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Topic Title: Grading Margin(time) between ACB and Fuse
Topic Summary: Determining time margin between fuse and ACB for correct fault discrimination
Created On: 09 November 2011 02:01 PM
Status: Read Only
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 09 November 2011 02:01 PM
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Posts: 1
Joined: 11 April 2011

Hi there,
This is the first time I'm posting. Please excuse my improper english.
I would like to ask what sort of margin is used for grading ACB/MCCB with downstream fuse vice versa fuse with downstream ACB/MCCB.

Let say my breaker is ABB PR112 and my downstream device is BS88 200A fuse.
What I've used -
Grading margin = 0.4(Fuse prearcing time) + 0.15s ,
thus my minimum ACB operating time = Fuse prearcing time + 0.4(Fuse prearcing time)+0.15s

What do you guys think?
Please also shed some light on what grading to use for grading fuse with downstream ACB/MCCB

 10 November 2011 05:32 PM
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Posts: 204
Joined: 18 January 2003

You can register and download the ABB software from here....

Download the Schneider software...

 10 November 2011 07:08 PM
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Joined: 28 August 2002

Hi Wee Kok,
I would join the website engineering questions & answers community, there's hundreds of followers there from engineers to electricians. Too many academics on the IET site.
From my recollection, I'm retired now, I presented the two ( or more) characteristics on the same graph, taking account of manufacturing margins and the pre-arcing time of the fuse link, and the maximum fault level below the fuse.
It might be worth mentioning that in my 50 years experience there's a fair percentage of protective devices out there that don't discriminate, even at 11 kV. It was common in the Middle East for faults to trip way upstream through one or two levels of protection. Sometimes it's not possible because there are too many levels close together. But there are ways if you study the old GEC measurements, Siemens and others support documentation.
 11 November 2011 11:57 AM
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Joined: 22 November 2010

I always used 0.4 second between Inverse Definate Minimum Time Lag relay settings taking the worst fault condition, eg standby in service for relay nearest the fault but standby not in service for next upstream relay. I do not know if this is an industry standard.

Best wishes

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