7-2
Maintenance and Testing
Date Code 20000421
SEL-251, -2, -3 Instruction Manual
SEL
OGIC
TM
Control
Equations
SEL
OGIC
Control Equations allow you to isolate individual relay elements.
See the
SET n p
command description in
Section 3: Communications
for
more information.
Low-Level Test Interface
The SLP chassis SEL-251 Relay has low-level test interface between the calibrated input module
and the separately-calibrated processing module. You may test the relay in either of two ways:
conventionally, by applying ac current signals to the relay inputs; or by applying low magnitude
ac voltage signals to the low-level test interface. Access the test interface by removing the relay
front panel.
Note:
The Low-level Test Interface is not available on LP chassis SEL-251 Relays (1-amp
nominal current input models).
Figure 7.1 shows the interface connections. Remove the ribbon cable between the two modules
to access the outputs of the input module and the inputs to the processing module (relay main
board).
You can test the relay processing module using signals from the SEL-RTS Low-Level Relay Test
System. Never apply voltage signals greater than 20 volts peak-peak to the low-level test
interface. Figure 7.1 shows the signal scaling factors.
You can test the input module two different ways:
•
Measure the outputs from the input module with an accurate voltmeter, and compare the
readings to accurate instruments in the relay input circuits, or
•
replace the ribbon cable, execute the METER command, and compare the relay readings
to accurate instruments in the relay input circuits.
Figure 7.1: Low-Level Test Interface for the Conventional Terminal Block Model
On the Plug-In Connector model, the low-level interface uses a 34-position connector rather than
a 20-position connector. The input module output has been changed from J2 to J1. Pins 21 - 30
are no-connects, pins 31 and 32 are +15 Vdc, and pins 33 and 34 are -15 Vdc.
Test Methods
There are two means of determining the pickup and dropout of relay elements: target lamp
illumination and output contact closure.