This document discusses vacuum circuit breakers and the selection of circuit breakers. It describes the construction of vacuum circuit breakers including their moving contacts, fixed contacts, and arc shields. It explains the principle of operation where an arc is produced and quickly extinguished in vacuum. It discusses the advantages of vacuum circuit breakers such as being compact, reliable, and having a long life. The document also outlines factors to consider when selecting a circuit breaker such as voltage, frequency, interrupting capacity, and continuous current rating. Finally, it provides a table comparing different circuit breaker types and their applicable voltages and breaking capacities.

1. Vacuum Circuit Breaker
&
Selection of Circuit Breakers
Prepared By:
Shubham Bharadia

2. Index
• Construction
• Principle of operation
• Working
• Advantages
• Disadvantages
• Conclusion
• Selection of Circuit Breakers
• Typology of circuit breakers
• Table for selection

3. Construction
• What is a circuit Breaker?
• A circuit breaker is an automatically operated electrical
switch designed to protect electrical circuit from damage.
• A vacuum circuit breaker is a circuit breaker where
vacuum is used to extinguish the arc.
• The main parts of a VCB :
 Moving contact
 Fixed contact
 Arc shield

4. Construction
• As vacuum offers the highest insulating strength. So it
has far superior arc quenching properties than any other
medium. The degree of vacuum is in the range from 10-7
to 10-5 torr.
• The movable member is connected to the control
mechanism by stainless steel bellows.
• A glass vessel or ceramic vessel is used as the outer
insulating body.
• The arc shield prevents the deterioration of the internal
dielectric strength by preventing metallic vapours falling
on the inside surface of the outer insulating cover.

6. Principle of VCB
• When the contacts of the breaker are opened in vacuum,
an arc is produced between the contacts by the
ionisation of metal vapours of contacts.
• The arc is quickly extinguished because the metallic
vapours, electrons and ions produced during arc rapidly
condense on the surfaces of the circuit breaker contacts.
• As soon as the arc is produced in vacuum, it is quickly
extinguished due to the fast rate of recovery of dielectric
strength in vacuum.

7. Working
• The circuit breaker are essential as isolator cannot be
used to open a circuit under normal conditions as it has
no provision to quench arc that is produced after opening
the line.
• It has to perform following function:
 Full load current is to be carried continuously
 Opening and closing the circuit on no load
 Making and breaking the normal operating current
 Making and breaking the fault currents of magnitude up
to which it is designed for.

8. Working
 Arc extinction is the main process of a circuit breaker.
 We know that circuit breakers work under the
instructions from relay. When the trip coils from relay
energize, circuit breaker identifies fault occurred and
hence the contacts are pulled apart.
 A high intensity current flows through the contacts of the
circuit breaker before they are opened by the system.
 VCBs are capable of interrupting currents with a very
high (di/dt), typically in the range of 150 - 1000A/μs and
consequently the high frequency current

9. Advantages of VCB
• They are compact, reliable and
have longer life.
• There are no fire hazards.
• They can interrupt any fault
current.
• No noise is produced while
operating.
• Require less power for control
operation.
• It has higher dielectric
strength.

10. Advantages
• Suitable for repeated operating duty.
• Usable at any voltage up to 230kV and higher voltages
where longer life and maintenance free operation is
desired.
• Non explosive and silent operation compared to air blast
and SF6 breakers.
• Because of the above reasons with economical
advantages added these circuit breakers are widely
used.

11. Disadvantages of VCB
• Loss of vacuum due to transit damage or failure makes
the entire interruption useless and it cannot be repaired
at site.
• It needs additional surge suppressors in parallel with
each phase for interruption of low magnetizing currents
in certain range.
• vacuum circuit breakers are uneconomical above 36kV.

12. Conclusion
• Thus, a circuit breaker is equipment that breaks a circuit
either manually or automatically under all conditions at
no load, full load or short circuit.
• Hence, we can say that Vacuum circuit breaker is really
the most reliable one with low cost, no fire hazards, no
gas generation, low arc energy, and little maintenance.
So this is the most widely used circuit breaker used for
high voltage transmission.
• Vacuum circuit breakers are employed for outdoor
applications ranging from 22kV to 66kV.

13. How to choose a circuit
breaker?
• There are a few different criteria to consider when
selecting a circuit breaker including
1. Voltage
2. Frequency
3. interrupting capacity
4. continuous current rating
5. unusual operating conditions
6. product testing

14. • Voltage Rating
 The overall voltage rating is calculated by the highest voltage that
can be applied across all end ports.
 It is important to select a circuit breaker with enough voltage
capacity to meet the end application.
• Frequency
 Circuit breakers up to 600 amps can be applied to frequencies of 50-
120 Hz.
 Higher than 120 Hz frequencies will end up with the breaker having
to derate.

15. • Maximum Interrupting Capacity
 The interrupting rating is generally accepted as the highest
amount of fault current the breaker can interrupt without
causing system failure to itself.
 Determining the maximum amount of fault current supplied
by a system can be calculated at any given time.
• Continuous Current Rating
 In regards to continuous current rating, modelled case circuit
breakers are rated in amperes at a specific ambient
temperature.
 This ampere rating is the continuous current the breaker will
carry in the ambient temperature where it was calibrated.
 A general rule of thumb for circuit breaker manufactures is to
calibrate their standard breakers at 104° F.

16. • Typology of circuit breakers
 Low Voltage Circuit Breakers
 Medium Voltage Circuit Breakers
 High Voltage Circuit Breakers

17. • Basic types of circuit breaker
 Air Circuit Breakers
a) Air circuit breaker
b) Air-blast circuit breaker
 Oil Circuit Breakers
a) Minimum oil Circuit Breakers
b) Bulk oil Circuit Breakers
 Vacuum Circuit Breakers
 SF6 Circuit Breakers

18. Selection of circuit breakers
TYPE Voltage Breaking
Capacity
MCB 400V-600V For small rating
current
Air break circuit
breaker
400V-11kV 5-750MVA
Minimum oil
circuit breaker
3.3kV-220kV 150-2000MVA
VCB Vacuum(10-5-10-
7torr)
3.3kV-33kV 250-2000MVA
SF6 SF6 at 5Kg/cm2 2.2kV-765kV 1000-50,000MVA
Air blast circuit
breaker
Compressed air at
high pressure
66kV-1100kV 2,500-60,000MVA
Table showing selection of different circuit breakers
for different voltage and breaking capacity

19. THANK YOU