3-tier series rating

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Grouch1980

Grouch1980

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Hi all... and for my last question of the weekend :)

The image below is from a Schneider Electric page. Let's say this set up was a 3-tier series rated system (which it is in their example). unfortunately the example didn't go into too much detail.

My 2 questions are:
1. for a 3-tier series rated system, does breaker A series rate with breaker B, and breaker B series rate with breaker C? or is it from A to B, and A to C?
2. Let's say you have a fault on the load side of breaker C... does breaker A or breaker B open for a 3 tier system?

IMG_1487.jpg
 
1. I believe that you would have to have a three breaker series rating. But consider that the impedance of the wiring run to the downstream panelboard may well reduce the available fault current to a point where series rating is not needed at that point.

2. The series rating, and even the relative trip currents, does not by itself give you any information on which breaker will trip first in case of a fault.
The operating principle here is coordination, which involves a careful examination of the trip curves of all the breakers and the magnitude of the possible fault current.
 
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That makes sense... you wouldn't know which breaker would trip. what about my first part? is it series rated from A to B and then B to C, or A to B and A to C?
 
GoldDigger said:
When I referred to a three breaker rating, I meant A-B-C listed together by part number.
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oh i know, that was my mistake.... for some reason my browser was only showing half your response. afterwards it showed your full response, and i saw your answer for my first question. I blame my browser.
 
Grouch1980 said:
oh i know, that was my mistake.... for some reason my browser was only showing half your response. afterwards it showed your full response, and i saw your answer for my first question. I blame my browser.
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Actually that was because I was still editing it after staring with the second part. My bad.
 
Grouch1980 said:
what about my first part? is it series rated from A to B and then B to C, or A to B and A to C?
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I agree with Larry and Goldie, I dont think it really makes sense to try to make either of those statements exclusive in a three tier system.

Surprisingly, there is scant information out there on applying series ratings. The best thing I have found is the document by EAton:


See page 4, "other applications of series ratings.

One very common thing I see all the time, is people "build their own" three tier rating by applying a series rating between A and B, and then take the B series rating as if it was the actually fault current at B, and series rate that to C. That document clarifies that that is not acceptable (unless it is indeed a three tier).
 
electrofelon said:
I agree with Larry and Goldie, I dont think it really makes sense to try to make either of those statements exclusive in a three tier system.

Surprisingly, there is scant information out there on applying series ratings. The best thing I have found is the document by EAton:


See page 4, "other applications of series ratings.

One very common thing I see all the time, is people "build their own" three tier rating by applying a series rating between A and B, and then take the B series rating as if it was the actually fault current at B, and series rate that to C. That document clarifies that that is not acceptable (unless it is indeed a three tier).
Click to expand...
Great! nice find... i went through page 4. I see where it says all 3 devices have to be tested and listed together, at the very bottom. And they even have a table at the very bottom of the article for the triple series rating.
 
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