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.
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”.
ISC = 3-phase short-circuit current
ISCB = rated 3-ph. short-circuit breaking current of the circuit-breaker
Ir (or Irth) = 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.
- ^ Both designations are commonly used in different standards.