Switchgear Switching devices

Cubicle type switchgear can be constructed in many different arrangements depending upon a variety of requirements. For example the following aspects may be important for a particular plant, environmental protection, ease of access to internal parts, ease of terminating cables, fixed or withdrawable switching devices, maintainability and level of personnel skill, cost and economics, expected life duration of the product, fitness for its purpose. The switchgear industry is very competitive and so it is essential to clearly specify what is required in the form of assembly and its construction. Otherwise a false or unsatisfactory decision may be made mainly based on a cost comparison. 

Part 4 of the lET Regulations deals with the application of protective measures for safety and Chapter 53 with the regulations for switching devices or switchgear required for protection, isolation and switching of a consumer s installation. 

Switchboards and switchgear include buses, conductors, control devices, protective devices, circuit switching devices, interrupting devices, interconnecting wiring, accessories, supporting structures, and enclosures. 

The rating of cuiTcnt-carrying equipment (switchgear assemblies, such as for the main bus system), devices (breakers, switches and contactors) and components (connecting links and wiring etc.) is defined by two parameters ... 

The mechanical endurance of the current-carrying parts of all the equipment, bus system, deviees and components, used in a particular circuit as well as the load-bearing members and supports on which they are mounted. The electrical parts of a device (breakers and switches, etc.) are the responsibility of the component manufacturers. The manufacturer of the switchgear assembly is responsible for the busbar systems, metallic links and wires. 

The bus system of a switchgear assembly, its interconnecting links and wires are the protected type components, whereas an interrupter (breaker, switch or a fuse) may be a protecting or protected type, depending upon their application and location in the circuit. A contactor and an OCR are therefore protected devices in the same context, for they provide no short-time protection. A protecting device may become protected when it is also provided with a back-up protection. 

The devices in switching equipment are referred to by numbers, according to the functions they perform. These numbers are based on a system which has been adopted as standard for automatic switchgear by IEEE. Numbers and their meanings are shown in Table 5.41. 

Low-voltage switchgears include miniature circuit breakers, residual current devices, fuses, switches, and relays and contactors. 

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