7-12 Maintenance and Testing Date Code 20011026
SEL-321/321-1 Instruction Manual
Using a Breaker Simulator
If the relay setting EN52A = Y, the relay Switch-Onto-Fault (SOTF) logic operates based on the
state of the relay 52A input. When EN52A = Y, we recommend testing the relay with a latching
relay to simulate line breaker auxiliary contact action. This ensures proper assertion and
deassertion of the 52A input on the back panel. If you do not have a means of simulating breaker
status, disable 52A-based SOTF protection by:
• Setting EN52A = N,
• Setting the MTO logic equation to N/A,
• or disabling Switch-Onto-Fault protection by setting ESOTF = N.
Low-Level Test Interface
The SEL-321 Relay has a 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.
Figure 7.2 shows the low-level interface connections. This drawing also appears on the inside of
the relay front panel. 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 9 volts peak-peak to the low-level test
interface. Figure 7.2 shows the signal scaling factors.
CAUTION
!
The relay contains devices sensitive to Electrostatic Discharge (ESD).
When working on the relay with front or top cover removed, work
surfaces and personnel must be properly grounded or equipment
damage may result.
You can test the input module two different ways:
1. Measure the outputs from the input module with an accurate voltmeter (measure signal
pin to GND pin), and compare the readings to accurate instruments in the relay input
circuits, or
2. Replace the ribbon cable, press the front-panel
<METER> button, and compare the relay
readings to other accurate instruments in the relay input circuits.
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