Cascading Of Incoming & Outgoing Breakers

This is just personal preference, but for the smaller MCCB's I'd be inclined to split the board with a section for smaller feeders protected by a suitably rated ACB.

I've had to do this when modifying older boards to accept small feeds.

but my client insists on a single 4000A acb and using another ACB just for smaller feeders will increase the cost too and the market is pretty hectic these days ......

ya the discrimination is time based right. i mean if fault occurs at the load side of a 100A breaker thn the 100A breaker must trip before the incomer ACB so tht there s power flowing through other feeders . this is what discrimination means right?

and cascading is like say for a 70 kA fault at the busbars (after the incomer) ,the incomer must trip before the fault affects the outgoing circuits which are of 50KA Icu..am i right?

i have taken schneider electric acb masterpact nw40bH1 as the incomer. and outgoing are nsx N type..........

yes sir, it is available but it is very very costly and the delivery time is more thn 12 weeks. so i cant use 150KA breakers. thats y i was talking about cascading. and yes total discrimination is provided between masterpact NW (current limiting type) and masterpact nt ,nsx breakers.

so the breaker sc rating posted on my question is that ok or do i have to go to one further level , say 70KA.

i mailed schneider customer care and they told me they ll come back with a solution..its been 2 days and yet no sign from them.

You don't seem to have studied ECT 167 which describes various systems of coordination.

If you understand and use cascading, you need a current-limiting circuit breaker as the incomer, and cheaper, normal MCCBs in the outgoing feeders. The Masterpact ACB also has a current-limiting version with (i think) 200kA capacity. Once you select this as the incomer, the rest of the downstream MCCB can be of lower capacity ... that is what 'cascading' or 'series rating' is all about.

Remember, proper selection of circuit breakers depends on ensuring that as far as possible, only the faulty section is isolated, and continuity of power is maintained to healthy feeders. Cascading may look like a compromise, but is it ? How likely is a 100kA fault ? Only the SC labs of CPRI and ERDA can deliver such a fault...that too to a solidly bolted short-circuit. So, you need to check whether your conditions are met at all levels - from overloads to high impedance short-circuits to I_cu.

Coordination is an interesting but complex subject. i would urge you to read that paper, study the coordination tables available on Schneider website, sit with their engineers and discuss in depth. You will learn a lot.

Isn't it time you registered on CR4 ? Many people (including me ) do not reply to anonymous posters.

i have studied ect 167 one time and i must say its pretty complex. but still i know some thing about discrimination. yes i am young man with just 10 months experience in lv design & working under a senior designer . my designer has gone out of country for his vacation and now its all upon me to design. since this s a tendering stage, i want to be little careful ........i had called the client regarding the short circuit rating & he said "hmm, i donot know, current, i ask my engineer and come ". His electric engineer told me that "the supply is from a 11kv/415v 2.5MVA step down transformer with secondary impedance of 6.25% and so from the calculation it is coming around 90-100k, so design accordingly".

i have seen the discrimination tables .if i want to use nw40bH1 there s no total discrimination between incomer n outgoing nw1000A acb. if i use NT acb i will get total discrimination.

but i dont know about the KA rating. NTH2 is 50KA & NTH1 42 KA. NTL1 which is a current limiting breaker is of 130KA.

The fault level at the outgoing terminals of the transformer may be 100kA, but will it not reduce at the outgoing of each feeder ? Something like this ....

This is on page 101 of this document from Schneider, cascading_2009_EN-2.pdf

You can select downstream MCCBs with lower Icu/Ics after calculating the various fault levels (using ETAP or some such software maybe?)

Here in the US, we call it "Series Ratings" when you want to use a system of protective devices down stream that have a lower interrupt capacity than the main ahead of them and rely upon the current limiting capability of the upstream device to keep the fault current levels below the levels of the lower rated devices. But for us, this can only be done if there has been some very expensive testing and third party approvals performed, you cannot do it in the field. So if you were constructing this as a one-off, you would either have to use a listed series combination, usually all from the same mfr., or be willing to sacrifice a test system at an approved lab. I'm not sure if IEC requires the same thing though, but is appears as though they do not. Still, it might be worth checking to see if a manufacturer has already done this testing for the US market on their components, because that would be an excellent argument that the combination of devices is sound.

Exactly JRaef

i remember a super designer from Westinghouse describing the logic of the "Series Rating"way back in 1984. i remember thinkig then 'What a nice concept! We need this in India, not that awful expensive discrimination system"

Nothing came out of that i regret, since my company was a 96 pound weakling against several 500 pound gorillas....

i will never forget what my illustrious American friend told me two years later.... "You really are a RICH country ! You don't want series rating ! You can afford full discrimination !!! "

Not really. Just too much CYA(cover your a$$)and NMJ (not My job) syndromes

Schneider, ABB, Siemens and many others have full discrimination and cascading ratings for their equipment on their websites...fully tested and certified by independant labs like KEMA and CESI.