Chapter G

Sizing and protection of conductors


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 {{FigRef|G3}} and {{FigRef|G4}} ).
A protective device is provided at the origin of the circuit concerned (see '''Fig. 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
<br>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>'''
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:
 
:I<sup>2</sup>t = k<sup>2</sup> S<sup>2</sup>
 
which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.
 
where
 
{{def
|t| Duration of short-circuit current (seconds)
|S| Cross sectional area of insulated conductor (mm<sup>2</sup>)
|I| Short-circuit current (A r.m.s.)
|k| Insulated conductor constant (values of k are given in {{FigureRef|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 {{FigRef|G5}}). θ means “temperature”.


which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.<br>where<br>
'''Note''':
:<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>'''I'''</big>: Short-circuit current (A r.m.s.)<br>
:<big>'''k'''</big>: Insulated conductor constant (values of k<sup>2</sup> are given in {{FigureRef|G52}})<br>


For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 &gt; θa2), Iz1 is less than Iz2 (see {{FigRef|G5}}). θ means “temperature”.
{{def
<br>Note: <br>
|ISC|3-phase short-circuit current
:'''ISC''': 3-phase short-circuit current<br>
|ISCB|rated 3-ph. short-circuit breaking current of the circuit-breaker
:'''ISCB''': rated 3-ph. short-circuit breaking current of the circuit-breaker<br>
|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.}}
:'''Ir (or Irth)<sup>(1)</sup>'''&nbsp;: 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.


[[File:FigG03.jpg|none]]
{{FigImage|DB422282_EN|svg|G3|Circuit protection by circuit breaker}}
'''''Fig. G3:'''''<i>&nbsp;Circuit protection by circuit-breaker</i>


[[File:FigG04.jpg|none]]
{{FigImage|DB422283_EN|svg|G4|Circuit protection by fuses}}
'''''Fig. G4:'''''<i>&nbsp;Circuit protection by fuses</i>


[[File:FigG05.jpg|none]]
{{FigImage|DB422284|svg|G5|I<sup>2</sup>t characteristic of an insulated conductor at two different ambient temperatures}}
'''''Fig. G5:&nbsp;'''''<i>I<sup>2</sup>t characteristic of an insulated conductor at two different ambient temperatures</i>


[[ru:Принципы защиты от токовых перегрузок]]
{{footnotes}}
[[zh:过电流保护原理]]
<references>
{{fn-detail|1|Both designations are commonly used in different standards.}}
</references>

Latest revision as of 09:48, 22 June 2022

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 k 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

  1. ^ Both designations are commonly used in different standards.
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