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7SR105 Rho Description of Operation
©2018 Siemens Protection Devices Chapter 1 Page 38 of 57
Section 4: Supervision Functions
4.1 Break Capacity Limit (50BCL)
An MCCB motor trip or contactor release should not be attempted if the short circuit current exceeds the set
Breaking Capacity Limit. The Breaking Capacity Limit setting is provided to prevent the current interrupting
capability of the primary switching device being exceeded.
The true RMS current of each phase is measured. If any phase current exceeds the Breaking Capacity Setting
then the operation of all output contacts assigned to ‘General Trip’ are blocked. When this break capacity limit
functionality is required all associated high current trips must be assigned in this way.
50BCL is a high speed element, it’s instantaneous operation can be used to interrupt protections assigned as a
general trip (OUTPUT CONFIG > OUTPUT MATRIX > General Trip). All contacts assigned as ‘Gn **** Trips’ in
the OUTPUT CONFIG > TRIP CONFIG menu (Thermal, P/F, E/F, Misc) are General Trips. A settings example is
provided for clarification – see Chapter 7 ‘Applications Guide’.
The ‘50BCL’ output can be used to trip an upstream CB. CB fail protection initiation can be selected by the user.
Gn 50BCL Setting
sets the pick-up current level for the element.
50BCL Block Tripping
is enabled where contacts assigned as a ‘General Trip’ are to be blocked for currents
above
Gn 50BCL Setting.
50BCL Initiate CBFail
is enabled where it is required to start CB Fail for currents above
Gn 50BCL Setting.
Inhibit 50BCL
&
I
A
I
B
I
C
Enabled
Gn 50 BCL Element
50 BCL Setting
Enable
Enabled
Gn 50 BCL Block
Tripping
Enabled
Gn 50 BCL Initiate
CBF
50 BCL Block
50BCL
50 BCL CB Fail
Block ‘Trip Contact’
>
Operate
Figure 4-1 Logic Diagram: Breaking Capacity Limit (50BCL)
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