Chapter 7) 7SR23 DAD Applications Guide
©2013 Siemens Protection Devices Limited
Chapter 7 Page 29 of 42
5.3.1
Use of the 87/50-2 Element
Where differential current continues to flow after operation of 87/50-1 the circulating current protection alone
cannot discriminate between a CB failure or protection ‘blind spot’ fault.
87/50-2 + DTL can be used to trip the remote ends of circuits where the CB has failed to clear the fault see Figure
5-5.
Where the fault has not been cleared because it is in a blind-spot then an interlocked overcurrent element (ILOC)
can be connected in series with the back-trip signal.
(e.g. the fault in Figure 5-4b will operate zone 2 protection but zone 1 protection is ‘blind’ to this fault).
Figure 5-5
CB Fail Back-Trip from 87/50-2 Element
5.4
CB Circuit Position
The position of up to 12 CBs can be monitored utilising either the IEC61850 comms or wiring between binary
inputs of the 7SR23 and CB auxiliary switches, in effect producing a replica of the busbar.
This feature can be used for example on double busbar installations – presented in schematic form in Figure 5-6
– to issue trip outputs to the relevant circuit breakers. Back-tripping logic from circuit CB fail inputs can also utilise
these busbar replica tripping outputs..
Where differential current continues to flow and CB status indicates that all CBs are open then this indicates a
blind-spot fault has occurred.
Figure 5-6
CB Position Monitoring – Double Busbar Arrangement
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