CB Short Circuit Rating

[Siyou]

Hello everyone!

How to evaluate CB short circuit rating based on the calculated short circuit current? How many percent must be the calculated fault current compared to CB short circuit rating?
Do you have standard CB short circuit ratings table for ANSI and IEC CB?
Thanks everyone

 

[Mbrooke]

This lists the max ratings for each voltage class:

https://assets.new.siemens.com/siem...e/version:1594977046/circuitbreakerfamily.pdf

40ka and 63ka is typical in major US substations 115kv and above.

 

[wbd]

What voltage are you at?

 

[cuky2000]

A couple of years ago, several circuit breakers rated for 90 kA were installed. At that time, 100 kA breakers were in testing and probably will be available.

https://library.e.abb.com/public/a147af5683d7462cc12579d00056e326/242PMI90_2GNM110092.pdf

 

[davidbeach]

I would not want to contemplate the need for 90 or 100kA breakers. Doing ground grid designs where we have to have 63kA breakers is bad enough.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cuky2000]

The available short circuit is imposed by particular system conditions an engineer has very little control over this situation. Large short circuits and large X/R ratios are typically closer to a generation station.
We solve the grounding system with no problem taking advantage of a fast clearing time than the usual 30 cycles.

 

[Siyou]

Do you use safety margin in comparing the calculated fault currents to the Cb rating?what if the safety margin is 330% what does it mean??
Do you have TABLE FOR standard kaic ratings IN ANSI and IEC?? THANK YOU

 

[Mbrooke]

@Cuky: I know the New Jersey 230kv system has plenty of 90ka breakers. Linden, Sewaren, Hudson are all generator pockets with the ties to neighboring systems make for some obscene fault current.

 

[Siyou]

Do you have tables for the short circuit rating of CB in ANSI at IEC used in high and medium voltage?

 

[Mbrooke]

I don't have any specific tables, sorry.

 

[cuky2000]

Below is a partial list of HV dead-tank breaker and MV SWGR used typically used in the ANSI marketplaces. A similar rating is available for live tank breakers used in the IEC regions.

It should be noticed that in Northamerica have a pocket of an unusual high short circuit in NY, NJ, PA in the PJM ISO, and other regions. That is forcing the planning team to start replacing over duties circuit breakers, reinforcing busses, and upgrade grounding grids. Special attention is required to surge arresters that until recently was not available for fault current in excess of 80 kA.

 

[Kiribanda]

1) With regard to safety margin after performing SC studies you have to make an engineering decision
about the required CB capability.
2) Due to harmonization of IEC & IEEE/ ANSI standards, the North American CB rating std (C37.06)
is now based on IEC 56. That means the cont current rating & short circuit current ratings are based
on R10 series, which means 20,25,31.5,40,50 & 63 in Amps or kA.

 

[Siyou]

Can I ask if the rated short circuit amps in nameplate is the interrupting symmetrical rms current rating? The nameplate has only the rated short circuit amp on the nameplate which is 25kA., with 3 cycles and voltage range factor of 1.0 . How can I know the momentary rating or the making current rating of this CB?
Thank You

 

[Kiribanda]

All HV & MV standard breakers are tested for X/R=17 at 60Hz means Time Constant=45ms.
Therefore, your bkr will have folllowing Momentary ratings.
Asy(rms) = 1.6x25kA=40kA
Asy(pk) = 2.6x25kA=65kA

 

[Siyou]

Why it is important to compute the momentary symmetrical rms current,momentary asymmetrical rms current and momentary asymmetrical peak in the evaluation of CB short circuit ratings?

Does the momentary asymmetrical peak short circuit current and the interrupting symmetrical rms short circuit current used for CB evaluation??

Thanks

 

[cuky2000]

It is common accepted that the manufacturer of the circuit breaker provides only the short circuit symmetrical current tested a particular X/R ratio since they cannot know the operating conditions of the equipment. However, virtually almost all fault currents are asymmetric. The owner application engineer is responsible to determine
.

[highlight #FCE94F]X/R_system+X/R_margin > X/R_test[/highlight]​

For those cases, the symmetrical short circuit rating on the nameplate needs to be derated accordingly with an accepted method.

_{NOTE: Indirectly the process described above considers the asymmetric and peak momentary value that the breaker should withstand the thermal and mechanical bracing of the breaker. For primarily for HV & UHV (less common on MV) application, the dielectric withstands capability of the circuit breaker should considering evaluate the impact of the Transient Recovery Voltage (TRV) and the Rate-of-Rise of Recovery Voltage (RRRV) imposed by the system on the circuit breaker.}

 

[Kiribanda]

1) Asymmetrical peak value depends on the DC% component at the fault location. Higher the X/R, greater
the DC%, therefore higher the Asy peak. A CB is having a Close & Latch capability. Therefore, one has to perform
1/2Cy sc study and calculate the Asy peak duty and has to be compared with the Close & Latch capability of the CB.
2) All ANSI CBs are type tested for X/R=17 at 60Hz for its interrupting capability based on its contact
parting time (CPT) & Interrupting time. During the sc study one has to perform 1.5-4Cy sc calculation and determine
the symmetrical interrupting duty depending on the X/R at the fault location. That interrupting duty has to be
compared with the name plate symmetrical interrupting capability of the CB.

 

[Siyou]

Does the momentary short circuit calculations also called the first cycle calculation???? Can you state why did sometimes its called half cycle calculation???
Does the momentary symmetrical rms current, momentary asymmetrical rms current and momentary asymmetrical peak current based in what cycle..Kindly explain to me their difference.

Thanks

 

[Kiribanda]

I have already mentioned that in my reply. All momentary (close & latch)calculations are based on 1/2 cycle
because the first peak of the sc current wave comes during the first cycle. I think you need some reading
about the subject.