Circuit breaker ratings 1

[wonderchi]

Hi,

I looked at a few circuit breaker catalogs and I want to verify what the catalogs are saying:

For a 630A circuit breaker at 11kV network, the Ip or Imax is 50kA. Is this value the rms value or the peak value?

Thanks

 

[waross]

It is the symetrical value as opposed to the isometrical value. RMS and peak values refer to a sine wave.
The symetrical value of the current is the steady state available fault current. The isometrical value is the initial current value under actual fault conditions.
Under fault conditions the current will be the sum of the available fault current plus the decaying DC ofset current.
The magnitude of the offset current depends on the x/r ratio of the system supplying the breaker. The breaker has an allowance for moderate x/r values. If the x/r of the system is outside the allowance, the breaker must be de-rated. A breaker with a greater Imax may be chosen, or some means employed to reduce the isometric current at the breaker. This topic has been addressed several times on this forum. A search of this web-site (x/r, available fault rating) may be productive.
respectfully

 

[wonderchi]

Waross,

Thanks for you post, however wht you telling me i already know...no need to search the forum.

Wht i want to know, which i am still not getting it, is that:

By using SKM, I obtained an asymmetrical rms value of 50kA at 0.5 cycles, and then I look at my circuit breaker it tells me Imax = 60kA. The question i want to ask is...is the value Imax- the asymmetrical rms or asymmetrical peak or symmetrical rms?

Thanks

 

[jraef]

Waross answered your question in his first line, although I would add this;

TYPICALLY, the breaker ratings listed are symetrical, but TYPICALLY the breaker manufacturer says that right on the label. Yours apparently does not, so the best idea would be to ask THEM!

Most manufactiurers also publish data giving the symetrical and asymetrical interrupt ratings. They probably have that data available as a download over the internet.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[wonderchi]

Thanks jraef

But why whould the manufacture publish symmetrical interrupt ratings and not asymmetrical?

Since symmetrical current is found in steady state and asymmetrical is the transeint part in the short circuit currnet with the sum of DC component. Hence asymmetrical is usually bigger then symmetrical.

Wouldnt this confuse people who are trying to rate circuit breakers?

In fault analysis, you are suppose to find the maximum fault current that the circuit breaker need to interrupt without causing damage to the itself and the protected equipment. And the maximum value is:

The asymmetrical peak value (Ip) which is the equivalent of asymmetrical RMS value (Imax RMS).

But providing symmetrical RMS, or symmetrical peak, does not mean to meet the purpose of a fault analysis.

???

 

[davidbeach]

Breakers are rated for symmetrical amps and a maximum X/R; that combination defines the maximum asymmetry. Makes the calculations much easier.

 

[wonderchi]

Is it true that only recently breakers are rated for symmetrical amps and a maximum x/r? Because there are still lot of books still refer to asymmetrical rms values.

 

[waross]

Calculations of available fault currents on smaller distribution transformers are easy to calculate from the impedance voltage.
Such calculations result in the symetrical currents. The breakers are rated this way so that electricians may safely select and install the appropriate breakers. Electrical inspectors who have a tradesmans background can easily check the plans and installations.
I understand that the x/r value that the breakers are rated for is conservative for small to mid sized distribution transformers. (I am open to correction and/or comments on this point)
In the case of distribution networks and transmission systems, fault currents and DC components become an engineering exercise.
This system has been in place for generations. In years past, many installations were planned by electricians. By the use of available fault currents (symetrical) and breakers rated for the available fault currents, electricians were able to effect safe installations.
respectfully

 

[wonderchi]

For confirmation:

Steady state fault analysis give you the maximum and minimum fault current that can be reached.

So what does transient fault analysis give you?

Thanks

 

[swgrmfg]

Mr. Wonderchi:

The transient analysis gives you the data to use for selection of the proper close and latch capability of the circuit breaker. Remember, circuit breakers not only interrupt faults, but they can also close on faults, or, selectively, let faults through. The initial asymmetrical values are at stake during closing (or holding during a let through). By the time relays react and the breaker mechanism operates, a minimum of 2 and a 1/2 cycles has elapsed and the fault current has decayed to something below the interrupting rating of the circuit breaker. The close and latch listed rating of a circuit breaker is usually higher than its interrupting rating. Therefore, it's prudent to check the definition of 'Imax' from the standard that applies (IEC, JEC, ANSI, ...)