Switchgear equipment in the distribution system is analogous to the steering wheel, accelerator and brakes of an automobile. Regardless of the age of the car, condition of the tires, and horsepower of the engine, without the steering wheel, brakes and accelerator, the car would not only be useless, but would be a positive menace. Similarly, any electrical power distribution system would be a dangerous menace without the necessary control, switching and protective features of the switchgear apparatus.
In a few words, then, the function of switchgear is to control electric power circuits. This can be subdivided into (a) switching and (b) protection against abnormal conditions.
As stated in NEMA Standard SG5-1.001, switchgear is the general term covering switching and interrupting devices, and assemblies of those devices with control metering and protective and regulating equipment, plus the associated interconnections and supporting structures.
The circuit breaker or load interrupter switch and fuse combination is often considered to be the main device in switchgear. The switching function requires the interruption of currents from zero to full continuous load. The circuit protection function requires the interruption of all currents from small overloads to the maximum short circuit current available at the point of application.
For indoor service from 2.4 kV to 35 kV and up to 1,000 MVA interrupting duty, magnetic type air breakers in metal-clad assemblies are most often selected.
Interrupting Duty
Momentary Duty
Metal-Clad Switchgear
Types Of Power Fuses
Example: Fault Currents At Medium Voltages
Fault Current Calculations At Medium Voltages
Protective Devices: Circuit Breakers And Fuses
Types Of Medium-Voltage Power Circuit Breakers
Medium Voltage Disconnecting & Load Interrupter Switches
