7SR11 & 7SR12 Applications Guide
©2017 Siemens Protection Devices Limited Chapter 7 Page 17 of 48
Block 50-1
INCOMER
51-1
50-
1
(
100
ms)
51-
1
F2
F
1
Power
flow
50-1
(0ms)
50-
1
(0ms)
50-1
(0ms)
50
-
1
(
0ms
)
Blocking outputs for circuit faults
Figure 2.5-2 Blocking Scheme Using Instantaneous Overcurrent Elements
Typically a time delay as low as 50ms on the incomer 50-1 element will ensure that the incomer is not tripped for
outgoing circuit faults. However, to include for both equipment tolerances and a safety margin a minimum time
delay of 100ms is recommended.
This type of scheme is very cost effective and provides a compromise between back-up overcurrent busbar
protection and dedicated schemes of busbar protection.
Instantaneous elements are also commonly applied to autoreclose schemes to grade with downstream circuit
reclosers and maximise the probability of a successful auto-reclose sequence – see section 4.
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