7.8.1 All Distribution Boards must be factory assembled, type-tested and comply with BS EN 60439.
7.8.2 Distribution Boards must be of robust construction, capable of withstanding expected electrical, thermal, and environmental stresses in normal service and during faults. [Note: Final Distribution Boards, including the incoming cable, will normally be rated for the Connected Load of the Circuits supplied; other Distribution Boards and Distribution Circuits
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may be rated for the Diversified load, using appropriate diversity factors, and with allowance made for future load growth where appropriate - see Guidance Note G2.]
7.8.3 Apparatus forming part of the assembly of Distribution Boards shall have electrical isolation clearances sufficient to withstand normal voltages, surge voltages and creepages as defined in BS EN 60439-1.
7.8.4 Each Distribution Board must have a neutral bar which is mounted on insulators and which has a sufficient number of terminal points of adequate size for the largest cable expected to be used.
7.8.5 Each Distribution Board must have an Earth bar which has a means of connection to the incoming Earth Conductor and cable gland of the incoming cable, see Appendix A5(e) and A12(c).
7.8.6 All Final Distribution Boards must, be arranged so as to provide for at least two zones of Earth Leakage Protection (e.g. 100mA and 30mA) and to avoid complete disconnection of power to the area being served, in the event of an earth fault. This may be achieved by either of the following methods:
(a) two or more busbar sections, each protected by an RCD;
(b) one busbar section, with individual RCBOs fitted on all Final Circuits (the incomer device may be a time- delayed RCD or an isolator switch where adequate fault protection is provided by an upstream Protective Device);
(c) two or more FDBs installed together to serve an area, each having one busbar section protected by an RCD of appropriate rating (e.g. one DB with an incomer 100mA RCD for lighting and another DB with incomer 30mA RCD for power circuits)
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Components
where the manufacturer's type tests are provided. Single phase DBs up to 12 ways, serving a limited area, may be provided with one busbar section, and one incomer RCD of appropriate rating, e.g. hotel rooms, pump room, central a/c room, etc.]
7.8.7 Other than for small 2-storey buildings each floor of a Premises shall be provided with at least one Final Distribution Board installed in an easily accessible location.
7.8.8 The number of Final Distribution Boards and Sub Distribution Boards provided in a Premises shall take into account the future accessibility, maintainability and safety of the system, whilst limiting the extent of possible power outages to serviced areas.
[Note: to meet the above requirement Final Distribution Boards should be limited to a maximum capacity of 14 ways 3-phase (or 42 ways single phase) and Sub Distribution Boards should be limited to 18 ways 3-phase.]
7.8.9 Single-phase Distribution Boards may be permitted in a Premises where adequate provision is made for balancing the total load at the Supply Intake between the three phases.
[Note: single-phase Distribution Boards would normally be used for small load applications such as hotel rooms, garages, small out buildings.]
7.8.10 The phase and neutral busbars in all Distribution Boards shall be identified by the colours given in Appendix A8.
7.8.11 The phase and neutral conductors in all Distribution Boards shall be of the same cross sectional area.
7.8.12 Neon indicators, voltmeters and ammeters must be provided for Distribution Boards rated at 400 A and above, and where reasonably practicable for other Distribution Boards. [Note: where practicable, maximum demand meters and power factor meters should also be provided.]
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7.8.13 All outgoing circuits of Distribution Boards must be provided with only circuit-breakers (such as CBs, MCBs, MCCBs, RCDs, RCBOs, etc) and shall not contain fuses of any kind, except for capacitor banks. See Regulation 10.2. [Note: the incoming device of a DB may be an isolator switch, where adequate fault protection is provided by an upstream Protective Device.]
7.8.14 All Distribution Boards must be installed in locations easily accessible for inspection, operation and maintenance, preferably at the entrance to the area they serve. Such locations must be secured from unauthorised interference. 7.8.15 Distribution Boards of rating above 200 A shall be installed
in a dedicated room. Distribution Boards of rating 200 A and below may be installed in a cupboard or other suitable enclosure complying with clause 7.8.14 and clause 3.5.9.
[Note: Main Distribution Boards of rating 200 A and above must be located in a Supply Intake room, see Appendix A12(b).]
7.8.16 Distribution Boards shall not be installed in locations where water is used (e.g. kitchens, bathrooms, pump rooms), within 2 metres of any pipe or other source of water, or in difficult-to-access positions (such as stairways).
7.8.17 Distribution Boards must be provided with sufficient numbers of Protective Devices so as to enable every outgoing Circuit to be connected to an individual Protective Device and shall include not less than 10% spare Protective Devices or blank positions.
7.9 LV switchboards
7.9.1 Where prefabricated Distribution Boards of sufficient rating are not available, LV switchboards, consisting of cubicle panel switchgear, may be provided. LV switchboards must consist of type-tested assemblies which satisfy the requirements of BS 5486 or BS EN 60439.
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Components
7.9.2 Switchboards must be located in a dedicated room to provide for safe access and egress at all times, including during maintenance, repair and replacement work. 7.9.3 LV switchrooms must meet the requirements of Supply
Intake rooms listed in Regulation 4.2. Typical layouts and minimum space requirements for LV switchrooms are given in Appendix A12(d).
7.9.4 Switchboards rated above 400 A must be provided with instruments for voltage, current, maximum demand and power factor measurement, as well as phase indicating lamps.
7.9.5 Where more than one incoming supply cable is provided at the Supply Intake these must be connected to separate switchboards (or separate sections of the same switchboard) which have the facility for interconnection through a bus coupler circuit-breaker or switch. In order to prevent parallel connection of the incoming supply cables the bus coupler circuit-breaker or switch must be interlocked to prevent its closure when both incoming supply cables are live. Bus switches and circuit-breakers must be capable of closure onto a fault unless interlocked so as to only be operated whilst dead.
7.9.6 All busbars in switchboards must be tinned copper and rigidly supported throughout their length. The neutral and earth busbars must run throughout the length of the switchboard. The neutral busbar must be of the same cross- sectional area as the phase busbars.
7.9.7 The main incomer circuit-breaker(s) must be clearly marked and left unlocked to allow immediate operation in an emergency, see clause 4.3.3.