Schweitzer Engineering Laboratories SEL-351 - Operation of Single-Phase Time-Overcurrent Elements (51 AT, 51 BT, 51 CT); Setting 51 PRS = N

3-22 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements Date Code 20001006
SEL-351 Instruction Manual
Setting 51PRS = N
If reset timing setting 51PRS = N, element 51PT reset timing is a 1-cycle dropout. If current I
P
goes above pickup setting 51PP (element is timing or already timed out) and then current I
P
goes
below pickup setting 51PP, there is a 1-cycle delay before the element fully resets. Relay Word bit
51PR (reset indication) = logical 1 when the element is fully reset.
Operation of Single-Phase Time-Overcurrent Elements (51AT, 51BT, 51CT)
To understand the operation of Figure 3.15, Figure 3.16, and Figure 3.17 follow the explanation
given for Figure 3.14 in the preceding part of this Phase Time-Overcurrent Elements subsection,
substituting phase current I
A
(or I
B
or I
C
) for maximum phase current I
P
and substituting like
settings and Relay Word bits. The settings ranges and accuracies for the single-phase time-
overcurrent elements settings are the same as the corresponding settings in Table 3.2.
Figure 3.15: A-Phase Time-Overcurrent Element 51AT (With Directional Control Option)
Courtesy of NationalSwitchgear.com