The Under-Time logic always uses the pickup signal from the step 4. The Under-Time logic will
normally be set to operate for a lower current level than the SOTF function. The Under-Time logic
can also be blocked by the 2
nd
harmonic restraint feature. This enables high sensitivity even if
power transformer inrush currents can occur at breaker closing. This logic is typically used to
detect asymmetry of CB poles immediately after switching of the circuit breaker. The Under-Time
logic is activated either from change in circuit breaker position or from circuit breaker close and
open command pulses. This selection is done by setting parameter
ActUnderTime
. In case of a
pickup from step 4 this logic will give a trip after a set delay
tUnderTime
. This delay is normally set
to a relatively short time (default 300 ms). Practically the Under-Time logic acts as circuit breaker
pole-discrepancy protection, but it is only active immediately after breaker switching. The Under-
Time logic can only be used in solidly or low impedance grounded systems.
UNDERTIME
HarmResSOFT
ActUnderTime
t4U
AND
t
tUnderTime
OR
Open
Closed
Close command
ActivationSOTF
t4U
StepForSOTF
STIN2
STIN3
AND
t
tSOTF
AND
BLOCK
2nd Harmonic
AND
Open
Close
Close command
STIN4
SOTF
SOTF or
UnderTime
TRIP
OFF
OR
UnderTime
SOTF
SOTF
ANSI06000643-6-en.vsdx
t
200 ms
t
200 ms
ANSI06000643 V6 EN-US
Figure 258: Simplified logic diagram for SOTF and Under-Time features
M13941-3 v4
EF4PTOC (51N/67N) Logic Diagram Simplified logic diagram for the complete EF4PTOC (51N/67N)
function is shown in figure
259:
1MRK 502 066-UUS B Section 8
Current protection
489
Technical manual
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