LV Circuit Breaker Data 1

[gvasiliou]

Hi again guys,

I would like to determine what is the maximum sinlg line to ground short circuit current that an existed LV Circuit Breaker is able to withstand.

The Circuit Breaker has the following characteristics:
Type: Merlin Gerlin, NS2500N , 4 Pole
Installation Conditions: 400 Volts - 50 Hz - 3phases + Neutral

The label of the circuit breaker refers the following data:

Although the circuit breaker short circuit capability is cleary mentioned to be 70kA for 3phase bolted short circuit, i can not find the CB capability for Single Line to Ground short circuit faults.

Anybody has an idea...? Thanks in advance.

 

[DRWeig]

Hi gvasiliou,

I believe the rating applies to any fault current, no matter the cause. If it's 70kA for three-phase, it's 70kA for single-phase.

Good on ya,

Goober Dave

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[gvasiliou]

The label refers 85kA for 220 Volts, and since the voltage reference for Line to ground short circuit could be considered 220 Volts i was hoping the particular CB to be able to support 85kA for Line to ground fault. It seems that i'm wrong....

Is anybody has some kind of experience in similar issue, please advise.

 

[DRWeig]

Wait for others' responses -- it is possible that I'm confused.

Good on ya,

Goober Dave

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[mgtrp]

If you're operating at 400V, the 380/415V rating applies, regardless of whether the fault is phase-phase of phase-earth.

Note that if this is the generator CB for the system mentioned in some of your recent posts then it may only be interrupting part of the total fault current - only the system contribution for faults in the generator, only the generator contribution for faults on the bus or system. If it is a feeder CB fed from the bus, you must consider the combined fault current.

 

[deltawhy]

Well besides the obvious fact that if your AFC at that point is above the ratings of that breaker at rated voltage it will not work, what is the purpose of finding this information (400V/1.732 = 231V > 220V)?. Are you anticipating you are only going to be seeing L-G faults? Definitely don't want to be counting on this fact.

Nonetheless, interesting topic if done hypothetically.

 

[gvasiliou]

Thank you guys for your replies.
@mgtrp
Yes, the circuit breaker described belongs to the system described in my recent posts.
Actually is the LV circuit breaker between Generator and Transformer.
If we count each contribution seperately as you advise then CB is OK. But if we have to add fault currents then it is not ok.....
And i honestly i don't know if fault currents must be added or not
PS : Is there any recognized code (ANSI/VDE/IEC) clarifying something like that...?)

@DeltaWhy
It is really an interesting topic, but for me is not theoritical, is a real problem. This LV Circuit was already installed before short circuit study of the whole system to be completed (don't ask why...) and now i need to know if this Circuit Breaker is capable to withstand the fault currents of the system or must be replaced.

Project Details:
SC Results for point F1 of bellow sketch
(ignore point F2 of the sketch):

3phase Short Circuit
Generator Contribution to F1 : 21,7 kA
Grid Contribution to F1: 50,8kA
Total Fault at F1: 72,4 kA

Line to Ground Short Circuit
Generator Contribution to F1 : 46 kA
Grid Contribution to F1: 40,5kA
Total Fault at F1: 86,5 kA

PS : Results of the short circuit study can be found attached to this post.

 

[davidbeach]

There is no condition for which the breaker has to interrupt the whole fault current. It has to interrupt the system contribution for generator faults and the generator contribution for system faults. But I certainly hope that it isn't really a four pole breaker switching the neutral. That machine needs to be grounded, even if high resistance grounded, the entire time there is voltage on it.

 

[gvasiliou]

DavidBeach thanks for your info.
Your consideration seems sensible, but my customer claims that correct sizing of the circuit breaker should be considering the total fault.
PS: Offcourse there is a possibility my customer to be wrong, but since is a goverment company i would like to prove somehow our sensible consideration using some kind of international standard.

Do you know if there is any kind of international standard (ANSI/IEC/VDE/whatever) that gives some guidance for this issue, defining for example if such a circuit breaker must be sized for total fault current or contribution fault current by each source....?

For neutral, unfortunatelly the breaker used is 4pole, meaning that if a fault will hapen the neutral will be disconnected by earth. Do you think that this is dangerous for the alternator...?

Regards

 

[jraef]

I can't speak for IEC ratings but here in the US where we have Delta power systems, we have what are referred to as "Slash Rated" MCCBs. They are considered unuseable for Delta power systems because the voltage to ground will be at line-to-line voltage, not line-to-neutral voltage, so therefore the energy in a grounded fault will be as high as the energy in a bolted fault. If that breaker were to have a UL lable on it, the label would read "415/240V" because the highest voltage on a single pole grounded fault would be 240V, not 415V. In that case, you would in fact use the 240V rating for interrupt capacity and it would be higher than the 415V phase-to-phase rating, as is reflected in the label of your breaker.

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[gvasiliou]

To all of you that you find this topic interesting:

I just found a document by AREVA that refers to sizing of the Generator Circuit Breaker. I'm luck enough that this document describes GCB sizing to a system almost identical to my system.
This AREVA document fully agrees with davidbeach and jraef consideration which is the most reasonable approach.

Although, i would be happy enough if somebody knows any international standard that confirms above considerations.
Unfortunately AREVA document describes normal practice but no reference to any standard is made.

Document can be found here:

http://ewh.ieee.org/r3/atlanta/ias/IEEE-Areva-Oct-09.pdf

Regards

 

[gvasiliou]

In order to close this topic forever, the relavant standard which describes exactly what we are looking for is the following:
IEEE std C37.013 - 1997: IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis, page 55/87, paragraph 7.3.5.1.

Part of paragraph 7.3.5.1:
"The generator-source short-circuit current has no direct relationship to the system-source short-circuit current.
Because the system-source short-circuit current is higher than the generator-source short-circuit current, it has been
specified as the rated short-circuit current of a generator circuit breaker."

 

[ScottyUK]

While that may be true in some instances it is not true in all circumstances. It is likely to be true for line-earth faults in those installations where the generator has a high resistance earthing scheme, but may or may not be true for line-line faults or for solidly earthed systems.

Do not forget that the generator can contribute a significant exponentially decaying DC component to the fault which may result in a long period until the first current zero crossing. This can make it difficult for for some types of breaker to extinguish the arc, and prolonged arcing can result in breaker failure.

Many MV distribution breakers are highly unsuited to generator duty, although those manufacturers whose product range doesn't include a generator CB don't make much effort to highlight this problem. Occasionally you will probably encounter some very questionable designs which introduce an intentional delay to the trip signal until the DC component decays. I am uncomfortable with the idea of deliberately allowing a fault to persist longer that necessary in order to allow cheap distribution breakers to be used in a more severe service than they are designed for.


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