1/2 Cycle Momentary Symmetrical vs Asymmetrical Fault Current can you clear up the confusion? 2

[bdn2004]

I thought the first 1/2 cycle is analyzed because there are asymmetrical currents and that's what drives up that value in that short time span.

The short circuit diagram on the left is the symmetrical 1/2 cycle momentary current selection. The one on right is the same circuits but using the asymmetrical currents - which are way higher.

Obviously I'm confused. Can someone explain this simply?

 

[bdn2004]

This is confirming on the MCCB what you are saying. They are inherently protected up to their rating with the instantaneous rating. It certainly isn't 1 to 3 seconds. In this case looks like .08 seconds.

 

[dpc]

For circuit breaker duty, you can run the Equipment Duty Report in EasyPower. It should be pretty definitive regardless of breaker type. Default is ANSI. If you want IEC duty calcs, I think it will cost additional $$.

Cheers,

Dave

 

[cuky2000]

It is suggested to read the enclosed article in the link below regarding the 1s and 3s short time rating.

I hope this bring some light to the discussion.

https://myelectrical.com/not

 

[bdn2004]

Cuky2000,

The page is gone when I click on that link?

Thanks.

 

[bdn2004]

hey Cuky... I think you posted the wrong thing here. It has nothing to do with this thread.

 

[cuky2000]

bdn2004,
Attached is a pdf file with the article mentioned and couple more references for the subject.
I hope this helps.

 

[cuky2000]

Below is an illustration of a typical 3-phase symmetrical (balance) fault with AC asymmetrical fault currents. The asymmetric fault current is the result of the DC offset which occurs to maintain the principle of energy conservation at the instant after and the pre-fault event.
[sub]For additional detail regarding this subject, see

https://www.eng-tips.com/viewthread.cfm?qid=422436

[/sub]

 

[waross]

A friendly comment Stevenal:
Your link posted on 4 Jan gives an accurate description of something other than fault currents, which is the issue originally under discussion.
It has opened up a rabbit hole.
cukie's last post illustrates the difference between a symmetrical fault, symmetrical in that three phases are involved.
and the resulting asymmetrical currents. eg; DC offset.
Asymmetrical fault: A fault where less than three phases are involved.
Asymmetrical fault current: A fault current that includes a decaying DC offset current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Waross,

I don't believe I opened any hole, I was simply attempting to help clear up the OP's confusion on the question (Jan 2), which no one else had answered:

I guess another part of my confusion is a 3-phase symmetrical fault is calculated for maximum fault Amps. But in the case of a line to line fault the currents in the phases are no longer “symmetrical”, as the magnitudes are not the same in all three phases. And this is also referred to as asymmetrical. Am I wrong?

So no, Bdn is not wrong. It's all a question of which noun is being modified. I posted the link after being challenged later. Shall we keep digging? I'm guessing that question has been adaquately answered, and the hole can be closed.

 

[cuky2000]

I guess that we can say as a concluding remark that: