IT system single phase breaking capacity
IT system single phase breaking capacity
(OP)
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?
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?






RE: IT system single phase breaking capacity
RE: IT system single phase breaking capacity
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 ...
RE: IT system single phase breaking capacity
RE: IT system single phase breaking capacity
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RE: IT system single phase breaking capacity
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.