Interrupting and Adj. Interrupting Short Circuit

[HamidEle]

I have a fundamental question about short circuit calculation. Not so clear about the definition of Interrupting and Adjusted interrupting symm. Is the adjusted sym the same as the assymmetrical short circuit? Which one should we use to compare with equpment rated interpting rating? Can anyone shed some llight on it? Thanks in advance.

 

[davidbeach]

It depends. What are the applicable standards?

 

[HamidEle]

ANSI Standards. My understanding is, the circuit breaker ratings should be based on the adjusted symmetrical short circuit current, which depends on the circuit breaker parting time.

 

[juleselec]

This is more about definitions than anything else, so I'll just outline the ones relevant to you:

A short circuit current is modelled by ANSI or IEC as a decaying sinusoidal ac waveform superposed on a decaying dc waveform. The sinusoidal part is typically called the symmetrical short circuit and the dc part is often called the dc decrement or just the dc component.

"Decaying" is the key word here because the magnitude of the short circuit changes with time.

The interrupting SC current is the magnitude of the short circuit at the time the circuit breaker is supposed to clear the fault (usually in the first few cycles).

So the interrupting symmetrical current is the ac part of the interrupting SC current. This is of significance because, and I'll paraphrase IEEE Std 142, "post-1964, the short-circuit (interrupting) capabilities of circuit breakers are rated on the symmetrical rms current".

ANSI doesn't mention the "adjusted" interrupting current, and maybe someone else can shed some light on this. But it does talk about the assymmetrical (total) interrupting current, which includes the decaying dc component. Older circuit breakers (pre-1964) are rated to this interrupting current.

 

[jghrist]

The calculated fault current has to be adjusted for comparison to the symmetrical current interrupting capability of a MV or HV circuit breaker. The adjustment is based on the interrupting time, the X/R ratio (derived from separate R and X networks), and whether or not the faults are fed predominately from remote or local generators. See Red Book IEEE Std 141 (¶4.5.4.2 in the 1993 edition).

 

[HamidEle]

Thanks, Guys,

That is exactly what I thoght.

 

[HamidEle]

IEEE circuit breaker short circuit rating methods are different than IEC, whereas IEC circurt brekaer has Symmetrical and Asymmetrical shortc breaking current ratings. In IEEE world, assymentrical short circuit (decayed) has to be used to compare with Circuit breaker Symmetrical interrupting ratings, which is defined in IEEE 141 4.5.4.2.

 

[HamidEle]

We are having difficulty finding appropriate circuit breakers with peak making duty. The maximum making current rating is 108kA. Is it possible to get higher than 108kA?