# Overcurrent protection principles

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 I
^{2}t 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:

- I
^{2}t = k^{2}S^{2}

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 (mm^{2})**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.