16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

  6.1. - Basic requirements for circuits 6.4 - Industrial socket outlet circuits
  6.2 - Maximum demand and diversity 6.5 - Other circuits
  6.3 - BS1363 socket outlet circuits 6.6 - Circuit segregation

6.1.1 - Basic requirements for circuits

The Regulations require that installations should be divided into circuits, the purposes being:

1. - to prevent danger in the event of a fault by ensuring that the fault current is no greater than necessary to operate the protective system. For example, a large three-phase motor must be connected to a single circuit because the load cannot be subdivided. If, however, a load consisted of three hundred lamps, each rated at 100 W, it would be foolish to consider putting all this load onto a single circuit. In the event of a fault, the whole of the lighting would be lost, and the fault current needed to operate the protective device (single-phase circuit current would be 125 A at 240 V) would be high enough to cause a fire danger at the outlet where the fault occurred. The correct approach would be to divide the load into smaller circuits, each feeding, perhaps, ten lamps.

2. - to enable part of an installation to be switched off for maintenance or for testing without affecting the rest of the system.

3. - to prevent a fault on one circuit from resulting in the loss of the complete installation (see {3.8.6} on the subject of discrimination).

The number of final circuits will depend on the types of load supplied, and must be designed to comply with the requirements for overcurrent protection, switching and the current-carrying capacity of conductors. Every circuit must be separate from others and must be connected to its own overcurrent protective fuse or circuit breaker in a switch fuse, distribution board, consumer's unit, etc. See {Fig 6.1 and Fig 6.2}.

Fig 6.1 Typical arrangement for feeding final circuits in a domestic installation

Fig 6.2 An arrangement for main and final circuits in a large installation

A durable notice giving details of all the circuits fed is required to be posted in or near each distribution board. The data required is the equipment served by each circuit, its rating, its design current and its breaking capacity. When the occupancy of the premises changes, the new occupier must be provided with full details of the installation (see reference to the Operating Manual in {8.8.1}). This data must always be kept up to date.


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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations