It is currently Tue Sep 28, 2021 7:27 am

Post new topic Reply to topic

Is correct to use arcing current to calculate FCT
Arcing current
Boltef short circuit current
You may select up to 2 options

View results
Author Message
ekstra   ara
 Post subject: FCT - use bolted or arcing current to calculate it
PostPosted: Wed Sep 19, 2012 3:31 am 

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!

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?

 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Sep 19, 2012 1:39 pm 
Sparks Level

Joined: Thu Jul 07, 2011 6:03 am
Posts: 66
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.

 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Sep 24, 2012 5:56 pm 

Joined: Sat Apr 18, 2009 6:31 am
Posts: 4
Location: Kuala Lumpur, Malaysia

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.

 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 3 posts ] 

All times are UTC - 7 hours

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883