18
2
Current and energy limiting curves
Ics = 100 % Icu
The exceptional limiting capacity of the BX range greatly
reduces the forces created by fault currents in devices.
The result is a major increase in breaking performance.
In particular, the service breaking capacity Ics is equal to 100
% of Icu.
The Ics value, defined by IEC standard 60947-2, is guaranteed
by tests comprising the following steps:
break three times consecutively a fault current equal to 100
% of Icu
check that the device continues to function normally, that is:
it conducts the rated current without abnormal temperature
rise
protection functions perform within the limits specified by the
standard
suitability for isolation is not impaired.
Longer service life of electrical installations
Current-limiting circuit breakers greatly reduce the negative
effects of short-circuits on installations.
Thermal effects
Less temperature rise in conductors, therefore longer service life for
cables.
Mechanical effects
Reduced electrodynamic forces, therefore less risk of electrical
contacts or busbars being deformed or broken.
Electromagnetic effects
Fewer disturbances for measuring devices located near electrical
circuits.
Economy by means of cascading
Cascading is a technique directly derived from current
limiting. Circuit breakers with breaking capacities less than the
prospective short-circuit current may be installed downstream
of a limiting circuit breaker. The breaking capacity is reinforced
by the limiting capacity of the upstream device. It follows that
substantial savings can be made on downstream equipment
and enclosures.
Current and energy limiting curves
The limiting capacity of a circuit breaker is expressed by two
curves which are a function of the prospective short-circuit
current (the current which would flow if no protection devices
were installed):
the actual peak current (limited current)
thermal stress (A
2
s), i.e. the energy dissipated by the short-
circuit in a conductor with a resistance of 1
:
.
Example
What is the real value of a 150 kA rms prospective short-circuit (i.e.
330 kA peak) limited by an BX250L upstream ?
The answer is 30 kA peak (curve page 19).
Maximum permissible cable stresses
The table below indicates the maximum permissible thermal
stresses for cables depending on their insulation, conductor (Cu
or Al) and their cross-sectional area (CSA). CSA values are
given in mm² and thermal stresses in A
2
s.
CSA
1.5 mm² 2.5 mm²
4 mm²
6 mm²
10 mm²
PVC
Cu
2.97x10
4
8.26x10
4
2.12x10
5
4.76x10
5
1.32x10
6
Al 5.41x10
5
PRC Cu
4.10x10
4
1.39x10
5
2.92x10
5
6.56x10
5
1.82x10
6
Al 7.52x10
5
CSA
16 mm²
25 mm²
35 mm²
50 mm²
PVC Cu 3.4x10
6
8.26x10
6
1.62x10
7
3.31x10
7
Al 1.39x10
6
3.38x10
6
6.64x10
6
1.35x10
7
PRC
Cu 4.69x10
6
1.39x10
7
2.23x10
7
4.56x10
7
Al 1.93x10
6
4.70x10
6
9.23x10
6
1.88x10
7
Example
Is a Cu/PVC cable with a CSA of 10 mm² adequately
protected by an BX160F?
The table above indicates that the permissible stress is
1.32x106 A
2
s.
All short-circuit currents at the point where an BX160F (Icu =
35 kA) is installed are limited with a thermal stress less than
6x105 A
2
s (curve page 19).
Cable protection is therefore ensured up to the limit of the
breaking capacity of the circuit breaker.
The limiting capacity of a circuit breaker is its aptitude to let through a
current, during a short-circuit, that is less than the prospective short-
circuit current.
The exceptional limiting capacity of the BX range is due to the rotating
double-break technique (very rapid natural repulsion of contacts and
the appearance of two arc voltages in-series with a very steep wave
front).
(t)
Prospective
Isc peak
(Isc)
Prospective
current
Prospective
Isc
Limited Isc
peak
Actual
current
Limited
Isc
Operating characteristics and performance
2-2 Breaking performance