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Guest Mark Magee

Short Circuit Calculations

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Guest Mark Magee

I am not entirely certain as to your procedure for short circuit calcs, however I am concerned that there is an error in the method you have used (according to your help screen).

 

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.

 

Thank you.

 

 

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You are right, there are a number of simplifications that we have built in to the fault calculations in Design Master. As we upgrade the software in the future, we plan to revisit this calculation to include both these refinements and others (such as motor contributions). I've made a note of this in our wish list.

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I agree with Mark - some what (although the only question I've had from a plans reviewer was what formula I used.) Given the Ohms-Law Brown Book formula DM uses, he was happy. As for motor load contribution, that could be made a selectible option, but generally, our reviewers do not expect to see it. Although we do have to ensure that motor full load is no more than 1% of AIC ratings of series-rated panels.

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Guest Mark Magee

Although it may be true for utilities in overhead line applications to use reactance diagrams rather than using full data of R+jX, this is not true of low voltage applications where the X/R ratios are much lower due to decreased separation distance between conductors... and decreased transformer sizes.

 

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

 

Respecfully,

Mark

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For anyone following this discussion, we are working on implementing this in the next release of Design Master Electrical. I have a question about utility and generator impedances in a new topic:

 

http://forums.designmaster.biz/index.php?showtopic=606

 

Your input would be greatly appreciated.

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