Chapter 11Switching and Control Devices

Prepared by:Lincoln A. AzupardoBSEE 5B

Purpose

• To make, break or change the connections in an electric circuit, under either normal or abnormal conditions

Switches

a) Knife switchesb) Disconnecting switchesc) Air-break switchesd) Control switchese) Auxiliary switchesf) Oil switches

a) knife switches

• Used to open and close circuits of low voltage and current capacity.

• Used extensively in lighting and small power circuit• To protect such low capacity circuits against

overloads and short circuits, it is necessary that proper fuses be connected in series with the switch blades

b) disconnecting switches

• Should never be opened until the circuit breaker in the same circuit has been opened, and should always be closed before the circuit breaker is closed

• May be operated by hand, either directly or by means of a wooden pole, or through a set of levers and bell cranks; or in the case of high voltage switches the control may e done electrically

c) air-break switches

• Designed to open circuits under load• Used generally outdoors for circuits of medium

capacity• Special arcing horns are provided so that arc may rise

and be ruptured.• Built for about 135,000 volts maximum but their use

is confined to lower voltages• Not designed to open under abnormal conditions

d) control switches

• Includes all switches that are used to control the operation of other equipment.

• Designed for operating voltage less than 250 volts and very small current capacities

e) auxiliary switches

• Includes all switches or contactors that are actuated by some other control switch or device.

• Control circuits may be designed for very small currents, while the operating circuits as a general rule must handle much larger currents and very often at higher voltages.

f) oil switches

• At high voltages and large current capacities it is desirable to immerse the switch contacts under oil.

g) magnetic-impulse switches

• arc is extinguished by blowing it magnetically into arc chutes where it is lengthened cooled, and interrupted.

Fuses

• Used to protect circuits of small capacity against abnormal currents such as overloads or short circuits.

Types: a)cartridge fusesb)transformer fuse block and cutoutc)expulsion fuses

a) cartridge fuses

• Composed of a strong fiber casing inside of which is enclosed a fuse wire, generally an alloy of lead

• Used as a protective device in low-capacity circuits, such as small lighting and power lines, and on the secondary of instrument-potential transformers, when used for metering or relay protection

b) transformer fuse block and cutout

• It is common practice to place these fuses in the secondary of such transformers, thereby protecting the transformer against short circuits or overloads.

• Two types, both include a porcelain housing, enclosing the fuse and contact points. In one type, the fuse is carried on a plug that is provided with an insulating porcelain knob in order that it may be removed for re-fusing. A second type consists of rectangular porcelain receptacle with a removable front door that carries the fuse.

c) expulsion fuses

• For higher voltages such as found in power circuits or main feeders.

• Consists of a hollow tube, made of some heat-resisting substance such as fiber with a lining of asbestos or some other material, through which is passed a fuse wire. One end of the tube is closed and connected to the line; the other end is opened and allows the fuse wire to project out and connect to the other terminal

• For outdoor use only

Classification of Circuit Breakers

Interrupting medium - air, air blast, oil, magnetic blastService - indoor, outdoorOperation – gravity opened, gravity closed, horizontal breakContacts – butt, wedge, laminated-flat contact, bayonet, explosion chamber, deion gridAction – nonautomatic, automaticMode of control – direct control, remote control (manual, electrical[motor/solenoid], pneumatic)Tank construction – all poles in one tank, one tank for each poleMounting – panel mounting, rear of panel, remote, remote from panel (framework, cell, floor)

Air Circuit Breakers

• “carbon circuit breaker” have 2 or 3 contacts• Protect the main contacts against arcing or pitting,

secondary copper contacts and also carbon contacts are provided

• Carbon contacts take most of the arcing, and since carbon is very refractory, the contacts are not damaged very much

• In case of damage to either the secondary or carbon contacts, it is possible to replace these easily.

Circuit Breakers

• Standard oil circuit breakers may be classified according to their method of operation as

a) Gravity openedb) Gravity closedc) Horizontal break• But several variations of these types are nevertheless

found

a) gravity-opened type

• Most common type wherein contacts are operated by an insulated rod which projects outside the tank

• Tendency is for the contacts to fall open due to gravity

b) gravity-closed type

• It will close under the action of gravity• Terminals are at the bottom of the tanks, the circuit

being completed through contact rods• Equipped with a set of main contacts located at the

top of the tank which opens before the arcing-rod contacts are opened and, hence, are not subjected to arcing and can, therefore, be located outside the tanks

c) horizontal-break type

• In both of earlier types, the circuit is broken by contacts moving in a vertical plane, however, this type has its contacts arranged for horizontal motion.

d) air-blast circuit breakers

• Introduced in the United States practice in 1939• Blow out the arc with a jet of air that is supplied

from a pressure tank• Arc is blown into a set of arc chutes, where it is

subdivided into several parts, making the task of extinguishing somewhat easier.

e) magnetic-blast circuit breakers

• Designed for 5,000V and less• These are air circuit breakers that extinguish their

arcs by blowing them magnetically into arc chutes where they are lengthened, cooled and interrupted.

f) impulse oil circuit breakers

• Arc is extinguished by an oil blast produced by a spring-driven piston.

Contact Details

• simplest type of contact is using of either flat or curved metal surfaces as terminals of CBs

• Butt contact– when moving contactors are held firmly against the

stationary terminals by means of heavy springs– For low voltages and capacity

• Wedge design– Wedge-shaped terminal on the moving element, which

when in the closed position is forced between spring jaws

• Bayonet or rod type– Arcing tips are connected to the contact arms through

heavy springs– Essential when contacts be opened at very high speed

• Oil-blast explosion chamber– Consist of an upper, an intermediate, and a lower element– In the closed position, these three elements are pressed

together

Circuit Breaker Construction

Mechanical details of CB vary a great deal, depending on the particular requirements that must be met•Indoor– In most cases, lower voltage ratings and therefore present

a different appearance. Generally, they are mounted in cells

•Outdoor– Must naturally be water and moisture proof, the bushings

must be designed for all weather conditions and, as a general rule must be given greater clearance

• For very low capacity and voltages all poles of a polyphase circuit breaker are sometimes placed in the same tank, but for medium- and high-capacity units it is necessary to enclose the separate poles in individual tank

Circuit Breaker Control

• Remote electrical control may be obtained by means of an operating motor or solenoid

• Operating voltages used are either 125 or 250 volts• DC is generally obtained from storage batteries,

which give continuous service w/ maximum possible reliability

• Pneumatic method of CB control, owing to its positive and unusually fast action

• relays are placed in ckt in order that CB may be opened automatically in case of abnormal condition.

Application of Circuit Breakers

To apply CBs: normal current of circuit, rated voltage, & the interrupting capacity in amperes must be known•Interrupting capacity of circuit breaker is also affected by the frequency of the circuit, p.f. prior to the short ckt, the temperature, and also the altitude of the breaker above sea level•Stored electrostatic and magnetic energy of system has definite effects upon•also limited by the safe operating temperature•Up to an elevation of 3,300 ft above sea level ther is no perceptible decrease in rating

Distance above sea level, feet Current-rating factor Voltage rating

4,000 0.99 0.98

5,000 0.96 0.95

8,000 0.94 0.86

10,000 0.91 0.80

12,000 0.89 0.74

14,000 0.87 0.70

• duty cycle is another considerationStandard duty cycles for CBs is CO-15 sec-COCB is closed, then opens due to relay action. It is then

reclosed in 15s and then again reopens