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 Post subject: To coordinate or arc-flash adjust??
PostPosted: Thu Feb 10, 2011 7:20 am 
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Location: Westminster, MD
I pose the question to you folks on the forum concerning a choice between a coordinated system, and one that has a slight miss in coordination but eliminates an HRC Dangeorus situation.
I've attached a TCC that shows two different fuse selections for the primary of a 750 kVA transformer, a 25E and a 50E.
The 25E yields an incident energy of about 23.2 calories/cm^2 at the transformer secondary, which is the service-entrance to a switchboard, but has overlap with two thermal-mag breakers distribution breakers, and the Main.
The 50E coordinates nicely, but yields 63 calories/cm^2 at the gear.
I'm inclined to tolerate/accept the miss in coordination since it eliminates an HRC Dangeorus potentially life-threatening situation. But if that fault occurs, a fuse is tripped, which is the last thing I want to happen with the system.
As a PE, I'm bound by the Code of Ethics which states "The engineer shall hold paramount safeguarding life, health and property and promoting the public welfare in the performance of his professional duties".
If I follow that to the letter, then perhaps the miss in coordination is deemed acceptable because the chance of ever having 63 calories at the gear is gone.
As far as the criticality of the load, which is certainly a mitigating factor in the decision, this one's not a hospital or Data Center, but a university facility.
I appreciate any opinions.
John M


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PostPosted: Thu Feb 10, 2011 7:39 am 
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It looks like the worst case mis-coordination is with the Main Breaker using the 25E fuse. That being the case, I would use the 25E to lower the arc flash. Besides making the system safer for workers, let's think about the difference between the main breaker tripping versus the primary fuse opening. The mis-coordination is a small region compared to the other regions where it is coordinated. What is the risk that the fault will be in that region?

The other considerations are that if the fuse opens, one does not know if the fault is in the transformer or on the bus since coordination is not perfect. On the other hand if the fault is in the main buswork, does it matter whether the breaker tripped or fuse opened. Still going to be major repairs whether bus or transformer.

I would go with the 25E and make sure it is well documented in the report and facility is aware of it.

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PostPosted: Thu Feb 10, 2011 10:32 am 
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Location: New England
Will the Main breaker line side be seperately enclosed? The secondary side of the main breaker will get you down to where you want to be, your concern is the line side of the main. How often will you be exposing the line side?

When I do coordination now, I find myself in a new school of thought. I am less concerned with coordination and more concerned with reducing arc flash. It remains a constant compromise. But if I am going to work routinely on equipment live, I prefer the lower arc flash. If its seldom to never I go for the coordination.

I would also look at the associated HRC levels and risks for if you were to blow the primary fuses. Is it connected directly to the utility, requiring a long delay to repair? Is there a means to isolate the medium voltage (assuming it is medium voltage)?


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PostPosted: Thu Feb 10, 2011 12:41 pm 
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I see a big reduction in IE with the faster fuse for faults on the line side of the secondary main. Considering a line side fault, loss of coordination isn't really an issue.

It looks like the region where coordination is compromised with the 25E fuse begins at around the 2 Sec line. Unless you are using an extended exposure time, there isn't much improvement in IE gained for the loss of coordination for faults downstream of the secondary main. 2 Seconds cutoff occurs long before Mr. 25E gets mixed up with those thermal magnetic trips.

With that said, could the ST on the main be reduced without compromising downstream coordination? This expands the window of faults allowing the main to trip prior to to blowing the primary fuse.

Coordination was always subjective enough, adding the arc flash issues makes for even more spirited discussions.


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PostPosted: Thu Feb 10, 2011 12:45 pm 
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Thanks for the replies guys. You swayed me.
And no haze, it doesn't have an isolated mcb.
I agree wbd, that the occurrence of a fault with magnitude and duration in the overlap region is highly unlikely.
It speaks to the importance of knowing your inrush requirements in a given system to allow for fine tuning of the instantaneous pickup.
Regards,
John M


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PostPosted: Fri Feb 11, 2011 10:05 am 
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Coordination between the main and the primary fuse is not critical because you don't lose any selectivity. You might blow a fuse instead of tripping a breaker, which is an inconvenience.

You lose coordination with some feeder breakers for currents between 2500 A and 3500 A. Are the faults in this range?


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PostPosted: Fri Feb 11, 2011 2:47 pm 
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jghrist writes:
You lose coordination with some feeder breakers for currents between 2500 A and 3500 A. Are the faults in this range?

No sir, the bolted fault currents are much larger, on the order of 10 and 16k amps. But I suppose an arcing fault could be in that range! That's probably all the more reason to limit the IE with the smaller fuse.

John M


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PostPosted: Fri Feb 11, 2011 3:09 pm 
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In addition to simple coordination. I also look at the arcing currents when I am doing the coordination. I pick a safety value like 85% of the arcing fault current, then I make sure that value gives me the response I want. For instance, on branch breakers, I want to make sure that the 85% or arcing current value gets me into the 'instantaneous' reaction zone of the breaker. If that value is close, but moves me into the 'short time' area, I might compromise some coordination to get back into the instantaneous. Its a compromise depending on the criticality of the equipment versus the quantity of interaction time by the electrician.


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PostPosted: Mon Feb 14, 2011 8:06 am 
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mayanees wrote:
jghrist writes:
You lose coordination with some feeder breakers for currents between 2500 A and 3500 A. Are the faults in this range?

No sir, the bolted fault currents are much larger, on the order of 10 and 16k amps. But I suppose an arcing fault could be in that range! That's probably all the more reason to limit the IE with the smaller fuse.

John M


In that case, I'd say use the smaller fuse. The chances of miscoordination are very small because the chances of a fault in the miscoordination range are very small.


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