Page 8 - Handbook of Electrical Engineering

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6.6 Transformer Inrush Current 140
References 142

7 Switchgear and Motor Control Centres 143
7.1 Terminology in Common Use 143
7.2 Construction 144
7.2.1 Main busbars 144
7.2.2 Earthing busbars 146
7.2.3 Incoming and busbar section switching device 146
7.2.4 Forms of separation 147
7.2.5 Ambient temperature derating factor 149
7.2.6 Rated normal current 149
7.2.7 Fault making peak current 149
7.2.8 Fundamental AC part 150
7.2.9 DC part 150
7.2.10 Double frequency AC part 150
7.2.11 Fault breaking current 152
7.2.12 Fault withstand duty 153
7.3 Switching Devices 154
7.3.1 Outgoing switching device for switchgear 154
7.3.2 Outgoing switching device for motor control centres 155
7.4 Fuses for Motor Control Centre Outgoing Circuits 156
7.5 Safety Interlocking Devices 157
7.6 Control and Indication Devices 158
7.6.1 Restarting and reaccelerating of motors 158
7.6.2 Micro-computer based systems 159
7.7 Moulded Case Circuit Breakers 162
7.7.1 Comparison with fuses 162
7.7.2 Operating characteristics 163
7.7.3 Cut-off current versus prospective current 164
7.7.4 i-squared-t characteristic 164
7.7.5 Complete and partial coordination of cascaded circuit breakers 165
7.7.6 Worked example for coordination of cascaded circuit breakers 167
7.7.7 Cost and economics 172
References 172

8Fuses 173
8.1 General Comments 173
8.2 Operation of a Fuse 174
8.3 Influence of the Circuit X-to-R Ratio 174
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8.4 The I t Characteristic 176
8.4.1 Worked example 179
References 181