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 | |||
*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: | ||
: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 | 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''': | |||
''''' | |||
---- | {{def | ||
|ISC|3-phase short-circuit current | |||
|ISCB|rated 3-ph. short-circuit breaking current of the circuit-breaker | |||
|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> |
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.
Notes
- ^ Both designations are commonly used in different standards.