ABOUT CIRCUIT BREAKERS

1.  A circuit breaker is an automatically operated
electrical switch designed to protect an
electrical circuit from damage caused by
overloading or short circuit.
 It protects the circuit by interrupting the
current flowing in the line, transformer etc…
 A breaker interrupts the current by
mechanically moving electrical contacts apart
inside an interrupter, causing an arc to occur
that is immediately suppressed by the high-
dielectric medium inside the interrupter.

2.  A simple circuit breaker has the following
components :
◦ Electromagnet /bimetallic strip
◦ Moving Contact
◦ Switch
◦ Stationary Contact
◦ Upper and lower terminal
◦ Catch

3.  Basically the circuit breaker consist of a simple
switch connected to either an electromagnet or a
bimetallic strip.
 When the current reaches onwards to the unsafe
levels then the electricity magnetizes the
electromagnet and the electromagnet is strong
enough to pull down a metal lever connected to
the switch linkage.
 Due to this operation the entire linkage shifts,
tilting the moving contact away from the
stationary contact to break the circuit and the
electricity shuts off.

4.  The figure below shows a low voltage circuit
breaker
http://circuitbreakerstips.com/wp-
content/uploads/2011/05/circuit-
breaker-diagram.gif

5.  The most important types of circuit breakers
are the following:
◦ Oil circuit breakers (OCBs)
◦ Air-blast circuit breakers
◦ SF6 circuit breakers
◦ Vacuum Circuit breakers

6.  They interrupt arc in clean oil.
 The interrupting contacts or interrupters are
inside the oil filled tanks.
 The oil here is basically used to extinguish
the arc.
 The intense heat of the arc decomposes the
oil and produces gases, mainly composed of
hydrogen, generating high pressure that
produces a fluid flow through the arc carrying
its energy away until its total extension

7.  Can be used in a range of low to high voltage.
 Dielectric strength of oil is high compared to
air.

8.  Environmental hazard if oil is spilled
 Oil needs to be replaced after certain time
because with passage of time it looses its
dielectric strength.
 Oil becomes contaminated, during arc
suppression, with gases.

9.  The contacts are enclosed in a sealed
chamber filled with SF6 gas.
 SF6 has very high dielectric strength, much
greater than that of oil.
 It interrupts the current faster than the OCBs.

10.  Gas used is non flammable
 Current is interrupted much quickly
 Equipment dimensions are comparatively
small because of the gas

11.  The gas turns into liquid at very low
temperatures.
 It is difficult to maintain correct gas pressure

12.  They extinguish the arc by opening the
contacts in vacuum.
 When the breaker is opened the arc is
extinguished very quickly.

13.  Smaller and lighter than air circuit breakers.
 Arc is extinguished very quickly
 If the vacuum leaks out you can’t vacuum it.
 The operation life is limited.

14.  They use compressed air to blow out the arc.

15.  Inexpensive
 Simple installation
 Simple construction
 Simple maintenance requirements
 Large size
 Time to extinguish the arc can be large
compared to other circuit breakers

16.  Circuit breakers equipped with a mechanism
that can automatically close the breaker after
it has been opened due to a fault.
 They can be programmed to trip at specific
over current conditions and reclose at specific
time intervals.
 Reclosers are typically set to trip and reclose
two or three times before a lock out condition
occurs. In a lock out condition you have to
manually reset the recloser.

17.  They are used in substations or power lines.
 They are used to isolate or deenergize
equipment for maintenance purposes,
transfer load from one source to another in
planned or emergency conditions.
 They cannot interrupt load currents and so
they are opened or closed when the current is
zero.

18.  The figure below shows a disconnect switch:
http://www.joslynhivoltage.com/pdffil
es/db710-204.pdf

19.  The figure below shows another disconnect
switch:
http://www.joslynhivoltage.com/pdffil
es/db710-204.pdf

20.  The simplest lightning arresters are metallic rods
that rise above the highest point of a building,
channeling the lightning toward a ground
electrode by means of a conducting wire.
 This prevents the high current from passing
through the building itself, which might cause a
fire or endanger its occupants.
 Much more sophisticated lightning arresters are
used on electrical utility systems. They divert
lightning and high-voltage switching surges to
ground preventing the damage to the costly and
critical electrical equipment.

