1-8 Introduction Date Code 20011026
SEL-321/321-1 Instruction Manual
Interference, or Impulse Tests. Stickers on the rear of the relay indicate which inputs should only
be connected to low-level electronic sources. We recommend that you use twisted-shielded-pair
(TSP) cable between the source and relay and that you ground the shield at one end of the cable.
The SEL-321 Relay provides several electronic-level input options. Option A provides the best
accuracy in each case. The 1.314 V secondary input-voltage option should be used in most
applications. Determine your secondary input-current option by multiplying your 1 per-unit
primary current by the sensor scaling factor.
Examples: Secondary Input-Voltage Option = 1.314 V
1 PU = 2000 A, Sensor Scaling Factor = 0.1 mV/A
Secondary Input-Current Option = 2000+0.1 = 200 mV
Since the SEL-321 Relay is set in secondary quantities and nonconventional sensor outputs are in
primary quantities you must scale the relay settings using PTR (Potential Transformer Ratio) and
CTR (Current Transformer Ratio). Determine the PTR setting by dividing the primary line-
neutral voltage by 67 V. The CTR is determined by dividing your 1 per-unit primary current by
5 A. Use the PTR and CTR to determine the settings in secondary quantities.
Examples: PTR = (525 kV
L-L
)/(√3+67) = 4524 ≈ 4500
CTR = 2000/5 = 400
Maximum Continuous Rating
Input: 15 V
peak
≈ 10.6 V
rms
Relay Test System (RTS)
The SEL-RTS Relay Test System is designed to bypass a relay's internal CTs and PTs (or
low-level input module) so that it can provide low-level signals directly to the relay's processing
module. You can use the SEL-RTS to test a relay's low-level input module at the terminal screws
by modifying Scale Factors in the SEL
TEST
TM
Configuration Window. To determine the current
channel ratio, divide 5 A by the secondary 1 PU input. To determine the voltage channel ratio,
divide 67 V by the secondary 1 PU input.
Examples: (5 A)/(200 mV) = 25
(67 V)/(1.314 V) = 50.99 ≈ 51
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