BEHIND BARS

C4.4.1 General

All circuit breakers shall not be of moulded case type. The neutral pole shall have cross-sectional areas not less than that of respective phase pole.

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ACB shall be of triple-pole with switch neutral (four pole) or triple-pole with bolted neutral, as specified, fully metalclad with earth terminals. It shall be of horizontally withdrawable type having spring assisted closing and, where specified, motorised spring charger and push-button electrical control.

ACB shall comply with and be type tested to IEC 60947-2.

Unless otherwise specified, ACB shall have built-in overload protection with IDMT characteristics and instantaneous short circuit interruption. Where an ACB is used as the main incomer, the IDMT characteristics shall be compatible with those of the Electricity Supplier.

C4.4.2 Performance Characteristic

ACB shall be certified, in accordance with IEC 60947-2, to have the following performance characteristics:

(a) Rated short-circuit breaking capacity - not less than 50 kA (b) Rated short-circuit making capacity - not less than 105 kA (c) Rated short-time withstanding current - not less than 50 kA for

1 second

C4.4.3 Closing and Tripping Operation

Unless otherwise specified, ACB shall be suitable for independent manual operation utilizing energy stored in a spring. For automatic closing ACB, the spring mechanism shall be wound by a universal motor fitted with limiting switches. In addition, the spring mechanism shall have manual charging facilities for emergency use. All ACBs shall have slow closing mechanism for contact checking.

Unless otherwise specified, trip coil of 30V d.c. shall be fitted for local, remote or relay tripping.

"Trip" push buttons shall be provided on the front plate of each ACB. These buttons shall be direct acting mechanically.

When automatic closing is specified, the control circuit of the ACB shall be fitted with suitable anti-pumping devices.

C4.4.4 Racking Gear

The enclosure shall be fitted with a set of racking gear which shall lock the ACB to the runner rails in three distinct positions, i.e. CONNECTED, TEST and ISOLATED. The ACB can only be closed fully when it is in the CONNECTED position. In the TEST position, it shall be possible to operate the ACB (for ACB with motorised charging spring only) to close and test the secondary and auxiliary contacts, but not the main isolating contacts.

The racking gear shall consist of rollers fitted to the ACB frame. All parts shall be accurately aligned and adjusted so that the ACB can be moved freely along the runner rails within the enclosure between the two extreme positions, i.e. CONNECTED and ISOLATED. Facilities shall be provided to padlock the ACB in its CONNECTED, TEST or ISOLATED position.

Position indicators and breaker-condition indicators shall be provided. These indicators shall be arranged so as to be prominently visible when the ACB is in its normally installed position.

C4.4.5 Interlocking Facility

Suitable interlocking facilities shall be provided such that:

(a) the ACB cannot be plugged in or withdrawn when it has been closed;

(b) the ACB cannot be closed until it is fully engaged in either the CONNECTED or the TEST position;

(c) the ACB cannot be slowly closed except in the TEST or ISOLATED position;

(d) it shall not be possible to initiate the closing action of the energy stored spring mechanism until the spring has been fully charged. (e) Where key interlocking is employed, tripping of a closed ACB

shall occur if an attempt is made to remove the trapped key from the mechanism.

C4.4.6 Main Isolating Contacts and Safety Shutter

The main isolating contacts shall be self-aligning and shall be protected by insulated barriers forming the safety shutter.

Two sets of safety shutters shall be provided for every ACB, one for the incoming terminals of the main isolating contacts and the other for the outgoing terminals. Each set shall be capable of being operated individually.

When the ACB is not in the CONNECTED position, the safety shutters shall close automatically, screening off the main isolating contacts. When the ACB is being racked into the CONNECTED position, the safety shutters shall automatically open the insulating barrier, thereby allowing the entry of the movable isolating contacts but without lowering the degree of protection.

Safety shutters shall be fitted with properly designed and securely fixed warning labels. In addition, facilities shall be provided to padlock the safety shutters after the ACB has been withdrawn from the enclosure.

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C4.4.7 Contact

All contacts shall be self-aligning, provided with spring accelerated opening mechanism.

Main contacts shall be fitted with detachable arc chutes for each pole. Sufficient auxiliary contacts shall be provided for connection of secondary wiring. These auxiliary contacts shall remain engaged when the ACB is in the TEST or CONNECTED position and shall be disengaged when the ACB is in the ISOLATED position.

All ACB shall be fitted with the maximum number of auxiliary contacts as the manufacturer's standard provision. In any event, not less than two pairs of normally-closed auxiliary switches and two pairs of normally-open auxiliary switches shall be provided. All auxiliary contacts and switches shall be wired to an easily accessible terminal strip for external connection.

Terminals for external connections of the secondary/auxiliary contacts and switches shall be of clamping yoke type for wires of minimum CSA of 1.0mm². All terminals shall be clearly labelled to identify the function of the circuits connected to the terminals.

Terminals for each ACB for secondary/auxiliary contacts shall be separated from all other terminals by means of a spacer of 20 mm width.