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50
4.7.4.3 SHORT CIRCUIT
IEEE 37.2 -51 AC time overcurrent relay (shutdown / electrical trip)
For overload conditions caused by a short circuit, the overcurrent alarm is not fast enough to protect the
system. The Short Circuit alarm performs this protection.
If the Short Circuit alarm is enabled, and the current is excessive, the DSE controller begins following the
IDMT ‘curve’. If the Trip is surpassed for an excess amount of time the Alarm triggers (Shutdown or Electric
trip as selected in Action).
The higher the short circuit current, the faster the trip. The speed of the trip is dependent upon the fixed
formula:
T = K x 0.14 / ((I / I
s
)
0.02
-1)
Where: T is the tripping time in seconds (accurate to +/- 5% or +/-
50ms (whichever is the greater)
K is the time multiplier setting
I is the actual current measured
I
s
is the trip setting value
I
S
(Trip setting value)
K
(time multiplier setting)
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