Page 341 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Due to these disadvantages, plain - break oil circuit breakers are used only for low - voltage not exceeding 11 KV
applications where high breaking- capacities are not important.
Vacuum circuit breaker (VCB)
Circuit breaker which uses vacuum as an arc quenching medium is called as vacuum circuit breaker.

Vacuum offers the highest insulating strength and have the superior arc quenching properties than any other
medium. When the contacts of a breaker are opened in vacuum, the interruption occurs instantly as the dielectric
strength between the contacts are many times higher than the other circuit breakers.
The technology is only suitable for medium voltage application. For higher voltage application, the vacuum
technology has been developed.

Principle of vacuum circuit breaker
• When the contacts of the breaker are opened in vacuum (107 to 105 torr), an arc is produced between the
contacts by the ionisation of metal vapours i.e, combination of electrons and ions of contacts. However, the
arc is quickly extinguished because the metallic vapours, rapidly cools resulting quick recovery of dielectric
strength.
• The salient feature of vacuum is, as soon as the arc is produced in vacuum, it is quickly extinguished due to
the rapid recovery of dielectric strength of vacuum.
Construction of vacuum circuit breaker
Fig 6 shows the typical parts of vacuum circuit breaker

• It consists of the fixed contact, moving contact and arc shield mounted inside a vacuum chamber.
• The movable member is sealed by a stainless steel bellows, is connected to the control mechanism . This
enables the permanent sealing of the vacuum chamber, to eliminate the possibility of leak.
• A glass vessel or ceramic vessel is used as the outer insulating body.

• The arc shield prevents the metallic vapours falling on the inside surface of the outer insulating cover.
Working of vacuum circuit breaker
• When the breaker opens, the moving contact is separated from the fixed contact and an arc is produced
between the contacts. The production of arc is due to the ionisation of metal ions and depends upon the
material of contacts.
• The arc is quickly extinguished because the metallic vapours , are diffused in a short time and condensed on
the surfaces of moving and fixed members and arc shields.
• Since vacuum has rapid Arc recovery rate of dielectric strength, the arc extinction in a vacuum breaker occurs
with a short separation (say 0.625 cm) of contacts.
Application of VCB
• Vacuum circuit breakers are employed for outdoor applications ranging from 22KV to 66KV.
• They are suitable for majority of applications in rural areas.
Sulphur hexafluoride (SF6) circuit breaker

Circuit breakers which uses the sulphur hexafluoride gas (SF6) as an arc quenching medium is called as SF6
circuit breaker.
The sulphur hexafluoride gas (SF6) is an electronegative gas and has a strong tendency to absorb the free
electrons. When the contacts of the breaker are opened in a high pressure sulphur hexafluoride (SF6) gas
medium and an arc is struck between them.

The SF6 gas capture the conducting free electrons in the arc and form immovable negative ions. This loss of
conducting electrons in the arc quickly improve the insulation strength to extinguish the arc.
The sulphur hexafluoride (SF6) circuit breakers are very effective for high power and high voltage applications.

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CITS : Power - Electrician & Wireman - Lesson 106-116