Choosing Breaking Capacity
Calculation of the required breaking capacity involves determining the supply impedance and voltage. Supply impedance is calculated from the impedance of the elements making up the supply system. Customers of an electrical supply utility can request the maximum value of prospective short circuit current available at their point of supply. Networks involving multiple sources of current, such as multiple generators, electric motors, and with variable interconnections may be analyzed using computer software. A system study will generally consider the maximum case of additions of generation and interconnection out to some projected horizon year, to allow for system growth during the useful life of the studied installation. Since practical calculations involve a number of approximations and estimates, some judgment is required in applying the results of a short-circuit calculation to the selection of apparatus.
Some applications have low short circuit current so any fault current protector will suffice. This is typical for supplies from miniature transformers as used in control circuits. In large systems such as industrial power centers or utility networks, fault currents can be very high and selection of a suitably rated device is important.
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