DONE BY MY COLLEAGUES IN COLLEGE BSEE-5

1. Chapter 11
Switching and Control Devices
Prepared by:
Lincoln A. Azupardo
BSEE 5B

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

3. Switches
a) Knife switches
b) Disconnecting switches
c) Air-break switches
d) Control switches
e) Auxiliary switches
f) Oil switches

4. 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

6. 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

8. 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

10. 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

11. 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.

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

13. g) magnetic-impulse switches
• arc is extinguished by blowing it magnetically into arc
chutes where it is lengthened cooled, and
interrupted.

14. Fuses
• Used to protect circuits of small capacity against
abnormal currents such as overloads or short
circuits.
Types: a)cartridge fuses
b)transformer fuse block and cutout
c)expulsion fuses

16. 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

17. 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.

18. 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

19. Classification of Circuit Breakers
Interrupting medium - air, air blast, oil, magnetic blast
Service - indoor, outdoor
Operation – gravity opened, gravity closed, horizontal
break
Contacts – butt, wedge, laminated-flat contact, bayonet,
explosion chamber, deion grid
Action – nonautomatic, automatic
Mode of control – direct control, remote control (manual,
electrical[motor/solenoid], pneumatic)
Tank construction – all poles in one tank, one tank for each
pole
Mounting – panel mounting, rear of panel, remote, remote
from panel (framework, cell, floor)

20. 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.

21. Circuit Breakers
• Standard oil circuit breakers may be classified
according to their method of operation as
a) Gravity opened
b)Gravity closed
c) Horizontal break
• But several variations of these types are nevertheless
found

22. 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

23. 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

24. 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.

25. 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.

26. 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.

27. f) impulse oil circuit breakers
• Arc is extinguished by an oil blast produced by a
spring-driven piston.

28. 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

29. • 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

30. 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

31. • 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

32. 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.

33. 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

34. 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 consideration
Standard duty cycles for CBs is CO-15 sec-CO
CB is closed, then opens due to relay action. It is then
reclosed in 15s and then again reopens