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Developed and maintained
by the NFCC

Three-phase low-voltage systems


Industrial or commercial premises

The electrical demand of commercial and small industrial premises may be such that electricity supplies will be provided through a three-phase supply.

The connection from the distribution system will be similar to a single-phase supply, but the size of cable providing the electricity will be able to take a greater level of voltage and current. The system will be provided with a cut-out, connected to the three-phase meter by single-core insulated and sheet conductors.

Past the meter, the supply is fed through a three-phase main switch to a distribution fuse board. In larger installations, the main switch may be connected to a busbar chamber, from which separate single-phase circuits may be taken to distribution boards at remote parts of the premises.

Residential premises

Blocks of flats are often supplied by a single three-phase service cable, which terminates in a multiway cut-out or fuse board.

A number of separate fuses (not normally more than eight) are connected to each line conductor and single-phase ring mains, which are taken to each flat. If each flat is provided with an individual meter, there will be another cut-out or main fuse before the meter and consumer unit. Alternatively, the meter will be located in a single meter position, adjacent to the service terminal equipment, and the rising main will terminate in each consumer unit for each flat.

In larger complexes there will be more than one service cable. For example, the first three-phase cable may terminate on the ground floor and the single-phase rising main will service the flats on that floor and the few floors above. Another three-phase cable will terminate at higher levels in the building to service flats on that level and the floors above, and so on. These supplies will have passed through the main intake room or switchgear. Isolation of the whole building will be achieved by locating the main intake switch room; alternatively, individual floors can be isolated.

It is imperative for fire and rescue service personnel to locate the main intake switch and the meters for each floor or group of floors.

Hazards (for further information refer to National Operational Guidance: Utilities and fuel)

  • Electrocution (especially if there is water at the electrical intakes)
  • Damage to switchgear can make isolation difficult - this will require specialist assistance

References and further reading