Switchgear/circuit breakers

FAILURE OF CLASS IE SAFETY RELATED SWITCHGEAR CIRCUIT BREAKER TO CLOSE ON DEMAND... 

Electrical switchgear, circuit breakers, switches, and small transformers are normally grouped together in a building or Motor Control Center (MCC). Climate controlled buildings may also be required to house minicomputers, transducer banks, and other instrument interfaces. Putting this equipment in a building protects it from fires and explosions in the process area. 

Metal enclosed low voltage power circuit breaker switchgear Metal clad and station type cubicle switchgear Metal enclosed interrupter switchgear Guide for calculating losses in isolated phase bus... 

Figure 19.21 The spiral arc in a rotating arc SFg circuit breaker (Courtesy South Wales Switchgear Ltd.)...

BS 5311, High voltage alternating current circuit breaker BS 6867, Code of Practice for maintenance of electrical switchgear for voltages above 36 kV BS 158, Insulating oil for transformers and switchgear BS 5622, Insulation co-ordination... 

The selection of operating mechanism will depend upon the control regime required for the circuit breaker. If the circuit breaker is to be operated locally (i.e. at the switchgear itself) then the manually charged spring option will be acceptable. If it is to be operated from a remote location then either motor-wound spring or solenoid operation will be required. 

Inclusion of thermography into a predictive maintenance program will enable you to monitor the thermal efficiency of critical process systems that rely on heat transfer or retention electrical equipment and other parameters that will improve both the reliability and efficiency of plant systems. Infrared techniques can be used to detect problems in a variety of plant systems and equipment, including electrical switchgear, gearboxes, electrical substations, transmissions, circuit breaker panels, motors, building envelopes, bearings, steam lines, and process systems that rely on heat retention or transfer. 

Figure 6.109 Flameproof switchgear for Group I application, with SFe power circuit breaker.

Figs 6.72 and 6.73). Sparking components (e.g. fuses, contactors, circuit breakers) inside or parts not complying with the temperature class of the assembly are individually protected by an independent type of protection (for the most part in a flameproof housing). More recently, attempts have been made to introduce pressurized switchgear assemblies onto the market (Fig. 6.19). In the USA, the classic solution has been a flameproof enclosure with the conduit technique, see Fig. 6.83. 

The secondary selective circuit arrangement is recommended for all critical loads (Fig. 2). It requires the installation of double-ended switchgear or a variation thereof. Double-ended switchgear includes two main circuit breakers that serve separate feeder buses with a tie circuit breaker in the middle. Each feeder bus has its own set of feeder or branch circuit breakers. The load should be evenly split between each of the feeder buses. 

Important loads can be connected to redundant feeder circuit breakers, one on each side of the double-ended switchgear, by connecting the load sides of the circuit breakers together with a feeder cable. This approach should be used with caution. Another approach is to connect the redundant feeder circuit breakers to a time-delayed automatic transfer switch. 

Unit substations, each consisting of a primary fused disconnect switch or circuit breaker, a dry transformer, and secondary switchgear should be in a central location near their loads. Unit substation transformers should be sized to carry at least 125% the maximum present and future connected load. 

Metal-clad switchgear equipment with air or oil circuit breakers should be selected for (1) starting and controlling motors for operation above... 

When standby or future capacity is required for transformers it is necessary to rate the secondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuit breakers and switchgear busbars need to be appropriately rated for the future demand. The decision to over-rate the primary cables or lines may be made at the beginning of the project or later when demand increases. Again this is a matter of economics and forecasting demand. 

Situations occur, especially with offshore platforms, where no physical space is available to install an extra generator and its associated equipment. Sometimes the main switchrooms cannot accept any more switchgear, not even one more generator circuit breaker. Therefore the potential for upgrading a prime mover without having to make major changes to the electrical system is an option that should be considered seriously at the beginning of a project. 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

The use of oil in switchgear has become unnecessary and discontinued. The choice of circuit breaker type for a particular power system depends upon several main factors -... 

The following description of integrated motor control systems (IMCS) is based upon Reference 7, for which permission to use the material therein was kindly given by Switchgear and Instrumentation Ltd. The principles described can be used for low and high voltage switchgear that contain plain feeders, interconnectors, incomers and busbar section circuit breakers, in addition to... 

Table 11.5 compares the cases in terms of cost, but without the cost of the extra switchgear being included. The cost of a circuit breaker would be in the same order of magnitude as its associated reactor. 

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