Chapter G

Sizing and protection of conductors

Location of protective devices

From Electrical Installation Guide

A protective device is, in general, required at the origin of each circuit

General rule

(see Fig. G7a)

A protective device is necessary at the origin of each circuit where a reduction of permissible maximum current level occurs.

Possible alternative locations in certain circumstances

(see Fig. G7b)

The protective device may be placed part way along the circuit:

  • If AB is not in proximity to combustible material, and
  • If no socket-outlets or branch connections are taken from AB

Three cases may be useful in practice:

  • Consider case (1) in the diagram
    • AB ≤ 3 metres, and
    • AB has been installed to reduce to a practical minimum the risk of a short-circuit (wires in heavy steel conduit for example)
  • Consider case (2)
  • Consider case (3)
    • The overload device (S) is located adjacent to the load. This arrangement is convenient for motor circuits. The device (S) constitutes the control (start/stop) and overload protection of the motor while (SC) is: either a circuit-breaker (designed for motor protection) or fuses type aM
    • The short-circuit protection (SC) located at the origin of the circuit conforms with the principles of Calculation of minimum levels of short-circuit current .

Circuits with no protection

(see Fig. G7c)


  • The protective device P1 is calibrated to protect the cable S2 against overloads and short-circuits


  • Where the breaking of a circuit constitutes a risk, e.g.
    • Excitation circuits of rotating machines
    • circuits of large lifting electromagnets
    • the secondary circuits of current transformers

No circuit interruption can be tolerated, and the protection of the cabling is of secondary importance.