333 / 408
11
11/19
Dimensions
Section 12
Short-circuit current limiting (cont.)
Technical advice
DB125769
Representation: Current limiting curves
The current limiting capacity of a circuit breaker is reflected by 2 curves which give,
as a function of the prospective short-circuit current (current which would flow in the
absence of a protection device):
b
b
the real peak current (limited)
b
b
the thermal stress (in A²s), this value, multiplied by the resistance of any element
through which the short-circuit current passes, gives the power dissipated by
this element.
The straight line "10 ms" representing the energy A²s of a prospective short-circuit
current of a half-period (10 ms) indicates the energy that would be dissipated by
the short-circuit current in the absence of limiting by the protection device (see
example).
Example
What is the energy limited by an iC60N 25 A circuit breaker for a prospective
short-circuit current of 10 kA rms. What is the quality of current limiting?
>
as shown in the graph opposite:
b
this short-circuit current (10 kA rms) is likely to dissipate up to
1,000 kA
2
s
b
the iC60N circuit breaker reduces this thermal stress to: 35 kA
2
s,
which is 22 times less.
Example of use: Stresses acceptable by the cables
The following table shows the thermal stresses acceptable by the cables depending
on their insulation, their composition (Cu or Al) and their cross section. Cross-
section values are expressed in mm² and stresses in A²s.
S (mm²)
1.5
2.5
4
6
10
PVC
Cu
2.97 x 10
4
8.26 x 10
4
2.12 x 10
5
4.76 x 10
5
1.32 x 10
6
Al
5.41 x 10
5
PRC Cu
4.10 x 10
4
1.39 x 10
5
2.92 x 10
5
6.56 x 10
5
1.82 x 10
6
Al
7.52 x 10
5
S (mm²)
16
25
35
50
PVC
Cu
3.4 x 10
6
8.26 x 10
6
1.62 x 10
7
3.21 x 10
7
Al
1.39 x 10
6
3.38 x 10
6
6.64 x 10
6
1.35 x 10
7
PRC Cu
4.69 x 10
6
1.39 x 10
7
2.23 x 10
7
4.56 x 10
7
Al
1.93 x 10
6
4.70 x 10
6
9.23 x 10
6
1.88 x 10
7
Example
Is a Cu/PVC cable of cross section 10 mm² protected by a NG125L device?
The above table shows that the acceptable stress is 1.32 x 10
6
A²s. Any short-circuit
current at the point where a NG125L device (Icu = 25 kA) is installed will be limited,
with a thermal stress of less than 2.2 x 10
5
A²s. (Curve on page 11/26).
The cable is therefore always protected up to the breaking capacity of the circuit
breaker.
10000
100000
1000000
100
1000
100
10
1
0,1
0,01
≤1
4
6
8-10
16
20-25
32-40
50-63
2-3
10 ms
400
3
Limited energy (A²s)
Prospective current (kA rms)