2.1 Restricted Earth Fault Types 169
2.1.1 Low Impedance Bias Characteristic 169
2.1.2 High Impedance REF Principle 170
3 Restricted Earth Fault Protection Implementation 172
3.1 Restricted Earth Fault Protection Settings 172
3.2 Low Impedance REF 172
3.2.1 Setting the Bias Characteristic 172
3.2.2 Delayed Bias 173
3.3 High Impedance REF 173
3.3.1 High Impedance REF Calculation Principles 173
4 Application Notes 175
4.1 Low Impedance REF Protection Application 175
4.1.1 Setting Guidelines for Biased Operation 175
4.1.2 Low Impedance REF Scaling Factor 175
4.2 High Impedance REF Protection Application 176
4.2.1 Setting Guidelines for High Impedance Operation 176
Chapter 8 CB Fail Protection 177
1 Chapter Overview 179
2 Circuit Breaker Fail Protection 180
3 Circuit Breaker Fail Implementation 181
3.1 Circuit Breaker Fail Timers 181
3.2 Zero Crossing Detection 181
4 Circuit Breaker Fail Logic 183
5 Undercurrent and ZCD Logic for CB Fail 185
6 CB Fail SEF Protection Logic 186
7 CB Fail Non Current Protection Logic 187
8 Circuit Breaker Mapping 188
9 Application Notes 189
9.1 Reset Mechanisms for CB Fail Timers 189
9.2 Setting Guidelines (CB fail Timer) 189
9.3 Setting Guidelines (Undercurrent) 190
Chapter 9 Current Transformer Requirements 191
1 Chapter Overview 193
2 CT requirements 194
2.1 Phase Overcurrent Protection 194
2.1.1 Directional Elements 194
2.1.2 Non-directional Elements 195
2.2 Earth Fault Protection 195
2.2.1 Directional Elements 195
2.2.2 Non-directional Elements 195
2.3 SEF Protection (Residually Connected) 195
2.3.1 Directional Elements 195
2.3.2 Non-directional Elements 196
2.4 SEF Protection (Core-Balanced CT) 196
2.4.1 Directional Elements 196
2.4.2 Non-directional Elements 196
2.5 Low Impedance REF Protection 196
2.6 High Impedance REF Protection 197
2.7 Use of Metrosil Non-linear Resistors 197
2.8 Use of ANSI C-class CTs 199
Chapter 10 Voltage Protection Functions 201
1 Chapter Overview 203
P24xM Contents
P24xM-TM-EN-2.1 v
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