1-4
Introduction
Date Code 970103
SEL-251, -2, -3 Instruction Manual
by relay elements. Check for load encroachment and unbalance through instantaneous, demand,
and peak-demand measurements.
Access SEL-251 Relay Information via the SEL-RD Relay Display
You can connect up to four SEL-251 Relays to one SEL-RD Relay Display. Access relay target,
meter, status, fault history, and circuit breaker information via the relay display. You can even
change the active setting group via the display.
G
ENERAL
D
ESCRIPTION
: P
LUG
-I
N
C
ONNECTOR
M
ODEL
The General Description presented for the Conventional Terminal Block model relay fully
applies to the Plug-in Connector model. In addition, the following information applies strictly to
the Plug-in Connector model.
Wiring Harnesses
Custom wiring harnesses can be pre-wired, which enables quick and easy relay installation. The
plug-in connectors attach to dc power, ct and pt inputs, and contact inputs and outputs. Ct
secondaries are automatically shorted inside the plug-in connector when removed from the relay.
During in-service testing, spare connectors can be wired to auxiliary test equipment and plugged
directly into the relay. The actual source and I/O connectors may simply be unplugged before
each test and reconnected afterwards. If there is a need to replace a relay, the connectors can be
unplugged and reconnected to the new relay in a matter of minutes. There is no need for a wiring
check because the connections were verified at installation and no wiring was disturbed during
the replacement process.
High-Current Interrupting Output Contacts
High-current interrupting contacts are standard on the SEL-251 plug-in connector model. These
contacts use an electromechanical relay with solid state circuitry to interrupt dc current far in
excess of a typical contact output. No SCRs are employed in this circuitry. The circuit is
designed to make 30 Adc, carry 6 Adc, and interrupt 10 Adc. The circuit can interrupt 10 A four
times in one second, and then must be allowed to cool for two minutes to prevent thermal
damage.