Reading an electrical standard in full is a tedious job which most of us don’t relish. At the same time, familiarity with the provisions of the standard is essential for effective engineering practice. Considering this requirement, I am planning to publish digests of the relevant standards applicable to electrical equipment and switchgear. This will be a heavily condensed form of the standard but without missing the major points. It should also be kept in mind that this is no alternative to reading the standard itself. The number followed by the symbol # in brackets at the end of a sentence refers to the clause number in the standard.
MCB distribution boards are used to distribute electrical power to downstream loads. IS 13032:1991 (reaffirmed 2016) regulates the statutory attributes.
The following discussion is based on IS 13032:1991 (reaffirmed 2001)
- MCB distribution board is defined as “An enclosure containing busbars and miniature circuit-breakers for the purposes of protecting, controlling or connecting more than one outgoing circuit fed from one or more incoming circuits. Miniature circuit-breaker boards are also known as miniature circuit-breaker distribution boards (MCB DB) or MCB boards (#3.1)
- The rated current of the MCB board is the rating of the busbars (#3.3)
- Three types of MCB boards are defined. They are a) Single pole and neutral (SPN), b) Triple pole (TP), and c) Triple pole and neutral (TPN) (#5.1)
- The preferred number of outgoing circuits per phase are specified as 4, 6, 8, 12 and 16 (#5.2)
- The preferred rated voltages are 240 V and 415 V (#6.1)
- The preferred rated currents of the MCB outgoing ways shall be 6, 10, 16, 20, 25, 32, 40, 63, 80, 100 A (#6.2)
- The rated current of the busbar shall be 100 A or 200 A. The neutral busbar shall have the same rating as the phase busbar for the SPN MCB board. For TPN MCB boards, the neutral busbar shall be of the same rating as the phase busbar for boards up to and including 32 A and of half the rating as the phase busbars (subject to a minimum of 32 A) for boards having other ratings (#6.3)
- The manufacturer shall state the short time withstand capability of the board both in terms of the magnitude of the current and the duration of withstand. Unless otherwise specified, the time of withstand shall not be less than 0.1 second. The magnitude of the current shall be at least equal to that of the MCB incorporated in the board having the highest category of duty (#6.6)
- Routine tests are intended to detect faults in materials and workmanship. They are carried out on every new MCB board after its assembly or on each transport unit. Another routine test at the place of installation is not required (#9.1.2)
- Routine tests include a) routine tests on MCBs as the case may be, b) Inspection of the board including inspection of wiring, and if necessary, electrical operation test, c) dielectric test and d) Checking of protective measures (#9.1.2)