Overcurrent protection principles

From Electrical Installation Guide

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.

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Fig. G3: Circuit protection by circuit-breaker

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Fig. G4: Circuit protection by fuses

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Fig. G5: I²t characteristic of an insulated conductor at two different ambient temperatures