4.2 MOULDED CASE CIRCUIT BREAKERS
Moulded case circuit breakers (mccb) shall have the number of poles as specified in the Drawings and/or Bill of Quantities. They shall comply with IEC 60947-1 and 60947-2. They shall be fully tropicalised and suitable to be used up to an ambient temperature of 40 0C, enclosed in glass-reinforced polyester moulded case and suitable for use on 240/415 V, 50 Hz. a.c. supply system.
They shall be of the quick-make, quick-break type having manually operable toggle type handle. Permanent position indicators shall be provided to show status of the breaker. When tripping occurs, the handle shall be in the `trip’ position midway between the ‘OFF’ and ‘ON’ or ‘0’ and ‘|’ position so as to provide positive indication of automatic interruption. The operating mechanism shall be non-tamperable. The mccb shall have trip-free feature to prevent the breaker from being closed against fault conditions. Multipole mccb shall have common-trip operating mechanism for simultaneous operation of all poles.
The tripping units shall be one of the following types: –
(a) Thermal-magnetic type with bimetallic elements for inverse time-delay overload protection and magnetic elements for short circuit protection.
(b) Solid state trip unit with adjustable overload protection and adjustable short circuit protection with or without adjustable time-delay.
An arc extinguisher shall be incorporated to confine, divide and extinguish the arc drawn between the breaker contacts each time a breaker interrupts current. The contacts shall be of non-welding type. Unless otherwise specified in the Drawings and/or Bill of Quantities, the minimum a.c. interrupting capacity of the mccb shall be 50 kA rms (symmetrical) for 1 s at 415 V for switchboards connected to transformer or Supply Authority’s or Licensee’s incomer and 30 kA for the subsequent switchboards.
The mccb shall be of utilization category B and the rated service short-circuit breaking capacity (Ics) shall be 100% of the rated ultimate short-circuit breaking capacity (Icu) at 415 volts for incoming feeder, and for all outgoing feeder Ics shall be 50% of Icu.
If current limiting types of mccb are used, they shall be equipped with current limiting device of either permanent self-resetting power fuse type or magnetic repulsion movingcontact type.
The current limiting device shall coordinate with the normal trip mechanism so that all fault and overload currents occurring within the safe capability of the mccb shall cause the mccb to open, and all currents occurring beyond the capability of the mccb shall cause the current limiting devices to operate.
If required, the mccb shall have facilities for shunt trip, under-voltage/no-volt trip, externally connected earth fault protection, externally connected overcurrent protection etc.. They shall also have auxiliary contacts, accessories etc. for indication, alarm and interlocking purposes if necessary. Panel mounting external operating handle with padlocking facilities and door interlocking facilities to prevent the panel door from being opened to access to the mccb in closed position shall be provided.