KA rating of circuit breakers 1

[govender1]

Could someone please explain the meaning behind the KA rating of a circuit breaker and how does one select a circuit breaker.

Thanks in advance.

 

[RalphChristie]

Govender1:

The symmetrical and asymetrical ratings (short-circuit current ratings) of the switchgear is expressed in kA.

See:

http://www.siemenstd.com/switchgear/pdf/TechTopics23Rev0A.pdf

http://www.siemenstd.com/switchgear/pdf/TechTopics21Rev0.pdf

For selecting circuit breakers see:

http://www.usace.army.mil/inet/usace-docs/armytm/tm5-811-14/chap3.pdf

X/R ratio see:

http://www.powerstudies.com/content/resources/ImportanceofX-over-RRatios.pdf

Hope it help
Good luck
RCC

 

[RajT]

In a nutshell note the following.
The CB rating (besides its continous duty rating in amps)has what is known as short circuit withstand rating in order to withstand the high currents (in kA) experienced during a short circuit fault on the system.
This short circuit withstand rating is expressed in two forms the Irms rating and the Ipk (peak) rating.
Ihe Irms rating is used to determine the CBs ability to withstand the fault (stated for either 1sec or 3 sec)and be able to break the fault during that period without suffering damage.
The Ipk rating which is an asymetrical current and stated as being either Irmsx2.1for LV swbds or Irmsx2.5 for MV swbd is the CBs ability to make onto a fault.

The selection of the correct CB can be determined based on a typical example as follows.
Consider a 415V swbd with which has 2000A busbars and that the max short curcuit current calculated on the system is 45kA rms/80kApk.
The correct CB (or the one with minimum technically acceptable limits)to be selected would be the one having following parameters (noting that for Lv system the CB sc ratings are usually defind by the manuf as 35kA/50ka/80kA/100kA for 1 sec).

CB selected to be:
3000A,50kA/105kApk for 1 sec.

You can ofcourse select a CB having a higher rating if it is felt that the LV system may in the future be upgraded or expanded which may result in higher system fault levels.

The CB continous rating of 3000A in this case is based on the ambient of say 35-40degC, as for higher ambient the CB has to be derated based on manuf data though the Irms/Ipk rating will remain the same.

I have not gone into describing how to calculate short circuit currents on distribution systems as it is some what a complex matter and also not asked for in the thread and kept to simply explaining how to select a CB as suggested once the SC levels have been assessed.

 

[cuky2000]

The meaning of kA (kilo-amp = 10^3 Amp) is the rating of the breaker obtained by standard procedures such as ANSI/IEEE C37-09 (Test Procedure), UL ( LV & MV), IEC –56, etc.

The prospective system short circuit must be less than the kA rating of the circuit breaker after considered derating or any unusual conditions.

Selection of circuit breaker should be based on in several electrical and other parameters such as continuous current, interrupting capacity, transient recovery voltage, capacitance current switching, etc.

Circuit Bkrs. manufactured after 1999 per ANSI standard to be used in metalclad swgrs. are not longer use MVA rating. Newest breaker rated for k=1 are certified with max. Interrupting rating for all operating voltage up to the max. rated voltage.

SUGGESTION: If you are in the ANSI marketplace check the standard such as IEEE Std 141 (Red Book), ANSI/IEEE C37.04, 06, 09, 010, 011 & 012.