Overcurrent protection principles: Difference between revisions
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{{Menu_Sizing_and_protection_of_conductors}} | {{Menu_Sizing_and_protection_of_conductors}} | ||
A protective device is provided at the origin of the circuit concerned (see | A protective device is provided at the origin of the circuit concerned (see {{FigRef|G3}} and {{FigRef|G4}} ). | ||
*Acting to cut-off the current in a time shorter than that given by the I<sup>2</sup>t characteristic of the circuit cabling | *Acting to cut-off the current in a time shorter than that given by the I<sup>2</sup>t characteristic of the circuit cabling | ||
*But allowing the maximum load current IB to flow indefinitely | *But allowing the maximum load current IB to flow indefinitely | ||
The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula: | |||
:'''<big>I<sup>2</sup>t = k<sup>2</sup> S<sup>2</sup></big>''' | :'''<big>I<sup>2</sup>t = k<sup>2</sup> S<sup>2</sup></big>''' | ||
which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor. | which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor. | ||
where | |||
:<big>'''t'''</big>: Duration of short-circuit current (seconds)<br> | :<big>'''t'''</big>: Duration of short-circuit current (seconds)<br> | ||
:<big>'''S'''</big>: Cross sectional area of insulated conductor (mm<sup>2</sup>)<br> | :<big>'''S'''</big>: Cross sectional area of insulated conductor (mm<sup>2</sup>)<br> | ||
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For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see {{FigRef|G5}}). θ means “temperature”. | For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see {{FigRef|G5}}). θ means “temperature”. | ||
'''Note''': | |||
:'''ISC''': 3-phase short-circuit current<br> | :'''ISC''': 3-phase short-circuit current<br> | ||
:'''ISCB''': rated 3-ph. short-circuit breaking current of the circuit-breaker<br> | :'''ISCB''': rated 3-ph. short-circuit breaking current of the circuit-breaker<br> | ||
:'''Ir (or Irth) | :'''Ir (or Irth)'''{{fn|1}}: regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level (see {{FigRef|G6}}) similar to that of a 30 A circuit-breaker. | ||
{{FigImage|DB422282_EN|svg|G3|Circuit protection by circuit breaker}} | |||
{{FigImage|DB422283_EN|svg|G4|Circuit protection by fuses}} | |||
{{FigImage|DB422284|svg|G5|I<sup>2</sup>t characteristic of an insulated conductor at two different ambient temperatures}} | |||
{{footnotes}} | |||
<references> | |||
{{fn-detail|1|Both designations are commonly used in different standards.}} | |||
</references> | |||
[[ru:Принципы защиты от токовых перегрузок]] | [[ru:Принципы защиты от токовых перегрузок]] | ||
[[zh:过电流保护原理]] | [[zh:过电流保护原理]] | ||
Revision as of 03:14, 27 November 2016
A protective device is provided at the origin of the circuit concerned (see Fig. G3 and Fig. G4 ).
- Acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling
- But allowing the maximum load current IB to flow indefinitely
The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula:
- I2t = k2 S2
which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.
where
- t: Duration of short-circuit current (seconds)
- S: Cross sectional area of insulated conductor (mm2)
- I: Short-circuit current (A r.m.s.)
- k: Insulated conductor constant (values of k2 are given in Figure G52)
For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see Fig. G5). θ means “temperature”.
Note:
- ISC: 3-phase short-circuit current
- ISCB: rated 3-ph. short-circuit breaking current of the circuit-breaker
- Ir (or Irth)[1]: regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level (see Fig. G6) similar to that of a 30 A circuit-breaker.
Fig. G3 – Circuit protection by circuit breaker
Fig. G4 – Circuit protection by fuses
Fig. G5 – I2t characteristic of an insulated conductor at two different ambient temperatures
Notes
- ^ Both designations are commonly used in different standards.
