Vacuum Air Circuit Breakers in MV Distribution System for Electrical Construction





The air vacuum circuit breakers are of the triple pole indoor type for rated voltage 24 KV.  They operate on the vacuum arc quenching principle and outstandingly cost effective. The vacuum breaker consists of the mechanism 2 housing complete with stored energy mechanism and control elements.  The three poles with vacuum interrupters, cast resin insulators and bracing struts, and the operating rods connecting the mechanism with the breaker poles.

Standard Specification

The air vacuum circuit breakers should conform to IEC 56 and 694, BS 5311 and DIN VDE 0670.

Technical Specification

For circuit breaker cubicles:

  1. Enclosure: Metal enclosed for indoor installation.
  2. Entry: Bottom cable entry.
  3. Access: Free standing front access.
  4. Protection: IP 4X tropicalized.
  5. Main bus bar: Air insulated copper.
  6. Circuit breaker: Withdrawable type with automatic closing shutters in withdrawn position.
  7. Position: Service/Test with necessary safety mechanical interlocks.
  8. Safety: Open arc test.

For circuit breakers:

  1. Maximum voltage: 24 KV.
  2. Frequency: 60 Hz.
  3. Power frequency withstand voltage: 38 KV.
  4. Impulse withstand voltage: 95 KV.
  5. Rated current: 400 A.
  6. Rated short circuit current: 16 KA.
  7. Rated short circuit making current: 40 KA.
  8. Ambient temperature: 55 deg. C.
  9. Relative humidity: 100%.

1. Construction

The air vacuum circuit breaker consists of the mechanism housing, three pole assemblies with vacuum interrupter, cast resin post insulators and struts and the operating rods.

The mechanism housing has a detachable cover, this has cutouts for the actuating and indicating devices.  The mechanism housing accommodates the operating and control elements.  Each of the three pole assemblies is supported by two cast resin post insulators which are bolted to traverse.

The pole assembly consists of the upper pole support with the upper terminal, the vacuum interrupter, the lower terminal and the flexible connection, the lever, and bracket for interrupter operation and the contact pressure opening springs.  The struts relieve the vacuum interrupter of external forces.

The operating rods of glass fiber reinforced epoxy resin transmit the action of the operating mechanism to the pole assemblies.  Their length can be adjusted by means of steel lugs fitted on the pole side.

A 24 way plug connector for connection of the control and signaling circuits is mounted on top of the mechanism housing.

2. Vacuum Interrupter

The vacuum interrupter is fixed rigidly to the upper pole support by means of the terminal post.  The lower part of the interrupter with the moving contact is located axially in the lower pole support by the guide.  A metal bellows seals off the interrupter from the surrounding atmosphere and enables axial movement of the contact.

3. Conducting Path

The conducting path is formed by the upper pole support with the upper terminal, the particular fixing and connection part of the vacuum interrupter with the fixed contact, the moving contact with the terminal bolt, the flexible connection and the lower pole support with the terminal.

4. Arc Extinction

A metal vapor arc discharge is initiated by the current to be interrupted as the contacts open.  Current flows through this plasma until the next current zero.  The arc is extinguished in the vicinity of the current zero and the metal vapor condenses within a few microseconds on the metal surfaces, this very quickly restores the dielectric strength of the contact gap.

To prevent local overheating of the contacts when heavy current are designed so that the arc does not stand still at one point of their surface but is caused to rotate by its own magnetic field.

A certain minimum current is needed to maintain the metal vapor arc discharge.  Should the current drop below this minimum, it is chopped before current zero.  To prevent excessive over voltage on switching of inductive loads, air vacuum break keep the chopping currents to a minimum.  This is achieved by using special contact material.

The arc is reliably extinguished even in case where contact separation takes place shortly before a current zero.  This is the result of the rapid restoration of dielectric strength of the contact gap.  The maximum arcing time of the last pole to clear is therefore only 15 ms.

The metal vapor plasma is highly conductive and the arc voltage is therefore very small - between 20 V and 200 V.  For this reason and because of the short arcing times, the amount of energy converted in the break is also very small.  This accounts for the long electrical endurance of the vacuum interrupter.

Because of the high vacuum (10E-9 bar) in the interrupter a contact gap of only a few millimeters is required for a high dielectric strength to buildup.

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