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Figure 2.4-2 Blocking Scheme Using Instantaneous Overcurrent Elements - Siemens 7SR21 Manual

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7SR21 & 7SR22 Argus Applications Guide

Figure 2.4-2 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.

Chapter 7 Page 15 of 39

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