Breaker Coordination and Avail Short Circuit Current

[sceepe]

Do you guys think there is any need to worry about breaker coordination at currents above the available short circuit current. (Assuming you have to worry about coordination in the first place?)

IMO you don't; but the available current may depend on the type of fault. Coordination needs to be correct reguardless of the type of fault. If your coordinated with the worse case fault is that good enough?

Semi related question: Why don't the gear manufactures have tools (software) avail to easily coordinate their products?

 

[tom baker]

Re: Breaker Coordination and Avail Short Circuit Current

Charlie B is the moderator who works with this, make sure he answers.
I would say no.
How could there be a current above the available?

 

[charlie b]

Re: Breaker Coordination and Avail Short Circuit Current

Sure, Tom. Put the pressure on.

To address your concerns, in order:

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• <font size="2" face="Verdana, Helvetica, sans-serif">You do not need to show coordination for a current level that is higher than the maximum available short circuit fault.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">If you are coordinated for the worst case fault, then you are coordinated for the lesser cases.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">It's not their product that needs coordination; it's your distribution system. The specs that I have seen for switchboards and distribution system equipment call upon the manufacturers to perform the coordination calculations, as part of their deliverables. So they have the software. But it might not be their own. They probably bought a commercially available analysis package. They could not offer that package to their customers, since they don't own the copyrights</font>

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[sceepe]

Re: Breaker Coordination and Avail Short Circuit Current

Charlie, you da man.

The specs that I have seen for switchboards and distribution system equipment call upon the manufacturers to perform the coordination calculations, as part of their deliverables.

I'm editing my specs tonight. Guess I am behind the times on this one. Should have known my peers would find a way to put this on the suppliers. I'm just disappointed I didn't think of it first. Can't wait to see the first set of shop drawings with coordination curves for everything from disconnect to MDP.

 

[ron]

Re: Breaker Coordination and Avail Short Circuit Current

The other option is to perform the short circuit and coordination studies in your office, generically during design, then specifically with approved shop drawings.
That way you as the designer get to control the "art" of coordination. Many times the coordination study is not simple decisions regarding OCPD closest to the fault clears first. Many times you have to decide reliability, safety and sequence of operations issues during setting selection. I don't think a manufacturer can make those decisions for MY client.
I use software by SKM and EDSA. I have associates that like ETAP, Easypower and ....... can't think of the last company name.

 

[charlie b]

Re: Breaker Coordination and Avail Short Circuit Current

You may be thinking about Power*Tools, or about the set of A_Fault, Dapper, and Captor. But those are all products of SKM.

 

[kevinsarnold]

Re: Breaker Coordination and Avail Short Circuit Current

For Selective Coordination, it should be done for all levels of overcurrent (overloads and faults) up to the Interrupting Rating of the OCPD. If you do it for what is available right now, then what happens when the tranformer is changed or the system increased? Then you have a system that is not what it once was. The software programs mentioned lay out the curves, but don't guarantee coordination, that is where you have to analyze and make the proper selections and settings. The 2005 NEC has defined Coordination (Selective) in Article 100 and in articles, 517, 620, 700 and 701 have Selective Coordinatin requirements. Hope this helps.

 

[templdl]

Re: Breaker Coordination and Avail Short Circuit Current

Do you guys think there is any need to worry about breaker coordination at currents above the available short circuit current. (Assuming you have to worry about coordination in the first place?)

The way I read this is if you have 25ka "short circuit current available" and you apply a 65kaic device (current above the available short circuit current.) The trip curves of all breakers are the same so you would be spending more money than needed.
Also,

IMO you don't; but the available current may depend on the type of fault. Coordination needs to be correct reguardless of the type of fault. If your coordinated with the worse case fault is that good enough?

Software to do fault studies for coordination studies have been available for years. I know the manufacturer that I had worked for made it available to those interesrted. The problem is if the person who does the study qualified even though the software is available to everyone. It has been of my experience that professional engineers (PEs) would be qualified to do the study. How's going to stand beside tneir calculations? A big responsibility. Then an OCPD could be applied for the calculated fault current at a specific point in that distribution system as you would like to do.

 

[jgrassi]

Re: Breaker Coordination and Avail Short Circuit Current

Charlie B wrote:

"If you are coordinated for the worst case fault, then you are coordinated for the lesser cases."

This is not always the case. With adjustable trip units, it is entirely possible to set breakers where the short-time (and even instantaneous) trip regions coordinate while the long time regions do not.