GE Industrial Solutions Entellisys 4.0 System User Manual - Manual (Page 59)

Overcurrent protection 59
5
The I 2 T option is available with each of the seven delay bands listed above. Note that the trip
times will not be shorter than the values listed above for the delay bands, even when the I 2 T
curve is selected. For example, suppose ST band 6 (60 Hz) is selected and I 2 T is enabled. On a
fault where (I
RMS
/C) is 4.0, the circuit breaker will trip according to the I 2 T curve. In this case the
system will initiate a trip after approximately 1.1 seconds. On a fault where (I
RMS
/C) is 10.0, the
system will not follow the I 2 T curve and will instead initiate a trip according to the constant-time
part of the trip curve (i.e., after 0.283 seconds). In this case, if the system followed the I 2 T curve,
the trip time would have been only 0.18 seconds.
5.1.2.2 Accuracy
Pickup accuracy is ± 10% over the entire protection range
5.1.2.3 Setup
The setup required for ST protection is done in the HMI and at the Messenger. There are three
options configured at each Messenger that control the operation of Short Time and
Instantaneous overcurrent protection:
•
Short Time option
•
Instantaneous option
•
Switchable Short Time/Instantaneous option
If the Short Time option or the Instantaneous option (or both) is enabled, the state of the
Switchable ST/IOC option is ignored. The Messenger and Entellisys CPU perform the protection
functions specified by the Short Time and Instantaneous options.
If the Short Time and Instantaneous options are disabled and the Switchable ST/IOC option is
enabled, the Messenger defaults to performing Short Time and digital Instantaneous functions
until instructed otherwise by the Entellisys CPU. In this case, the Messenger does not perform
the analog Instantaneous function. The Entellisys CPU instructs the Messenger to turn on or off
Short Time.