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

Thanks.

You can register and download the ABB software from here....
http://www.abb.bg/cawp/db0003d...c12578160049a171.aspx

Download the Schneider software...
http://www.global-download.sch...0E3BE852576B900696FC0

M

Hi Wee Kok,
I would join the CR4globalspec.com 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.

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
John