Symmetrical/Asymmetrical short circuit current

[jacphi]

We have a requirement for the 480V switchears and Motor control centres to have rating for 65kA (symmetrical) short circuit current and the main bus bracing for 100kA momentary short circuit current (Asymmetrical). I would like to know if the vendor provided 65kA (symmetrical) short circuit rated Switchgears and mcc's, will the requirement for bus bracing for 100kA momentary short circuit current (Asymmetrical) be met?

 

[RRaghunath]

Yes.

LV switchgear up to 80kA symmetrical short circuit with stand (1sec) is widely available.
The Asymmetrical or momentary or making current rating is higher by a factor of 2.1 for switchgear at that voltage.
So, there shouldn't be any problem in procuring the switchgear with the said specifications.

 

[electricpete]

Where does 2.1 come from?

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[RajT]

The 2.1 factor is a type test requirement defined in IEC.
It is a factor to which all the low voltage switchgear (upto 1000V)must satisfy for the purpose of meeting the IEC requirement.
Incidently the factor increases to 2.5 for all the medium voltage switchgear (between 1kV - 56kV) having to meet the same Asy. current requirements.

 

[light1]

480V power distribution system and equipment rating are not typical for IEC. ANSI Stds for LV switchgear imbed the worst asymmetrical value of the short circuit current associated with the symmetrical rating, i.e. 65kA rms symmetrical. 100kA rms asymmetrical or peak (it is not clear from the original posting) bus bracing will not suffice.

 

[alehman]

ANSI and UL standards for low voltage equipment are in terms of symmetrical amps. Fault X/R ratio is specified by the standards which vary based on equipment load rating and type of contstruction. The symmetrical rating of the equipment must be adjusted if X/R is outside of the specified standard.

 

[luis2107]

Hello,

I think the topic you have talked is very interesting and sometimes not very clear.

My question is:

Do you know if there is any standar from ANSI/NEMA/UL concerning to busbars in any voltage rating?

I guess there should be something such as with the circuit breakers. . .

Regards

 

[asco1]

UL891, Supplement B provides construction requirements for bus supports and bus bracing for up to 100,000. These are very specific and can be used by any UL Switchboard house.
A manufacturer may also have their own design subbmitted, tested, and aproved by UL.

 

[Flashover]

whats the difference between Symmetrical/Asymmetrical faults?
sorry for my ignorance

Flashover

 

[cuky2000]

Flasover, the term asymmetrical is used to describe the symmetry of the current sine wave about the zero axis.
The worst SC case scenario will occur at the zero point of the voltage wave. An exponential decaying unidirectional current component (DC decrement) will be added to the AC symmetrical component forming the total asymmetrical current wave at a particular instant.

See the enclose article

http://polk-burnett.apogee.net/pd/dshe.asp