6-6 Installation Date Code 20000421
SEL-251, -2, -3 Instruction Manual
The actual IRIG-B input circuit is a 56 ohm resistor in series with an optocoupler input diode.
The input diode has a forward drop of about 1.5 volts. Driver circuits should put approximately
10 mA through the diode when "on."
The IRIG-B serial data format consists of a one second frame containing 100 pulses and divided
into fields. The relay decodes the second, minute, hour, and day fields and sets the internal relay
clock accordingly.
When IRIG-B data acquisition is activated either manually (with the IRIG command) or auto-
matically, two consecutive frames are taken. The older frame is updated by one second and the
two frames are compared. If they do not agree, the relay considers the data erroneous and
discards it.
The relay reads the time code automatically about once every five minutes. The relay stops
IRIG-B data acquisition ten minutes before midnight on New Year's Eve so the relay clock may
implement the year change without interference from the IRIG-B clock. Ten minutes later, the
relay restarts IRIG-B data acquisition.
INSTALLATION: PLUG-IN CONNECTOR MODEL
Mounting
The relay is designed for mounting by its front vertical flanges in a 19" vertical relay rack. It
may also be mounted semi-flush in a switchboard panel. Use four #10 screws for mounting.
This section includes front and rear panel drawings.
Frame Ground Connection
Terminal 46 on the rear panel must be connected to frame ground for safety and performance.
These terminals connect directly to the chassis ground of the instrument.
Power Connections
Terminals 44 and 45 on the rear panel must be connected to a source of control voltage. Control
power passes through these terminals to the fuse(s). The power continues through a surge filter
and connects to the switching power supply. The control power circuitry is isolated from the
frame ground. A power connector is supplied with the wiring harness to connect to terminals 44
and 45. The connector is available with #18 - #14 AWG conductor.
Secondary Circuits
The relay presents a very low burden to the secondary potential and current circuits. It requires
three currents from the power system current transformer secondaries.
The current connector supplied with the wiring harness is a shorting type connector. When
removed from the relay, the connector automatically shorts CT secondaries. The connector can
be ordered with #16 - #10 AWG conductors.
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