GE Industrial Solutions Entellisys 4.0 System User Manual - Manual (Page 55)

Overcurrent protection 55
5
5 Single point functions
5.1 Overcurrent protection
The Entellisys Low-Voltage Switchgear system provides four different kinds of protection: Long
Time (LT), Short Time (ST), Instantaneous Overcurrent (IOC) and Ground Fault (GF). The system
provides overcurrent protection by monitoring the phase currents at each circuit breaker. When
an overcurrent condition is detected, the system opens the circuit breaker.
There are two levels of overcurrent protection in the Entellisys system. The first level is provided
by the Entellisys CPU, and is based on phase-current data provided by each Messenger. The
parameters for these functions are set by the customer through the HMI (except for long time
pickup). The individual Messengers, based on local phase-current data, provide the second level.
The parameters for these overcurrent functions are fixed according to the safe operating region
for a given circuit breaker and are not programmable by the customer (except for long time
pickup).
The system rejects any parameter input that is not valid or that places the system in an unsafe
or inconsistent state. The system performs a range-checking operation on its programmed
settings and does not allow a setting that exceeds the circuit breaker's design limit, safe
operation, or the range specified by the applicable standard.
With the exception of Ground Fault Tripping Priority, each of the single point functions supports
up to 16 topologies, from 0 to 15. Topology 0 is the Reduced Let-Thru Mode topology and
topology 1 is the default topology. If no topology circuit breakers are defined, Entellisys supports
only topologies 0 and 1. See
Zones, buses, and topologies on page 101 for more information.
5.1.1 Long Time Overcurrent protection
There are two levels of Long Time (LT) Overcurrent Protection in the Entellisys system. The first
level is provided by the Entellisys CPU. The second level is provided by the Messenger.
I 2 T Long Time Overcurrent Protection curves
The equation for the I 2 T Long Time function limit is:
The parameters in this equation are defined as:
T = Time to trip in seconds.
C = LT current setting in amps and is equal to the Rating Switch value times the LT pickup
multiplier (see Table 5-1).
K = LT delay band constant in seconds, user adjustable.
I
RMS
= Value of the fault current in amps.
K T
C
I
2
RMS = Ч ⎟
⎠
⎞
⎜
⎝
⎛