ComPact NSX100-630 - Limitation Curves

Presentation

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 ComPact NSX 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).

Ics = 100% Icu

The exceptional limiting capacity of the ComPact NSX range greatly reduces the forces created by faults 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 Icu value, defined by standard IEC/EN 60947-2, is guaranteed by tests comprising the following steps:

• Break the circuit three times consecutively with a fault current equal to 100% of Icu

• Check that the circuit breaker 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:

  Reduced temperature rise in conductors, therefore longer service life for cables.

• Mechanical effects:

  Reduces electrodynamic forces, therefore less risk of electrical contacts, or busbar being deformed or broken.

• Electromagnetic effects:

  Reduction in disturbances for measuring devices located near electric 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 circuit breaker. It follows that substantial savings can be made on downstream equipment and enclosures.

Nevertheless, the following limitation curves cannot be used to estimate cascading performance of two circuit breakers. For more information on reinforced breaking capacity, refer to the cascading tables in LVPED318033EN Selectivity, Cascading, and Coordination Guide.

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 (A²s), that is, the energy dissipated by the short-circuit in a condition with a resistance of 1 Ω.

Example: What is the real value of a 150 kA rms prospective short-circuit (i.e. 330 kA peak) limited by an NSX250L upstream?

The answer is 30 kA peak. Refer to Current-limiting Curves.

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 A²s.

Example: Is a Cu/PVC cable with a CSA of 10 mm² adequately protected by an NSX160F? The table above indicates that the permissible stress is 1.32x10⁶ A²s.

All short-circuit currents at the point where an NSX160F (Icu = 35 kA) is installed are limited with a thermal stress less than 6x10⁵ A²s. Refer to Energy-limiting Curves.

Cable protection is therefore ensured up to the limit of the breaking capacity of the circuit breaker.

Current-limiting Curves

Energy-limiting Curves