P12x/EN AP/A96
Application Guide
Page 16/80
MiCOM P120/P121/P122/P123
5.
BLOCKING LOGIC FUNCTION (BLOCKED OVERCURRENT PROTECTION)
This type of protection can be applied to radial feeder circuits where there is little or no back
feed. For parallel feeders, ring circuits or where there can be a back feed from generators,
directional relays should be considered.
The blocking logic function allows the upstream IDMT relay to be blocked by the start output
of a downstream relay that has detected the presence of fault current above its threshold.
Thus both upstream and downstream relays can have the same current and time settings,
and the blocking feature will automatically provide grading. If the breaker fail protection is
active, the blocking order on the upstream relay will be removed if the down-stream circuit
breaker fails to trip.
Thus for a fault downstream from relay C, the start output from relay C will prevent relay B
from operating and the start output of relay B will prevent relay A from operating. Thus all 3
relays could have the same time and current threshold settings and the grading would be
obtained by the blocking signal received from a relay closer to the fault. This gives a
constant, close time grading, but there will be no back-up protection in the event of pilots
being short circuited.
However, in practice it is recommended to set the upstream relay to a value that is 10%
higher than the downstream relay setting. This ensures that the downstream relay
successfully blocks the upstream relay when required.
A
B
C
P0024ENa
FIGURE 3: BLOCKING LOGIC
To assign the "Blocking Logic" functions, go under the AUTOMAT CTRL/Blocking Logic
menu.
MiCOM P120 & P121 relays have only one blocking logic function.
MiCOM P122 & P123 relays have two blocking functions, which can be used to block the
Earth and Phase thresholds.
Summary of Contents for MiCom P120
Page 1: ...MiCOM P120 P121 P122 P123 Overcurrent Relays Version 11 Technical Guide P12X EN T A96...
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Page 4: ...P12x EN T A96 Technical Guide Contents Page 2 2 MiCOM P120 P121 P122 P123 BLANK PAGE...
Page 13: ...Introduction P12x EN IT A96 MiCOM P120 P121 P122 P123 INTRODUCTION...
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Page 16: ...P12x EN IT A96 Introduction Page 2 8 MiCOM P120 P121 P122 P123 BLANK PAGE...
Page 22: ...P12x EN IT A96 Introduction Page 8 8 MiCOM P120 P121 P122 P123 BLANK PAGE...
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Page 35: ...User Guide P12x EN FT A96 MiCOM P120 P121 P122 P123 USER GUIDE...
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Page 108: ...P12x EN FT A96 User Guide Page 72 72 MiCOM P120 P121 P122 P123 BLANK PAGE...
Page 109: ...Menu Content Tables P12x EN HI A96 MiCOM P120 P121 P122 P123 MENU CONTENT TABLES...
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Page 112: ...P12x EN HI A96 Menu Content Tables Page 2 68 MiCOM P120 P121 P122 P123 BLANK PAGE...
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Page 221: ...Application Guide P12x EN AP A96 MiCOM P120 P121 P122 P123 APPLICATION GUIDE...
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Page 370: ...P12x EN CT A96 Communications COURIER DATABASE Page 2 248 MiCOM P120 P121 P122 P123 BLANK PAGE...
Page 514: ...P12x EN CT A96 Communications COURIER DATABASE Page 146 248 MiCOM P120 P121 P122 P123...
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Page 518: ...P12x EN CT A96 Communications IIEC 60870 5 103 Page 2 248 MiCOM P120 P121 P122 P123 BLANK PAGE...
Page 536: ...P12x EN CT A96 Communications IEC 60870 5 103 Page 20 248 MiCOM P120 P121 P122 P123...
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Page 540: ...P12x EN CT A96 Communications DNP 3 0 Database Page 2 248 MiCOM P120 P121 P122 P123 BLANK PAGE...
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Page 573: ...Connection Diagrams P12x EN CO A96 MiCOM P120 P121 P122 P123 CONNECTION DIAGRAMS...
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Page 582: ...P12x EN RS A96 Commissioning Test Record Sheets Page 2 60 MiCOM P120 P121 P122 P123 BLANK PAGE...
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