Location of protective devices: Difference between revisions
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A protective device is, in general, required at the origin of each circuit | |||
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== General rule == | |||
(see '''Fig. G7a''')<br>A protective device is necessary at the origin of each circuit where a reduction of permissible maximum current level occurs.<br> | (see '''Fig. G7a''')<br>A protective device is necessary at the origin of each circuit where a reduction of permissible maximum current level occurs.<br> | ||
== Possible alternative locations in certain circumstances | == Possible alternative locations in certain circumstances == | ||
(see '''Fig. G7b''')<br>The protective device may be placed part way along the circuit: | (see '''Fig. G7b''')<br>The protective device may be placed part way along the circuit: | ||
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[[File:FigG07a.jpg]] '''[a]'''<br><br> | |||
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[[File:FigG07b.jpg]] '''[b]'''<br><br> | |||
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[[File:FigG07c.jpg]] '''[c]'''<br><br> | |||
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Revision as of 13:10, 15 November 2013
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.
[a]
[b]
[c]
Fig. G7: Location of protective devices
