Protection switchgear

This part deals w ith three- and single-phase a.c. machines, and their protective switchgears. However, reference is inade to and comparisons drawn of a d.c, motor with an a.c. motor, to assist a user to inake a proper choice of machine. 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

In outdoor type switchgear or controlgear assemblies the normal practice is to provide a double door in the front to house the front panel and protect the door knobs, meters, lights, pushbuttons, reset knobs or other accessories mounted on the door and thus prevent water or dust leaking through joints, knockouts and fitments etc. It is also recommended to have a canopy on the top of the enclosure to protect the panel from direct rain. Figures 13.6 and 13.28 illustrate this type of construction. 

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. 

This may be 50 or 60 Hz as the system may require. The permissible variation may be eonsideied as 2% for measuring as well as protection VTs. These limits are based on the recommended variations applicable for a switchgear assembly (lEC 60429-1) or an electric motor (Section 1.6.2),... 

Equipment may be designed for more than one BIL values as noted in the various tables referred to above for motors, switchgears and other equipment. The choice of BIL for equipment for a paniciilai application will depend upon the extent of exposure the equipment may be subject to in normal service and the security level required by the system and the surge protection. For more details refer to Section 13.4.1(3). 

The criteria to determine the safe protective level of an arrester are the BIL of the equipment, as shown in Table 11.6 for motors. Tables 13.2 or 14.1 for switchgears, Table 32.1(A) for bus systems, and Tables 13.2 and 13.3 for all other systems. Motors have a comparatively lower BIL, but they are not connected directly on an outdoor... 

This is the most common scheme in normal use for any power system with more than one feeder, connected to a common bus, such as for distribution and sub-distribution power networks, having a number of load points, controlled through a main incoming feeder. In a switchgear assembly, for instance, common protection may be provided at the incoming for a ground fault or combined 0/C and G/F protections as discussed above. In such cases, a restricted G/F protection may not be appropriate or required, as the protection now needed is sy.stem protection, rather than individual equipment proteelion. The incomer must operate whenever a fault occurs at any point on the system. Moreover, for an LT system, where it may not be desirable or possible to provide individual protection to each feeder, such a scheme is adopted extensively. 

Instrument transformers (CTs, class PS, CT, VTs and CVTs etc.) form important components of a switchgear assembly for measurement and protection. They are covered for their specifications, selection and application. 

This part is complementary to Part II and provides technical support to switchgear assemblies and machines fed by them for surge and overvoltage protection. It is a very useful part for all those handling HV and EHV power systems and their surge and overvoltage protection. 

Purpose Unfavourable operating conditions Fault conditions Protection Single-device motor protection relays Summary of total motor protection Motor protection by thermistors Monitoring of a motor s actual operating conditions Switchgears for motor protection Selection of main components Fuse-free system... 

This chapter introduces the basic items of design and specification for the principal systems and components of an electrical industrial installation. Electrical supply systems are discussed with regard to interface with the supply authorities and the characteristics. Salient features of switchgear, transformers, protection systems, power factor correction, motor control equipment and standby supplies are identified and discussed together with reference to the relevant codes of practice and standards. The equipment and systems described are appropriate to industrial plant installations operating at typically 11 kV with supply capacities of around 20MVA. 

Switchgear and relay rooms are required to have smoke detection per NFPA 850, section 5.8.4 and IEEE 979, section 2.7. The activation of the fire alarm should shut down the air handling system. If the facility is especially critical to the continued hydrocarbon process consideration of a fixed fire protection system should be evaluated. 

For electrical apparatus outside of this scope, e.g. switchgear, IEC 60079-15 defines a protection technique restricted breathing enclosure. ... 

In case of repair or maintenance, a single switchgear compartment will be pneumatically isolated, i.e. the individual protective gas inlet and outlet valves are closed, the isolating switch is set in its OFF position and an (electrical or pneumatical) interlock with the door of the compartment is set off. Thus, the door can be opened to enable access to the internal components. 

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