IT Double fault Breaking Capacity

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electrics

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İn a schneider NG125H mcb data table it says

Type Voltage Breaking Capacity
1P 220...240 36000
1P 380....415 9000 (1)

(1) IT İsolated neutral system double fault 1P breaking capacity


so why is it so ?
is there any relation between the system and the capacity of a MCCB ?
 
A comment

A comment

Hi, Perhaps my post couldnt be understood since it is not so much clear..
I cant see why circuit breaker breaking capacity is defined with IT double fault and not with a simple NT single phase fault (voltage will be Phase-neutral of course ) ?
I wonder if this (1) Note is relevant to both 220 and 400 volt or just 400 volt?
If just 400 volt what might be the thing meant with it?
Can u see why 1P breaking capacities are given for IT schemes especially?
 
You need to hang tight and wait for zog, others here may chime in, but zog is the resident breaker expert IMO.
 
With an IT system (see Wikipedia) a phase to ground fault that will not trip the breaker, as the ground is isolated, so there will be a negligible fault current flow under these circumstances. But when a second phase to grounds fault happens (on a different phase, obviously) then there will be a phase to phase short, a double fault situation, and then the breaker will open.

Which is the what. But I dont know why...
 
electrics said:
Hi, Perhaps my post couldnt be understood since it is not so much clear..
I cant see why circuit breaker breaking capacity is defined with IT double fault and not with a simple NT single phase fault (voltage will be Phase-neutral of course ) ?
I wonder if this (1) Note is relevant to both 220 and 400 volt or just 400 volt?
If just 400 volt what might be the thing meant with it?
Can u see why 1P breaking capacities are given for IT schemes especially?
Click to expand...

Your post is even less clear now. I have no idea what you are asking.
 
dbuckley said:
With an IT system (see Wikipedia) a phase to ground fault that will not trip the breaker, as the ground is isolated, so there will be a negligible fault current flow under these circumstances. But when a second phase to grounds fault happens (on a different phase, obviously) then there will be a phase to phase short, a double fault situation, and then the breaker will open.

Which is the what. But I dont know why...
Click to expand...

I agree with you an the what.

The why is:
During a normal fault the 1 pole of the breaker is interrupting the L-G current at the L-N neutral. If the system is an IT, a single L-G fault does nothing, but during a second L-G fault the 1 pole of the breaker must now interrupt the L-L fault at the L-L voltage, which is a more severe situation.
 
electrics said:
okay forget it...
Click to expand...

Don't forget it, I just don't know much about small breakers like this, someone else might. Also keep in mind this is a mostly North American forum, you are using IEC terminology we don;t see much over here. Jref is pretty sharp with tis stuff, PM him and ask him to comment.
 
electrics said:
İn a schneider NG125H mcb data table it says

Type Voltage Breaking Capacity
1P 220...240 36000
1P 380....415 9000 (1)

(1) IT İsolated neutral system double fault 1P breaking capacity


so why is it so ?
is there any relation between the system and the capacity of a MCCB ?
Click to expand...

Keep in mind that this is primarily a forum for people in the US who are licensed Electricians and Electrical Engineers, IEC questions are not going to get a lot of responses. I happen to have worked for Siemens and Moeller for in the past here in the US so I have a little more exposure to these issues, but generally few people in the US know what "IT" means.

First off, you say "MCB" in the beginning and then ask about "MCCB" in the end, they are different beasts.

What this footnote is saying is that in the case of an IT system, one where there is no connection from Neutral to Earth in the distribution system, the amount of current that can flow through a 1 pole breaker to earth is inherently limited by the high relative resistance, and because it is a 1 pole breaker, there is no other path for fault current flow. So in that case, the MCB (Miniature Circuit Breaker) can withstand the forces of breaking a fault current of 18,000A as opposed to only 9,000A if it were a TT or TN system. Don't read or extrapolate anything else from that.

This would not automatically apply to MCCBs (Molded Case Circuit Breakers) either, they generally have much higher fault interrupting capacity to begin with.
 
Two-pole circuit-breakers (for phase and neutral) with one protected pole onlyThese CBs are generally provided with an overcurrent protective device on the
phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme,
however, the following conditions must be respected:
b Condition (B) of table G67 for the protection of the neutral conductor against
overcurrent in the case of a double fault
b Short-circuit current-breaking rating: A 2-pole phase-neutral CB must, by
convention, be capable of breaking on one pole (at the phase-to-phase voltage) the
current of a double fault equal to 15% of the 3-phase short-circuit current at the point
of its installation, if that current is y 10 kA; or 25% of the 3-phase short-circuit current
if it exceeds 10 kA"
so here what do u understand?? I finally found the reason for the footnote above, so can anyone interpret these lines??
 
dear JRAEF I read what you wrote but I think it doesnt make sense, especially after what I cited here above, how can you explain 15 % ??? anyway also please keep in mind that IT scheme has here two pole it means two breaking contact and the rating might be 1/2 times its first rating no surprise...
I see that I couldnt satisfy my curiosity of this subjecti nevertheless thank you..
 
Hello again

Hello again

Hi lads, Let me ask you one thing of this subject, Now we have a circuit and we calculate that this circuit will have 10 kA short circuit current, now if we use a P+N double pole MCB , so what will be the s.c rating of this mcb?
İf the answer is 10 kA so does it mean one of the poles will break the current even the other one doesnt or it means both pole have 5 kA breaking rating so that they break just half of the current since they operate at the same time? so will this MCB 5 OR 10 KA? can you englighten me?
 
I think there must be a guy who can answer this question, I am confused of this subject, pls tell me your opinion..
 
electrics said:
Hi lads, Let me ask you one thing of this subject, Now we have a circuit and we calculate that this circuit will have 10 kA short circuit current, now if we use a P+N double pole MCB , so what will be the s.c rating of this mcb?
İf the answer is 10 kA so does it mean one of the poles will break the current even the other one doesnt or it means both pole have 5 kA breaking rating so that they break just half of the current since they operate at the same time? so will this MCB 5 OR 10 KA? can you englighten me?
Click to expand...

I think this is not a bad question, by the way, is there anyone who can tell me about my first question, I couldnt find the answer still... I wonder MUCH....
 
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