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The fault-current limitation capacity of a CB concerns its ability, more or less effective, in preventing the passage of the maximum prospective fault-current, permitting only a limited amount of current to flow, as shown in diagram (a). (Refer to attached file Current-Limitation.png)
The current-limitation performance is given by the CB manufacturer in the form of curves [see diagrams (b) and (c)]
Diagram (b) shows the limited peak value of current plotted against the RMS value of the AC component of the prospective fault current (“prospective” fault current refers to the fault-current which would flow if the CB had no current-limiting capability)
Limitation of the current greatly reduces the thermal stresses (proportional I2t) and this is shown by the curve of diagram (c), again, versus the RMS value of the AC component of the prospective fault current.
The limiting capacity of the Compact NSX range, for example, is due to the rotating double-break technique, which is a very rapid natural repulsion of contacts and the appearance of two arc voltages in-series with a very steep wave front. This greatly reduces the forces created by fault currents, and thus, enables breaking at much lower capacities.
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