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Overcurrent protection principles

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)

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)[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. G3Circuit protection by circuit breaker

Fig. G4Circuit protection by fuses

Fig. G5I2t characteristic of an insulated conductor at two different ambient temperatures


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