Short Circuit Calculations
Posted 04 January 2008 - 11:14 AM
If the effective impedance is used according to NEC Table 9, rather than the complex impedance values of Z=R+jX, errors in fault calculations may be introduced.
Reasons are as follows:
1. The effective impedance (used in wire size customization) is for voltage drop calculations with a power factor of 0.85 lagging. I.e. they are making an assumption that you are using this value for steady state currents with a current angle of -acos(0.85) with Voltage angle of 0deg.
2. The summation of complex impedances can result in a significantly different value than that of scalar summation. This is especially true if the two impedances are on separate axis, such as with a conductor (primarily on the real axis) and a transformer (primarily on the imaginary axis).
3. There is no opportunity to enter X/R ratios for transformers, which is important because transformers have varying X/R ratios based on their KVA. See IEEE Red book 141-1993.
Posted 07 January 2008 - 05:32 AM
Posted 07 January 2008 - 01:37 PM
Posted 10 January 2008 - 09:22 AM
Reactance diagrams (scalar value calculations) can only be used (with a moderate level of accuracy) in situations where the simplifications are valid.
Remember that inductance (and inductive reactance) are proportional to separation distance.
See IEE std 141-1993 Ch 4
Posted 21 January 2008 - 08:16 AM
Your input would be greatly appreciated.
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