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Is correct to use arcing current to calculate FCT
Arcing current
Boltef short circuit current
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ekstra   ara
 Post subject: FCT - use bolted or arcing current to calculate it
PostPosted: Wed Sep 19, 2012 3:31 am 
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Joined: Fri Aug 03, 2012 1:44 am
Posts: 9
Relay and general protecion setting are calculated based upon short circuit calculation.
But during Arc-Flash calculation the TCC drawings (selectivity curves) are compared with arcing current.

Sometimes can happen that a protection calculated for both maximum and minimum short circuit current seems not suitable in case of arc-flash.

Consider an example like this:
Ibolted short circuit current = 8.32 kA
Istantaneous treshold of protection 4.8 kA.

this seems suitable with a margin of 173%. Definitely good and acceptable!

But:
I arcing = 5.17 kA
I arcing wiht 15% reduction 4.17 kA

The situation is the setting are not suitable for an arc-flash.

Is correct proceed to calculate (like some commercial software and IEEE 1584 too) the FCT with the arcing current?


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PostPosted: Wed Sep 19, 2012 1:39 pm 
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Joined: Thu Jul 07, 2011 6:03 am
Posts: 59
Location: Netherlands
Depends on what happens to selectivity with downstream devices and load inrush. If your sole criterion for the protection settings is the short-circuit current, then yes, lowering it to trip for arcing currents will give you (a lot) lower hazard levels. Just keep in mind that you may run into coordination issues or unwanted trips when energizing when you lower the instantaneous setting.

Alternatively you can look into maintenance switches that temporarily reduce the instantaneous setting when work is carried out. There are some retrofit solutions that provide it for low voltage circuit breakers, or as an option for a new trip unit.

Not sure what 'FCT' means by the way.


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PostPosted: Mon Sep 24, 2012 5:56 pm 
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Joined: Sat Apr 18, 2009 6:31 am
Posts: 4
Location: Kuala Lumpur, Malaysia
Hi

Many of my clients here want relay coordination should based on bolted fault current and arcing fault current.

I always make coordination using definite time characteristics. So whatever the fault type (maximum bolted, minimum bolted, minimum arcing fault), the tripping time is always constant. I use IDMT for overload protection only. Definite time for short circuit protection.

Maintenance switch is always the last option for me. I prefer to have a busbar blocking scheme to achieve fast bus protection.


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