Location of protective devices

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)
  • The upstream device P1 protects the length AB against short-circuits in accordance
• 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

Circuits with no protection

(see Fig. G7c)

Either

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

Or

• 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.

Fig. G7 – Location of protective devices

• 

  [a]

• 

  [b]

• 

  [c]