Coordination Study Level

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anbm

Senior Member
Are there are codes or state board require coordination study down to 0.01s
level? It's hard to archive this unless we use fuses... Thank you!
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The NEC does not define how fast things must operate to be considered 'selectively coordinated', therefore it is up to the AHJ.

My company regularly coordinates systems down to .01s using breakers. Yes, it does take electronic trip circuit breakers and involvement at the design stage.

One of the hardest circuits to coordinate is a 480Y/277 panel with a 200A MB and a 125A branch breaker feeding a 75KVA transformer to a 208Y/120V 225A MB panel.

One problem I have with using fuses for selective coordination (besides GF hassles) is that 'fuse ratio tables' are only good for those specific fuses. You can install Brand A fuses, but then someone will purchase replacement fuses from a different manufacturer because 'the sales person says they are interchangeable', causing a loss of selectivity.
 

dkidd

Senior Member
Location
here
Occupation
PE
If you search through the ROPs for proposals for 700.27 you will see that the Code panel considers 0.01 to be the limit, although there is disagreement. There have been numerous proposals to get the limit stated in the Code to be 0.1. One of the biggest concerns is that to achieve coordination at 0.01, the arc flash energy increases.
 

zog

Senior Member
Location
Charlotte, NC
If you search through the ROPs for proposals for 700.27 you will see that the Code panel considers 0.01 to be the limit, although there is disagreement. There have been numerous proposals to get the limit stated in the Code to be 0.1. One of the biggest concerns is that to achieve coordination at 0.01, the arc flash energy increases.

The problem with that is it is all based on when the breaker is supposed to trip, not based on actual test results, and even those are not perfect unless first trip testing methods are used.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Last year, after some interesting debates, the City of Seattle adopted a limit of 0.1s. I seem to recall, though I am not sure how to verify this, that the State of Washington's inspectors have declared that they will only ask for proof of selective coordination for that portion of the emergency and LRS system that is in operation when the backup power source is on line. In other words, coordination is not required for the normal source lineup.
 
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