IT system single phase breaking capacity 2

[pisimatza]

Hi

IT systems are systems conected to earth through an impedance. To lower short circuit curent, right ?

I have a situation, 380 V, 50 HZ, transformer 2MVA, where i have to choose circuit breakers for an IT system, and in CB catalog , the values for "single phase breaking capacity Iit " are very low: 6, 12, 20 kA. Or max 36 kA for "strongest" MCCBs with Icw = 150 kA for 1 s. The required "short time current" is 50KA /0,5 s.
1. Are those MCCB-s suitable, or low single phase breaking capacity makes then un-usable?
2. Aditional question: in attached diagram 10 A is the maximum short circuit current value, limited with the impedance(resistance)?
3. Then why they(engineers) ask 50kA?

 

[davidbeach]

Phase-phase and 3-phase fault currents are not limited by the grounding resistor.

 

[pisimatza]

You are right, you answered to no. 3, (and 2).
So that means MCCB-s with lower single phase breaking capacity in IT systems are good? In IEC 60947-2 tests for single phase breaking capacity in IT systems are described : "A short-circuit test on the individual poles of the multi-pole circuit breaker".
I still cannot understand this: a robust CB with Icu= 150KA/1 s, in IT has the " single phase breaking capacity in IT" only 25 KA ? This means that: The CB can take 150 KA, but in IT, the same CB can take only 25, under same voltage.
Enlight me, please ...

 

[davidbeach]

Sorry, I'm not conversant in IEC, someone else will have to explain.

 

[ScottyUK]

This isn't an IEC-only issue, it seems to apply to both IEC types and the odd UL breaker we see over here. Maybe it's a universal problem inherent in MCCB designs. Cooper Bussmann have some discussion in this link, obviously a little biased toward using their fuses to back up the MCCBs:

http://www.bussmann.com/pdf/f61e3a11-e814-4700-8a7a-9d90d4b19963.pdf

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If we learn from our mistakes I'm getting a great education!

 

[pisimatza]

Thanks ScottyUK, -nice explosion, you must be right, single phase fault probably are more harmfull to MCCB, due to their design.
So-in IT case , when first ph-gnd fault occures, current path is through resistor, current is < 10A. 2-nd ph-gnd fault if occurs, on a different phase, then fault current return path is not through the resistor, and current value can be up to 87% of 3p fault current, and voltage is phase to phase voltage.
Am i right that the fault current after second fault is 2p fault current?


Back to my case... if designer does not give a certain value
of maximum 2 phase short circuit current , i cannot choose the right MCCB.