Chapter A General rules of electrical installation design

G - Sizing and protection of conductors
The protective device must fulfill:
b instantaneous trip setting Im < Isc min for a
circuit-breaker
b fusion current Ia < Isc min for a fuse
5 Particular cases of short-circuit
current
Conditions to be fulfilled
The protective device must therefore satisfy the two following conditions:
b Its fault-current breaking rating must be greater than Isc, the 3-phase short-circuit
current at its point of installation
b Elimination of the minimum short-circuit current possible in the circuit, in a time tc
compatible with the thermal constraints of the circuit conductors, where:
2 2
K S (valid for tc < 5 seconds)
tc i
I 2
sc min
Comparison of the tripping or fusing performance curve of protective devices, with
the limit curves of thermal constraint for a conductor shows that this condition is
satisfied if:
b Isc (min) > Im (instantaneous or short timedelay circuit-breaker trip setting current
level), (see Fig. G45)
b Isc (min) > Ia for protection by fuses. The value of the current Ia corresponds to
the crossing point of the fuse curve and the cable thermal withstand curve
(see Fig. G44 and G45)
Fig. G45 : Protection by circuit-breaker
Fig. G46 : Protection by aM-type fuses
Fig. G47 : Protection by gl-type fuses
Schneider Electric - Electrical installation guide 2008
t
t
t
Im
Ia
Ia
t = k2 S 2
I 2
t = k2 S 2
I 2
t = k2 S 2
I 2
I
I
I
G31
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