Switches circuit interrupters

Theory of circuit interruption with different switching mediums (theory of deionization) 19/629... 

Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Division 1 and Division 2 locations. It is not required to seal IM in. or smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their current-interrupting contacts are hermetically sealed or under oil (having a 2-in. minimum immersion for power contacts and 1-in. for control contacts). 

For every piece of equipment in a substation, manual switches—called disconnect switches—are provided to enforce complete electrical isolation from equipment before any service is pertornied. Disconnect switches are placed in clearly visible locations so maintenance personnel can continuously confirm that the equipment is isolated. The disconnect switch cannot interrupt current, so it is opened only when the current has already been interrupted by an automatic switch such as a circuit breaker. 

When a disconnect switch, circuit breaker, or set of fuses is included in the same enclosure as the contactor, the starter is then called a combination starter. In addition to the fault-current-interrupting function, the breaker or fuses serve as the disconnecting device. Figure 29-5 illustrates schematically combination starters of various types. The latch is arranged to open the disconnect before the door can be swung open. There are also provisions for padlocking the disconnect open with the door closed so that maintenance wonc on the motor may proceed in safety. 

Most circuits also contain a switch that controls the flow of electrons along the pathway. Another component that may be included in the pathway is a fuse or circuit breaker. Fuses and circuit breakers are safety devices. When the flow of electrons—amperage—exceeds the value of the fuse, then the extra current flow causes the fuse to overheat and melt, thus breaking the circuit. In a circuit breaker, the extra current flow initiates an electromagnet that opens a switch and interrupts the flow of electrons. 

Circuit protection devices, such as fuses, breakers, or fault-interrupters, may be required in case of excessive current draws as a result of a short circuit, overcurrent, or earth fault. Several options exist with circuit breakers preferred over fuses. In some of the newer devices several functions are combined into one, thereby reducing the total number of components. An example of this is a combined power-switch/circuit-breaker. The switch/breaker is used as a switch and breaker and senses each line and opens all lines, except the grounded line, simultaneously when a fault current is present. Also, the switch/breaker looks and functions as a standard power ON/OFF rocker switch. In some standards simultaneous interruption of the phase and neutral is required, thereby, precluding the use of fuses. 

Circuit interrupting devices in power systems fall into three categories fuses, switches, and circuit breakers. Design ratings of these devices are... 

A disconnect switch or a nonautomatic circuit interrupter can be used to provide an electrical circuit disconnect function. Manual motor controllers marked Suitable as Motor Disconnect may also be used. Short-circuit protection is provided for these devices when fuses are added to the disconnect switch or thermal and/or magnetic trip units are added to the nonautomatic circuit interrupter. Self-protected combination controllers may be used to provide both the disconnect function and the short-circuit protection for motor circuits. 

The components are closely coordinated to work together, and the unit is rated for a particular value of short-circuit interrupting current. The unit may contain a thermal-magnetic or magnetic-only circuit breaker and a starter, or a fusible or nonfusible switch and a contactor or starter. The suitability of the enclosure (either provided with the combination starter or into which the combination starter is mounted) to its environment is an important consideration. 

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

A primary section provides for connection of incoming medium- or hi -voltage circuits, usually with disconnecting and circuit protective devices such as switches and circuit breakers. A transformer section includes one or more transformers. A secondary switchboard section provides for connection of secondary distribntion feeders, each with a circuit protective switching and interrupting device. 

Line Starters and Combination Starters A line starter consists of a contactor (motor-starting switch) and motor-overload relays, (Contactors are capable of canving and interrupting normal motor-starting and -running currents they are not, hovv ever, normally capable of interrupting short-circuit currents. They must be backed up by fuses or a circuit breaker for this function,... 

FIG. 29-5 Siiiiplificd schciiiatic dia raiii of a coiiilunation line starter with a circuit lirciLker as the fault interrupter and disconnect. Alternative fuses and disconnect switch are shown as siilistitiites for the circuit lirciLker,... 

With the availability of 3.3 and 6.6 kV vacuum contactors the control of HT motors up to 6.6 kV systems has now become easier and economical, compact and even more reliable. For 11 kV. systems, vacuum as well as SF (Sulphur hexafluoride) breakers can be used. The HT motor s switching and protection through a vacuum contactor provides a replica of an LT system. The earlier practice of using an HT OCB, MOCB, or an air blast circuit breaker for the interruption of an HT circuit is now a concept of the past. 

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 logic is the same as above. In the trip circuit of each interrupter is wired the NO contacts of the other two interrupters. Obviously only two of the three interrupters can be switched at a time. 

The 2NO contacts of the limit switch are wired in series with the closing coil of the interrupter (Figure 13.45). As soon as the spring is fully charged these contacts change over to NC and the closing coil circuit gets ready to clo.se. 

If it is very high, so that its power loss Vf/Rj is low then it may be left permanently in the circuit. Otherwise it should be introduced into the circuit only during interruption through the switching device) (wired across the N(D contact of the switching contactor). This will provide the resistance across the capacitor banks when being interrupted and short-circuit it when closed. 

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