distribution

System Listings

Mains Distribution

System Overview
The mains distribution is the system by which electrical energy is transmitted around the building. The mains distribution originates at the mains intake and radiates out throughout the building in a branching or tree like fashion.

The mains distribution design is dependent upon the following:

Type of building
Load requirements (size, importance and use)
Type of supply
Environmental conditions

Systems Available and Applications

In addition to be served by and using items of LV Switchgear, the mains distribution system will commonly contain some or all of the following:

Distribution Cables. Called "mains cabling", "sub-mains cabling" or "distributors" depending upon your background and what books you read! Distribution cabling will run from the mains intake to various equipment situated throughout the building. Often intermediate distribution board or switchpanels may be installed - generally referred to an "sub-distribution boards".
Many forms of cable are used for mains distribution including :
- Steel Wire Armoured (SWA) - Possibly the most common form of sub-main distribution cabling.
- Mineral Insulated Copper Clad (MICC) - Very long life, and robust nature make this a good form of cabling for sub-mains distribution, although its application is limited by it's high cost.
- Singles - Standard single core cables can be used for distribution cabling. But require mechanical damage. This means that cable containment is required along the entire route.
  
  In defined situations is is necessary to ensure that sheathing is of a Low Smoke & Fume (LSF) type.
Busbar Distribution Systems. Normally in the form of a set of copper conducting bars enclosed within a steel housing. The system is linear and would be installed in long runs either along the building or up through it. The bars would be rated at a high current and protected by a suitable fuseswitch at the mains intake or other distribution centre.
Where required, a "tap off" box is installed and the required switchgear is installed fed from the busbars. Many item of equipment can be served from the same busbar trunking run. It often represents an economical system, particularly in industrial premises or tall buildings where a central riser is used to take the busbar trunking vertically up through the building.
Sub-distribution board. These are distribution boards which although similar construction, are larger than a final circuit distribution boards. The boards are installed half way through the mains distribution system, generally at the point where a large distribution cable terminates, and several smaller sub-circuits start.
The board may be fitted with HRC fuses, MCBs or MCCBs. It may hence take the form of a large fuseboard or and LV panelboard.
Final Circuit Distribution board. The distribution board is generally installed locally to where the electrical power is used (point of utilisation). It contains protective devices, which protect final circuits such as lighting & power circuits.
These devices may be Cartridge fuses, HRC fuses, MCBs or even rewirable fuses!.
The board is normally fitted with an incomer - this may be a fuseswitch, isolator or MCCB (without certain protection setting)
Miniature Circuit Breaker (MCB). This is a protective device that is installed primarily to protect final circuits against overload and short circuits. An MCB is a small compact device, generally limited to cables no bigger than say 16mm² c.s.a. The MCB is normally installed within a distribution board and may be a single or three phase device. Various types of MCB are available (type B,C,D) these types relate to the tripping characteristic of the device. The MCB nowadays largely replaces the cartridge fuse and semi enclosed (rewirable) fuse as a safer and more convenient alternative.

Legislation & Guidelines

Mains distribution is governed by a range of regulations and legislation

British Standards.

Also refer to The British Standards Institution

BS 7671:1992 "Requirements for electrical installations". IEE Wiring Regulations. Sixteenth edition. This is the all encompassing "Bible" in relation to the design, installation and use of electrical installations and equipment in buildings.
BS 4752:1977 "Circuit Breakers". This covers the manufacture and application of Circuit Breakers for use with voltages up to and including 1000V a.c and 1200V d.c. (Supersedes BS862:1939 & BS936:1960)
BS 5486:1989 "Low-voltage switchgear and controlgear assemblies". Specification for particular requirements of consumer units
BS 7767:1994"Method for assessing the short-circuit withstand strength of partially type-tested assemblies (PTTA)" - Method of extrapolation which may be used for partially type-tested assemblies to prove compliance with the requirements of 7.5.1 of IEC 439-1.
BS 3036:1958 "Specification. Semi-enclosed electric fuses (ratings up to 100 amperes and 240 volts to earth)" Sizes of fuse carriers and bases for fuses where element is replaceable wire. For use in a.c. circuits. Three categories according to prospective current and time constant. For use at altitudes not exceeding 2000 m and temperatures averaging 35°C. Selection of fuses for different applications.
BS 1361:1971 "Specification for cartridge fuses for a.c. circuits in domestic and similar premises" Requirements, ratings and tests for fuse links, fuse bases and carriers. Dimensions and time/current zones for fuse links. Type I-rated 240 V and 5 A to 45 A for replacement by domestic consumers; Type II-rated 415 V and 60 A, 80 A or 100 A for use by the supply authority in the incoming unit of domestic and similar premises.
BS 6724:1997 Specification for 600/1000 V and 1900/3300 V armoured electric cables having thermosetting insulation and low emission of smoke and corrosive gases when affected by fire Construction and test methods for cables, which, in specified tests, produce less smoke and corrosive products than cables conforming to BS 5467 and BS6346; for use in fixed installations in industrial areas,buildings and similar applications.

Combined Standards

BS EN 60439-5:1996 "Specification for low-voltage switchgear and controlgear assemblies" - Particular requirements for assemblies intended to be installed outdoors in public places. Cable distribution cabinets (CDCs)for power distribution in networks

BS EN 60947-2:1996 "Specification for low-voltage switchgear and controlgear" - Circuit-breakers Characteristics, conditions for operation, methods of test and marking requirements for circuit breakers rated up to 1000 V a.c. or 1500 V d.c. Also covers integrally fused circuit breakers.

BS EN 60947-1:1992 "Specification for low-voltage switchgear and control gear" - General rules. Defines as far as practicable all rules and requirements of a general nature to obtain uniformity of requirements and tests throughout the range of applicable equipment and avoid the need for testing to different standards.
BS EN 60439-2:1993 Low-voltage switchgear and controlgear assemblies. Particular requirements for busbar trunking systems (busways) Definitions, calculations of resistance and reactance, types of mechanical load, connection of tap-off units, type tests, temperature rise tests, verification of structural strength and endurance of systems with trolley type tap-off facilities.

International Standards