Electrical ACB installation

This section lays out the major performance requirements for the ACB installation (electrical circuit breakers of the air-break type) inside switchboards.

Switchboard circuit breakers shall be of the air-break type suitable for indoor use and withdrawable complete with all necessary instruments, transformers, closing and tripping devices and protective instruments. They shall be easily accessible and operable from the front and provided with padlocking facilities. Both phase and neutral switching shall be required. Circuit breakers shall comply with IEC 60947-2 regarding their test performance and number of operating cycles, and shall have a rated short-circuit making and breaking capacity specified in the drawings and complying with IEC 60947-2 Table II. They shall be trip-free.

Locking facilities shall be provided on the ACB and control switch so that in any position the circuit breaker can be prevented from being directly and manually operated. Mechanical and electrical lockouts shall be provided to prevent closing of the electrical ACB after an over-current trip.

All operating mechanisms shall have mechanical ‘ON’ and ‘OFF’ indicators. ‘CLOSED’, ‘OPEN’ and ‘TRIPPED” indication lamps shall be provided. Hand charged or motor charged spring mechanism shall have mechanical indicators to show ‘SPRING CHARGED’ and “SPRING DISCHARGED’.

All ACB supplying loads from the main switchboards shall have motor charged operating mechanisms, which shall be arranged so that release of the spring to close the circuit breaker can only be done by deliberate action. It shall not be possible for shocks to release the charged spring. Remote controlled switching shall also be provided for motor charged operating mechanism ACB.

Closing and trip control switches shall turn clockwise for closing and anti-clockwise for tripping and labeled ‘OPEN-NEUTRAL-CLOSE’ with spring return to neutral.

The shunt trip coil shall be operated by 240 Vac supply unless otherwise stated in the design drawings.

All circuit breakers shall be provided with interlocks to ensure that:

a) a circuit breaker cannot be connected or withdrawn while closed;
b) a circuit breaker cannot be closed until they are fully plugged in or is completely isolated;
c) a spring charged mechanism cannot be discharged until it is fully discharged.

Mechanical and electrical interlocks shall be provided in cases where multiple incoming supplies are not to be paralleled. Tripping of a closed ACB shall not occur if an attempt is made to remove the trapped key or when it is in the withdrawn positions.

Facilities for operational tests of the circuit breaker shall be provided when in the isolated or withdrawn positions.

Provide auxiliary contacts for remote monitoring of all incoming and sectionalizing breakers as well as all distribution breakers on switchboards supplied from the emergency power system.

ACB used for distribution on switchboards supplied from emergency power system shall have provision for remote operation (OPEN and CLOSE) for load management.

All protective relays and instruments shall be provided as shown in the design drawings.

Where specified in the design drawings, the ACB shall have integral solid-state trip units with built-in over-current and earth-fault protection having time-current characteristics. A combination of the following time-current curve-shaping adjustments shall be provided:

a) Ampere setting
b) Long time delay
c) Short time pickup
d) Short time delay
e) Earth fault pickup
f) Earth fault delay

The solid-state trip-unit shall have a digital information system that provides circuit current values and mode of trip indication (i.e. overload, short circuit, and earth fault) with fault current levels displayed following an automatic trip operation.

All solid state trip units shall be provided with test jacks for in-service functional testing using a small hand-held test kit.


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