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 Post subject: Breaker Duty Example
PostPosted: Tue Jan 04, 2011 7:10 am 

Joined: Thu Oct 14, 2010 8:04 am
Posts: 11
Hello All,

I have a question that is coming up in regards to equipment we are designing.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
480V 3Ø, 4W. MCC Line up- Main - Tie - Main Scheme
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Now the fault current is maxed at 53kA on either section of the bus in this scheme. The distribution breaker duties are all 35kA. Now, using EASYPOWER, it says there is a equipment duty violation on the breaker because it is over duty by 40% or so.

Now the software says roughly 25kA is passing through the breaker towards the motor load. Another 1.5kA is traveling from the motor towards the breaker. So that sums to roughly 26.5kA through the breaker on a 1/2cycle calculation. So why is there a violation? The line side of the breaker is seeing the bus's max current at 53kA, but the breaker is only passing through 26kA?

My guess is, despite whats passing through, the 53kA available on the bus can fry the terminals on the breaker, but then by does is say 26kA is only passing through.

I just need some clarifcation on the theory of how to approach this scenario.

Thank you


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PostPosted: Tue Jan 04, 2011 9:10 am 
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Where it the location of the fault on the motor feeder? Is at at or near the motor i.e. down stream from the MCC?. The software consideres what happens if the fault is at the MCC bus OR at the load side terminals of the branch/feeder devices.

In your case it sounds like the 53 kA is available from the source to the MCC so if the fault is on the bus, on the main or on the load side of the feeders, it will still be 53 kA and the 26 kA sounds like it is for a remote fault downstream of the MCC.

Check and see if the 26 kA case is for a remote feeder fault and not a fault at the MCC.

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PostPosted: Tue Jan 04, 2011 9:27 am 

Joined: Thu Oct 14, 2010 8:04 am
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The example is using a "Fault All Buses" scenario. So the bus between the motor and circuit breaker is faulting and the bus in the MCC is faulting. A worst case scenario.

So if there is a bus fault in the MCC of 53kA for some duration, a distribution breaker out of this MCC, rated at 35kA, would/could catostrophically fail seeing that fault on its line side terminals? Even though only motor contributions will be traveling towards the fault location?? I'd like to understand why the software says its over duty when it only has a large fault on the line side. Is it because the frame of the breaker can't handle that on a line side?

Now if there is a fault between the motor breaker and the motor, it will only draw 25kA from the utility.. That seems ok.


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PostPosted: Tue Jan 04, 2011 9:49 am 
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The "Fault all busses" usually means the simulation will fault EVERY bus One at a Time. That being the case, you have 2 different events happening.

Event One The fault is at the MCC. The 53 kA current flows from the source to the MCC bus so the MCC main and bus will see this current for a fault on the bus.

Now picture a fault on the load side terminals of a feeder / branch breaker in an MCC bucket. If the fault is on the load side terminals of the feeder breaker, the fault current will still be 53 kA since the feeder is electrically close to the main bus (i.e. no appreciable impedance between the feeder and main bus) . The same 53 kA will flow through the feeder breaker to the load side terminals where the fault is. If the breaker is rated 35 kA and sees 53 kA, it could fail.

Event Two The fault is somewhere down toward the motor.

In this case if the fault is remote from the MCC i.e. located towards the motor, there is now impedance from the feeder conductor between the MCC feeder breaker and fault location that will limit the short circuit current - in your case that sounds like 25 to 26 kA. This current flows through the MCC feeder for a remote fault and the 35 kA breaker would appear adequate - but only for the remote fault. The feeder still has a problem for the fault AT the mcc.

Hope this helps.

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PostPosted: Tue Jan 04, 2011 10:23 am 
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Are there two sources when the tie breaker is closed, one on each bus? If so, then not all of the current would be interrupted by the tie breaker. If all of the 53 kA is from one source, then the tie breaker would have to interrupt 53 kA.


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