interface board has two groups of nine (9) common
contacts (18 total) and six (6) independent control inputs. The INT2, INT7,
and INT8 I/O interface boards have eight independent control inputs. All
independent inputs are isolated from other inputs. These control inputs are
optoisolated and hence are not polarity sensitive, i.e., the relay will detect
input changes with voltage applied at either polarity, or ac signals (when
properly configured, see
is a comparison of the I/O board input capacities; the table also
shows the I/O inputs on Main Board A or Main Board B. See
for complete control input specifications.
.
I/O Interface Board Outputs
NOTE:
Form A control outputs
cannot be jumpered to Form B.
The I/O interface boards vary by the type and amount of output capabilities.
lists the outputs of the additional I/O interface boards; the table also
shows the I/O outputs on the main board. Information about the Standard and
Hybrid (high-current interrupting) control outputs is in
Table 2.3
I/O Interface Boards Control Inputs
Board Number
Independent Contact Pairs
Common Contacts
INT1
a
a
Main Board A, INT1, INT5, and INT6 control inputs are direct coupled and are polarity
sensitive.
8
INT2
b
b
Main Board B, INT2, INT3, INT4, INT7, and INT8 control inputs are optoisolated and are not
polarity sensitive.
8
INT3
6
Two sets of 9
INT4
6
Two sets of 9
INT5
8
INT6
8
INT7
8
INT8
8
Main Board A
5
2
Main Board B
5
2
!
CAUTION
!
CAUTION
Substation battery systems that have
either a high resistance to ground
(greater than 10 k
Ω
) or are
ungrounded when used in conjunction
with many direct-coupled inputs can
reflect a dc voltage offset between
battery rails. Similar conditions can
exist for battery monitoring systems
that have high-resistance balancing
circuits or floating grounds. For these
applications, SEL provides optional
ground-isolated (optoisolated)
contact inputs. In addition, SEL has
published an application advisory on
this issue. Contact the factory for
more information.
NOTE:
The SEL-421-0 and the
SEL-421-1 do not support Main Board B
I/O and INT2, INT3, INT7, and INT8 I/O
interface boards. See
Versions and Supported Features on
page U.1.7
for details.
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