4.1 AIR CIRCUIT BREAKERS
Air circuit breakers (acb) shall be of withdrawable metalclad, flush mounted, horizontal drawout isolation and air break type suitable for installing on cubicle type of switchboard. They shall be three or four poles type as specified and shall comply fully with IEC 60947-1 and 60947-2. They shall be ASTA or KEMA certified for minimum rupturing capacity of 50 kA at 415 V for 1 s or otherwise specified.
They shall consist of quick-make, quick-break, mechanically and electrically trip free mechanism arranged to give double break in all poles simultaneously. The closing mechanism shall be of stored energy type, either manually or electrically charged. Mechanical `ON’ and `OFF’ or ‘|’ and ‘0’ indicators shall be provided. The tripping mechanism shall be equipped with push button for independent manual tripping and shall be stable and not being opened by shocks. Short circuit protection shall be provided by the built-in magnetic element. Each pole of the circuit breaker shall be provided with an arc chute and interpole barriers to reduce arcing time for rapid deionization of the arc and
guard against flash over. The contacts shall be renewable type.
The operating mechanism and carriage shall have the following positions: –
(a) Service – In this position the main and control contacts are engaged.
(b) Test – In this position the main contacts are isolated but the control contacts are still engaged. It shall be possible to check the correct operation of the control circuits without energising the main circuit.
(c) Isolated – Both main and control contacts are isolated.
They shall be provided with marking to show the breaker positions with facility for padlocking the carriage in the Test and Isolated positions. They shall be equipped with the following interlock devices: –
(a) Prevent withdrawal of breaker while the breaker is in closed position.
(b) Prevent closure of breaker while the carriage is in any position between `fully isolated’ and `fully home’.
The arrangement of the busbar connections shall be such that with the circuit breaker withdrawn, the life parts shall be protected, either by suitable shrouding or lockable shutters. Minimum four numbers (2-Normally-Open, 2-Normally-Close) double break type auxiliary contacts shall be provided. Mechanical interlocks and/or electrical interlocks, where specified, shall be provided. Mechanical interlock shall be of code key type, arranged to mechanically operate the trip mechanism latch so that the breaker can only be closed when the key is trapped in the lock.
Electrical interlock shall be controlled by means of operation of auxiliary switches of another breaker designed to cut out the closing coils and mechanism of the parent breaker.Where used as bus-coupler, they shall be of 4 pole type and provided with electrical and/or mechanical interlocks as required so that it is not possible for the coupler to close with its associated main incoming supply breakers closed.