Date Code 20000421 Installation 6-7
SEL-251, -2, -3 Instruction Manual
The four-wire wye potential from potential transformer secondaries is optional. It is used for
fault locating, voltage and MW/MVAR metering, and undervoltage torque control.
A PT connector is supplied with the wiring harness. The connector can be ordered with #18 -
#14 AWG conductors.
Control Circuits
The control inputs are dry. For example, to assert the IN5 input, you must apply control voltage
to the IN5 input terminals. Each input is individually isolated, and a terminal pair is brought out
for each input. There are no internal connections between control inputs.
Control outputs are dry relay contacts rated for 30 Adc make, 6 Adc carry, and 10 Adc interrupt
with an L/R = 40 ms at 125 Vdc. The contacts can interrupt 10 Adc, L/R = 40 ms, 125 Vdc four
times in one second after which the contact must be allowed to cool for two minutes. Each
contact is protected by a metal oxide varistor. Note, the contacts are polarity sensitive.
Communications Circuits
Connections to the two EIA-232 serial communications ports are made via the two 9-pin
connectors labeled Port 1 and Port 2R on the rear panel and Port 2F on the front panel. Pins 5
and 9 connect directly to frame (chassis) ground. See Table 3.2 for pin assignment.
Warning: Do not rely upon pins 5 and 9 for safety grounding, since their current-carrying
capacity is less than control-power short circuit current and protection levels.
The communications circuits are protected by low-energy, low-voltage MOVs and passive RC
filters. You can minimize communications-circuit difficulties by keeping the length of the
EIA-232 cables as short as possible. Lengths of twelve feet or less are recommended, and the
cable length should never exceed 100 feet. Use shielded communications cable for lengths
greater than ten feet. Modems are required for communications over long distances.
Route the communications cables well away from the secondary and control circuits. Do not
bundle the communications wiring with secondary or control circuit wiring. If these wires are
bundled, switching spikes and surges can cause noise in the communications wiring. This noise
may exceed the communications logic thresholds and introduce errors. The IRIG-B clock cable
should also be routed away from the control wiring and secondary circuits.
Jumper Selection
Jumpers JMP103, JMP104, and J6 are on the front edge of the main board. They are easily
accessed by removing the top cover or front panel. Jumper JMP3 is toward the back of the main
board and is accessed by removing the top cover.
EIA-232 Jumpers
J6 provides EIA-232 baud rate selection. Available baud rates are 300, 600, 1200, 2400, 4800,
and 9600. To select a baud rate for a particular port, place the jumper so it connects a pin
labeled with the desired port to a pin labeled with the desired baud rate.
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