21.  A bus is a conductor, or group of conductors,
that serves as a common connection between
two or more circuits.
 The purpose of the electrical bus in
substations is to connect equipment
together.

22.  A fuse is a short piece of metal inserted in the
circuit which melts when excessive current
flows through it and thus breaks the circuit.
 The action of a fuse is based upon the
heating effect of the electric current.
 The fuse element is generally made of
materials having low melting point and high
conductivity e.g. silver, copper etc…
 It is inserted in series with the circuit to be
protected.

23.  Under normal operating conditions when the
current flowing through the circuit is within
the safe limits, the heat developed in the fuse
is dissipated to the surrounding air and so
the fuse element remains at a temperature
below its melting point.
 However when some fault occurs and the
current exceeds the limiting value, the heat
generated due to this excessive current
cannot be dissipated fast enough and the
fuse melts and breaks the circuit.

24.  By breaking the circuit it protects the machine
or equipment from damage due to excessive
current.

25.  The time for blowing out of the fuse depends
upon the magnitude of the excessive current.
 Larger the current, the more rapidly the fuse
will blow.

26.  It is the cheapest form of protection available
 It needs no maintenance
 Its action is completely automatic compared
to a circuit breaker
 It can break heavy short circuit currents
without noise or smoke
 The inverse time current characteristics of a
fuse makes it suitable for over current
protection
 Minimum time of operation can be made
much shorter than with the circuit breaker.

27.  Time is wasted in rewiring or replacing the
fuse.
 On heavy short circuits discrimination
between fuses in series cannot be obtained
unless there is sufficient difference in the
sizes of the fuses concerned.
◦ Discrimination between two fuses is said to occur if
on the occurrence of a short circuit or over current
fault, only the desired fuse operates.

28.  To perform its function satisfactorily the fuse
element should have the following desirable
characteristics
◦ Low melting point e.g. tin, lead
◦ High conductivity e.g. silver, copper
◦ Free from deterioration from oxidation e.g. silver
◦ Low cost e.g. lead, tin, copper
 From the above we can see that no material
possesses all the desired characteristics.
Therefore a compromise is made in the
selection of material of a fuse.

29.  The most commonly used materials for fuse
are lead, tin, copper, zinc and silver.
 For small current up to 10 A tin or an alloy of
lead and tin is used for making the fuse
element.
 Zinc is used if a fuse with considerable time-
lag is required.

30.  Silver is preferred despite its high cost due to
the following reasons:
◦ It is comparatively free form oxidation.
◦ It does not deteriorate when used in dry air
◦ Conductivity of silver is very high

31.  Some terms which are used in analysis of
fuses are as below:
1. Current rating of fuse element: it is the
current which a fuse element can normally
carry without melting.
2. Fusing Current: It is the minimum current at
which fuse element melts and thus
disconnects the circuit protected by it.

32. 3. Fusing Factor: it is the ratio of minimum
fusing current to the current rating of the
fuse element.
4. Prospective Current: it is the r.m.s value of
the first loop of the fault current obtained if
the fuse is replaced by an ordinary
conductor of negligible resistance.
5. Cut-off Current: it is the maximum value of
fault current actually reached before the
fuse melts.

33. The value of the cut off current depends
upon:
◦ Current rating of the fuse
◦ Value of Prospective current
6. Pre-Arcing Time: it is the time between the
commencement if fault and the instant
when the cut off occurs. Generally a small
value of about 0.001 second.
7. Arcing time: this is the time between the
end of pre-arcing time and the instant
when the arc is extinguished.

34. 8. Total operating time: it is the sum of pre
arcing and arcing times.
9. Breaking Capacity or interrupting capacity:
is the current that a fuse is able to interrupt
without being destroyed
Figure 2 gives a clear picture of the above
mentioned terms.

