MCB installation (miniature circuit breakers) in the distribution boards shall comply with IEC 60898-1. They shall be incorporated with overcurrent protection. Their selection for use shall match their operating characteristics.
1. Type test certificates shall be produced for the MCB selected from international recognized testing laboratories.
2. Performance criteria
a) Breaking capacity
The rated short-circuit capacity, Icn, shall be higher or at least equal ot the maximum prospective short-circuit current at the points of installation as stated in the single-line drawings.
The rated service short-circuit capacity, Ics, of 6 kA and 10 kA breakers shall be 100 % Icn and 75 % Icn respectively.
All MCB in the distribution boards shall be from the same manufacturer and total discrimination with the upstream breakers under over-current conditions shall be guaranteed. Details of the operating characteristics and the coordination with other protective devices shall be submitted to the Engineer for approval. The details shall include the manufacturer’s full current discrimination tables showing the over-current discrimination levels.
Back-up protection is permitted when a current limiting circuit breaker is installed in the upstream. Details of this coordination shall be submitted to the Engineer for approval. The details shall include the manufacturer’s back-up protection tables showing the upstream/ downstream devices and the coordinated level of protection.
The electrical MCB shall be fully rated for uninterrupted continuous service based on an ambient temperature of 30 degree Centigrade.
The MCB shall be of 35 mm DIN symmetrical rail type and available in one, two, three and four pole version.
The MCB shall be suitable for mounting in any positions with no derating.
a) Trip characteristics
The trip characteristics of the MCB shall be Type B, C or D as indicated in the drawings.
b) Operating mechanism
The operating mechanism shall be of the quick-make, quick-break type with the speed of operation independent of the operator. In addition, the breaker mechanism shall mechanically trip free from the operating handle so as to prevent the contacts from being held closed against short circuit and other abnormal conditions.
The operating mechanism shall be designed to operate all poles simultaneously during opening, closing and trip conditions. The individual operating mechanism of each pole of a multi-pole MCB shall be directly linked within the MCB and not only by the operating handles alone.
All MCB shall be tested for isolating function as stipulated in IEC 60898. The isolation function shall guarantee that the breakers when in open position, should have isolation distance in accordance with the requirements necessary to satisfy the need for users safety.
Means of positive contact identification to mimic the exact position of the contacts shall be provided. The indication of the open and closed position of the main contacts shall preferably be provided by the position of the actuator. Color-coded bands and lettering on the actuator shall be incorporated to aid identification.
The operating handle shall be of the toggle type with facility for installation of padlock device.
All MCB shall have optional field installable accessories like shunt-trip coil, under-voltage release, ON/ OFF indications, alarm switch, etc. The installation shall be flexible such that to allow for fitting of accessories to either the left or right side of the MCB.
All MCB must be suitable for remote controlled ON/ OFF operation via an electrical motor operator.
The MCB shall have double function terminals to allow for correct termination of busbars and cables. The top terminal shall be suitable for PIN busbar, the bottom terminals shall be suitable for both PIN and FORK busbars.
The cable terminals shall be equipped with safety guards to protect against misconnection. The safety guards shall function in a manner such that there is only one possibility of cable insertion into the MCB terminals. This will eliminate all possibilities of loose cable terminations due to wrong insertions.
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