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E-3
Current-limiting curves
Ics = 100 % Icu
The exceptional limiting capacity of the Compact NS 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 operations:
break three times consecutively a fault current equal to 100 % of Icu
b
check that the device continues to function normally:
b
it conducts the rated current without abnormal temperature rise
v
protection functions perform within the limits specified by the standard
v
suitability for isolation is not impaired.
v
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 bus bars
being deformed or broken.
Electromagnetic effects
Less 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-limiting curves
The current-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),
b
thermal stress (A
b
2
s), i.e. the energy dissipated by the short-circuit in a conductor
with a resistance of 1 Ω.
Example
What is the real value of a 200 kA rms prospective short-circuit (i.e. 440 kA peak)
limited by an NS630bLB upstream ?
Answer: 70 kA peak (see next page).
The limiting capacity of a circuit breaker is its aptitude
to limit short-circuit currents.
The exceptional limiting capacity of the Compact NS 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).
E28800
(t)
Prospective
Isc peak
(Isc)
Prospective
current
Prospective
Isc
Limited Isc
peak
Actual
current
Limited
Isc