35. Figure below gives a clear picture of the above
mentioned terms.

36.  Fusses may be classified into
◦ Low voltage fuse
◦ High voltage fuse
 Low Voltage Fuse: it can be further divided
into two classes
◦ Semi-enclosed rewireable fuse
◦ High rupturing capacity (H.R.C) cartridge fuse

37.  Semi-Enclosed Rewireable Fuse: it is used
where low values of fault current are to be
interrupted.
 It consists of a base and a fuse carrier. The
base is of porcelain and carries the fixed
contacts to which the incoming and outgoing
phase wires are connected.
 The fuse carrier is also of porcelain and holds
the fuse element between its terminals.
 When a fault occurs, the fuse element is
blown out.

38.  The figure below shows a semi enclosed
rewireable fuse.
http://en.wikipedia.org/wiki/File:MEM_
1957_fuseholders.jpg

39.  The detachable fuse carrier permits the
replacements of fuse element without any
danger of coming in contact with live parts.
 The cost of replacement is negligible.

40.  Possibility that the wire which is replaced may
of wrong size or improper material.
 This type of fuse has low breaking capacity
and cannot be used in circuits of high fault
level.
 The heating element is continuously heated
so it gets deteriorated. So the current rating
of the fuse decreases.

41.  It consists of a heat resisting ceramic body
having metal end-caps to which is welded
silver current carrying element.
 The space within the body surrounding the
element is completely packed with a filling
powder.
 The filling material may be chalk, plaster of
paris, quartz or marble dust. It acts as an arc
quenching and cooling medium.

42. http://www.gwsupplies.co.uk/electrica
l/c/1359/industrial+fuses.htm
http://www.indiamart.com/rosma-
enterprises/
http://www.tlc-direct.co.uk/Images/Products/size_3/MM25SB3.JPG

43.  Capable of clearing high and low fault
currents
 They do not deteriorate with age.
 They have high speed of operation.
 They required no maintenance
 They are cheaper than other circuit
interrupting devices of equal breaking
capacity

44.  They have to be replaced after each operation

45.  Fuses are used on power systems up to
115,000 volts AC.
 High-voltage fuses are used to protect
instrument transformers used for electricity
metering, or for small power transformers
where the expense of a circuit breaker is not
warranted.
 For example, in distribution systems, a power
fuse may be used to protect a transformer
serving 1–3 houses.

46.  Some of the high voltage fuses are:
◦ Cartridge Type HV HRC Fuse
◦ Liquid Type HV HRC Fuse

47.  It is similar in general construction to the low
voltage type except that some special design
features are incorporated.
 In some design the element is wound in the
shape of a helix so as to avoid corona effect
at higher voltages.

48.  It consists of a glass tube filled with carbon
tetrachloride solution and sealed at both ends
with brass caps.
 The fuse wire is sealed at one end of the tube
and the other end of the wire is held by a strong
spiral spring fixed at the other end of the glass
tube.
 When the current exceeds the prescribed limit,
the fuse wire is blown out.
 As the fuse melts the spring retracts part of it
through liquid director and draws it well into the
liquid.

49.  The figure below shows a liquid type hv hrc
fuse.
Fuse link
cork
Liquid Director
Spring
Glass Tube

50.  Fuse:
◦ It performs both detection and interruption
functions
◦ It is completely automatic
◦ Small in size.
◦ Operating time is very small about 0.002 sec or so
◦ Requires replacement after every operation

51.  Circuit Breaker:
◦ It performs interruption function only, the detection
of fault is made by the relay system
◦ It can requires relays for automatic action
◦ Large in size
◦ Operating time is comparatively large 0.1 to 0.2 s
◦ No need for replacement after operation.

52.  Capacitor Banks, Reactors and Static Var
Compensators would be covered later (At the
end of the course)
 Read Your self:
◦ CONTROL BUILDINGS
◦ PREVENTATIVE MAINTENANCE