Operations - Short Time Fault Protection
31
Adjustable Short Time Pickup and Adjustable Short Time Delay
Adjustable Short Time Pickup
The Short Time Pickup adjustment is used to set the level of
high current the breaker will carry for a short period of time
without tripping. This adjustment, together with the Short
Time Delay, allows downstream breakers time to clear short
circuit faults without tripping the upstream breakers. On
Siemens Electronic Trip Units, the Short Time Pickup may be
set to 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, or 9 times I
r
.
Short Time Pickup
Adjustable Short Time Delay The Short Time Delay adjust-
ment is used to set the time interval the breaker will wait
before responding to the current value selected on the Short
Time Pickup adjustment. There are two modes of operation
of this adjustment on all Siemens Electronic Trip Units; one is
a fixed delay, the other is an inverse 1
2
t ramp delay. The 1
2
t
Delay has the characteristic of being inversely proportional to
the square of the magnitude of the overcurrent condition. This
means that higher overcurrent conditions have shorter delays
and conversely lower overcurrent conditions have longer
delays. This characteristic allows for better coordination with
downstream circuit breakers and fuses. In the fixed delay
mode, the Short Time Delay may be set to .07, .1, .15, .2, or
.3 seconds. In the inverse 1
2
t ramp Short Time Delay mode,
the delay may be set to a calibrated value of .07, .1, .15, .2,
or .3 seconds at a current equal to 8 times I
r
.
Short Time Delay
Courtesy of NationalSwitchgear.com
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