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Old 01-27-2019, 10:26 PM   #1
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Default Schneider Arc Flash Analysis

I went Friday to a plant that I often do work in to meet with maintenance about an arc flash study they had performed in December. I only had the page from the report listing equipment with inadequate AIC for the available current.

Honestly, it looked like some of the numbers were pulled out of thin air. Many of the listed breaker ratings weren't accurate to the breaker labels. Two identical panels on different sides of the same column, fed from the same switchboard, with identical feeders from identical breakers had two wildly different calculated short circuit currents. Has anyone else seen this kind of thing from a study? I am waiting to talk to the engineer that evaluated the system.
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Old 01-27-2019, 10:52 PM   #2
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I use this one for quick calcs, if you wanted to try it out for yourself and see what you come up with:


http://www.cooperindustries.com/cont...urces/fc2.html
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Old 01-28-2019, 06:48 AM   #3
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I use this one for quick calcs, if you wanted to try it out for yourself and see what you come up with:


http://www.cooperindustries.com/cont...urces/fc2.html
Thanks. I use that app often. Even with that I couldn't get close to the numbers they had. I'm just wondering if anyone else had seen a situation like this and what the explanation was. Do they derate in some cases for... reasons? For example, a 30 kAIC breaker is on the list as 18 kA. Another 35 kAIC breaker is listed as 10 kA. I'm suspicious.
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Old 01-28-2019, 09:11 AM   #4
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No, I've never seen a breaker listed as anything other than what was marked on the breaker.
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Old 02-25-2019, 05:44 PM   #5
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Originally Posted by CoolWill View Post
I went Friday to a plant that I often do work in to meet with maintenance about an arc flash study they had performed in December. I only had the page from the report listing equipment with inadequate AIC for the available current.

Honestly, it looked like some of the numbers were pulled out of thin air. Many of the listed breaker ratings weren't accurate to the breaker labels. Two identical panels on different sides of the same column, fed from the same switchboard, with identical feeders from identical breakers had two wildly different calculated short circuit currents. Has anyone else seen this kind of thing from a study? I am waiting to talk to the engineer that evaluated the system.
Identical feeder from the same switchboard would yield identical short-circuit currents. Unless one of the panels could be fed from an emergency generator or some other alternate feed?
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Old 02-25-2019, 10:32 PM   #6
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Agreed. I saw a job where the markings seemed to be backwards as you went downstream from the main. The farther from the source the higher the afc. Isn't that backwards?
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Old 02-26-2019, 10:18 AM   #7
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Agreed. I saw a job where the markings seemed to be backwards as you went downstream from the main. The farther from the source the higher the afc. Isn't that backwards?
Not necessarily. Protective devices have an inverse time curve. A higher short circuit could trip instantaneously, while a lower short circuit could take 2-3 seconds to trip. This could mean a lower short circuit would yield a higher incident energy.
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Old 02-26-2019, 10:25 AM   #8
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Not necessarily. Protective devices have an inverse time curve. A higher short circuit could trip instantaneously, while a lower short circuit could take 2-3 seconds to trip. This could mean a lower short circuit would yield a higher incident energy.
If the higher short circuit is the first device on the line doesn't that lower the rating for all devices downstream?
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Old 02-26-2019, 10:32 AM   #9
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With Coordination Studies wrong CB settings that do not exist.

Have seen Arc Flash studies that appear to be not 100% on the mark for sure.
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Old 02-26-2019, 10:36 AM   #10
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If the higher short circuit is the first device on the line doesn't that lower the rating for all devices downstream?
No, because the downstream devices will have an impedance between the first device and the downstream equipment.

Take for example a feeder breaker and a downstream MCC. The short circuit current at the feeder breaker is 30 kA. At the MCC it is 20 kA due to a long cable.

The breaker is set to trip instantaneously for faults above 25 kA. It has a long time pickup setting of 2 seconds for faults below 25 kA.

For a fault at the feeder breaker, the breaker trips instantaneously. The short circuit is 30 kA, but incident energy is low due to the instantaneous pickup.

For a fault at the MCC, the upstream feeder breaker trips at 2 seconds. The short circuit is 20 kA, but incident energy is high due to the 2 second pickup.

This is why IEEE Std. 1584 requires IE calculations for both the expected short circuit and 85% of that - in case your protective device has a long time pickup setting that is right at your expected short circuit current.
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Old 02-26-2019, 04:00 PM   #11
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I went Friday to a plant that I often do work in to meet with maintenance about an arc flash study they had performed in December. I only had the page from the report listing equipment with inadequate AIC for the available current.

Honestly, it looked like some of the numbers were pulled out of thin air. Many of the listed breaker ratings weren't accurate to the breaker labels. Two identical panels on different sides of the same column, fed from the same switchboard, with identical feeders from identical breakers had two wildly different calculated short circuit currents. Has anyone else seen this kind of thing from a study? I am waiting to talk to the engineer that evaluated the system.
Sounds like poor data collection, you don't pay an engineer to do data collections, would be too expensive, so either the plant supplied them with poor data or they subbed out someone that did a crappy job. Without accurate data, the whole arc flash study is invalid.
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