Location of protective devices
A protective device is, in general, required at the origin of each circuit
(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 with the Calculation of minimum levels of short-circuit current
- 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.