Coordination Study

[Red Wiggler]

I am sure this has been asked before but...

From what I understand, "Selectively Coordinated Protection" is when a breaker closest to a fault "opens" first ahead of the up-stream breakers and a "Series Rated" coordination uses 2 or more breakers coordinated together for the protection of the downstream loads.

I probably worded wrong, but is this the gist of the two systems?

Why would a customer select one system above another?

Is it purely econimcally in their decision process?

Or is it a maintenance issue?

Any help understanding this issue would be appreciated.

 

[don_resqcapt19]

They are two totally different issues.

Selective coordination is used to limit an outage to a single OCPD.

Series rated systems are used to permit the use of breakers with a lower interrupting than the available fault current at that point on the circuit. This often results in a system that is cheaper to install than a fully rated system where all of the breakers have an interrupting rating equal to or greater than the available fault current. With high current faults the use of a series rated system results in the loss of selective coordination as two or more breakers will open to clear the fault.

 

[jim dungar]

With high current faults the use of a series rated system results in the loss of selective coordination as two or more breakers will open to clear the fault.

No, it may result loss of selectivity and it may not. Although, in general I would not count on selectivity above 2-3kSCA.

However, I just finished (this morning) giving a selective coordination seminar. One of the examples we had was a Square D LA-MC breaker feeding a QO breaker. This combination is both selectively and series-rated up to 18kSCA.

 

[ron]

Jim,
I agree.
Not to be a nit picker, and attempting not to high jack the thread, but I applied the LA-MC and an QO a few years ago as series rated at 22kAIC per a document given by Square D.

 

[jim dungar]

Jim,
I agree.
Not to be a nit picker, and attempting not to high jack the thread, but I applied the LA-MC and an QO a few years ago as series rated at 22kAIC per a document given by Square D.

My turn.
http://ecatalog.squared.com/pubs/Circuit%20Protection/0100DB0501.pdf
page 38 table 12.

Yes they series rated to 22K, they are only selective to 18K.

 

[ron]

Well said.
As a further side note, I've stopped specifying the LA-MC breaker recently because of the high incident energy that is let through by the high instantaneous fixed setting. However it is always a balance to decide .... high incident energy or better selectivity.

 

[jim dungar]

However it is always a balance to decide .... high incident energy or better selectivity.

Hey, after all, isn't that what you get paid the big bucks for doing?

 

[ron]

To describe the selectivity that Jim was referring to, here's an article written by one of my previous associates about that LA-MC breaker.
http://ecmweb.com/powerquality/electric_achieving_discrimination_circuit/

 

[Red Wiggler]

VE possibility?

VE possibility?

They are two totally different issues.

Selective coordination is used to limit an outage to a single OCPD.

Series rated systems are used to permit the use of breakers with a lower interrupting than the available fault current at that point on the circuit. This often results in a system that is cheaper to install than a fully rated system where all of the breakers have an interrupting rating equal to or greater than the available fault current. With high current faults the use of a series rated system results in the loss of selective coordination as two or more breakers will open to clear the fault.

In your opinion could the "Series Rated" system be offered as a VE to a customer? And if so what could be the main selling point? Is a coordination study that much cheaper being "Series Rated" than a "Fully Rated" system? Do breakers cost differ between systems?

 

[brian john]

However, I just finished (this morning) giving a selective coordination seminar.

Who did you give the seminar to? Where was this?

 

[ron]

Red,
A series rated system will almost always be cheaper overall cost because the downstream equipment will be at lower AIC ratings (when viewed for its own standalone AIC ratings).
The coordination study is generally the same or a bit more difficult for a series rated system versus a fully rated system because it would involve looking at the manufacturers tables as linked to by Jim to find short circuit ratings and selectivity levels.