Fault Current Analysis Motor Contribution

[Chapmeister]

Hey all,
I am currently performing an arc flash analysis on our industrial plant. Am in the fault current calculation stage but am having trouble dealing with motor contribution. I understand the 4x factor on the FLA of the motor. It's the flow of the available fault current (AFC) from the motor in to parallel branches of the one-line that has me puzzled. What is the best way to incorporate the motor contribution throughout the system?

I was thinking that I could neglect the line impedance seen by the motor contribution (mc) and sum all the mc current values upstream to the utility. Add the total mc current to the utility AFC, and then carry that value downstream degrading it due to impedances of the lines and transformers. Does this seem like a valid method? Or is there a better way.

Unfortunately, I don't have fancy software, just using the one-line and a custom spreadsheet.

Thanks for any help you can offer!

 

[DanDel]

The motors have to be modeled as separate sources, just like the utility contribution. If you sum all the motors and the utility, you will not get accurate results. The fault at each bus will be a combination of all sources as seen from that bus through the impedances in between.

 

[Chapmeister]

Hmm, I see what you mean. That makes things complicated. I have 13 distribution switchboards each with a dozen or more switches, and besides lights and heat, the bulk of the loading is due to motors. Is there a way I can simplify the process of finding fault currents at bus points throughout the electrical system without tracing from every motor source in the system?

 

[DanDel]

You could lump the motors at each switchboard to sum the fault currents there if you were calculating only to those points. This way you would not be including the motor cable impedances, and have a higher, and therefore more conservative (though less accurate), fault current result at those switchboards. If you are going further than that, like down to the individual panels and motor buses, this wouldn't work.

Hand calculations for systems like this (more than 10-15 buses) are usually not done anymore; look at software packages.

 

[davidbeach]

Hand calculations for systems like this (more than 10-15 buses) are usually not done anymore; look at software packages.

What he said. You need one of the software packages designed for this purpose. Do a search on this forum for system analysis software.

 

[powerjunx]

Fault calculation a low voltage system having a motor loaded circuits, it appears the fault current contribution of SMALL MOTOR and LARGE MOTOR were approximated to a value of 20% to 25% reactance, respectively.

 

[Chapmeister]

My boss is reluctant in perchasing any software for me. Most of the software packages I have seen are fairly pricey, considering I'm mostly doing this as a preliminary study of the plants electrical system. I got a trial for Easypower 7.0 and it's a great program. I've been using it to check my spreadsheet results. My downstream results are all fine, but it threw me for a loop when I realized motor contribution in parallel branches needed to be considered.

I guess I'll have to tough it out and figure out a way to deal with it! Thanks for the tips, all.

 

[jghrist]

The IEEE Red Book, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Std 141, explains basic short-circuit calculation methods with a short example including motor contributions.

 

[ijl]

Chapmeister, you might consider a freeware program for your preliminary studies. See

http://pp.kpnet.fi/ijl

or

http://www.dpannhorst.de/

, for example.

 

[Chapmeister]

Thanks, I'll try those freeware apps. Hopefully they'll work out for me.