Date Code 20000421 Maintenance and Testing 7-17
SEL-251, -2, -3 Instruction Manual
Method: 1. Using the SET command, set a programmable output contact (A1-A4) to follow
one of the demand ammeter threshold alarms from Relay Word row R4 (e.g.,
NDEM). Also, set the respective demand threshold to some secondary current
value level and DATC (Demand Ammeter Time Constant) to a value from 5 to
60 minutes (e.g., NDEM = 4.50 and DATC = 15).
2. Using the METER RD command, reset demand ammeters to zero.
3. Note the time and apply a constant current to one phase. The current should be a
factor of 10/9 = 1.11 times greater than the NDEM setting (constant current =
1.11 x 4.50 A = 5.00 A). See Demand Ammeter in Section 2: Specifications
for background on the 1.11 factor.
Note: With current applied to only one phase (e.g., I
A
= 5 A, I
B
= I
C
= 0),
the following sequence currents result:
[ ]
[ ]
( ) ( )
[ ]
I I I I
A A A
A
I I a I a I
A B C
A B C
0
1
3
1
3
2
1
3
2
5 0 0 0 0 0
167 0
= + +
= Ð °+ Ð °+ Ð °
= Ð °
= + +
.
Where: a and aº Ð ° º Ð °1 120 1 240
2
( ) ( )
[ ]
[ ]
I A A A
A A A
A
2
1
3
1
3
5 0 0 0 240 0 0 120
5 0 0 120 0 120
167 0
= Ð °+ Ð °+ ° + Ð °+ °
= Ð °+ Ð- °+ Ð °
= Ð °.
The negative-sequence and residual demand threshold elements respond to 3xI
2
and 3xI
0
currents, respectively. Thus, for this test all demand threshold elements
see 5 A.
4. If the current remains constant at 5.00 A, the programmable output contact
assigned to the NDEM element asserts 15 minutes after you apply current when
DATC = 15.
5. Repeat these tests for the phase- and negative-sequence demand threshold alarms
(PDEM and QDEM).
Reclosing Relay Tests Setup
Purpose: Make settings for subsequent reclosing relay tests.
Note: These tests should be performed using a circuit breaker simulator to
simulate the action of the circuit breaker auxiliary contacts. A simple
latching relay works well as a circuit breaker simulator.
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