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 Post subject: Short Circuit Calculations and Conduit MaterialPosted: Thu Jul 21, 2016 3:48 am

Joined: Mon Nov 05, 2007 10:10 am
Posts: 48
Location: Mid-West
I recently came across a previous study that was completed for a facility that assumed a specific type of conduit [non-metallic/magnetic; PVC] vs what was actually installed [Rigid Meal Conduit], in digging into this deeper and looking at some of the short circuit calculations information available to me it appears that in the calculation they type of conduit [magnetic vs non-magnetic] can play a role in the amount of available fault current. When doing some quick simplistic calculations it appears that the conduit material does play a role and in larger systems could make a difference. I did some digging in my software's reference material and some other material but could not find adequate explanation / technical data.

I was hoping someone could share or point me to some additional resources on how conduit material affects available fault current? It appears there are different constants for conductors based on the type of installation [conduit type for systems of .208-13.2kV] also what if you have conductors installed in tray [3-1/C cables] does the spacing and configuration of how the cables are installed come into play and where could I find some information on this. Thanks in advance.

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 Post subject: Re: Short Circuit Calculations and Conduit MaterialPosted: Thu Jul 21, 2016 5:47 am
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
First, the proper model for a cable system is to model it as a transmission line. This would include a parallel capacitance (two conductors separated by an insulator makes a capacitor) which even if it's a single cable, the ground forms at least one parasitic capacitance. Second the cable itself has an "eddy current" which creates a series inductance. Just as with the old experiment where you make a coil of wire and then stick a nail through it, adding magnetic material within the field surrounding cable increases the inductance. Finally the cable has impedance unless it is a superconductor.

For short distances and/or low currents, these transmission line properties don't matter and you can model it as a short circuit. However with high currents (faults) and/or long distances, it makes a major difference. The net effect is for instance that when you run even 480 VAC cables over long distances (over about 250+ feet) with high currents, you will find that the voltage drop even under normal operation is unacceptable. Transmission line effects also impact fault currents, so yes it is important to model these correctly. Since all of these effects are based on the square of the current, higher voltages are generally unaffected by it...although when you are running transmission lines over several miles as utilities do, it is again important to model this correctly.

Most of the time an example of where you commonly see problems is if you have say 3 conductors per phase and the cable installer runs them in 3 separate conduits but runs say all of phase A in a single conduit instead of running one of each phase in each conduit. The other time you see it is when you run single conductors through CGB's or TNC's through a steel enclosure. The eddy currents between them can make the steel plate wicked hot. If you ever need to do this cut out a window and replace the steel window with an aluminum or fiber (nonmagnetic) plate. If you run aluminum be sure to insulate it with some rubber and then bond it with proper fittings to avoid galvanic corrosion issues.

So if you are running cable in conduit then yes, it has an impact. There are even charts that indicate how far you can run cable in steel conduit before parasitic inductance will cause excessive voltage drop (3%) thus limiting the length of the run. It is not so important the exact material such there is an order of magnitude difference between magnetic and nonmagnetic conduits, enough that you can pretty much ignore whether it is aluminum or PVC but it makes a major difference if it is steel. Similarly if the cables are all run in cable tray since much less of the magnetic field is impacted by cable tray, the effect is much less.

For myself this is one of those things where I just run one of those rule-of-thumb calculations when doing designs just to verify that there are no problems. If there is then I include the exact equation. When doing power system analysis since I have to key in all the important information anyways (cable length, size) it's a simple matter to add in the conduit information as well and the software produces a more exact answer. It might make a 0.1% difference or it might make a 10% difference but I just treat it as more of the data that needs to be gathered while doing the survey.

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