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#1

# Different of short circuit current 20 kA/3s and 12,5kA/1s

Please help me, what is the meaning and different of short circuit current 20 kA/3s and 12,5kA/1s
09-06-2014 06:08 AM
Top #2
A. 20,000 Amp for 3 Seconds and breaker opens
B. 12500 Amp. for 1 Second and breaker opens
09-06-2014 08:31 AM
Top #3
Thanks.....if there are radial network system and intsall Recloser 24 kV, 630 Amp with 12,5 kA and then we install CB, my question, is it better use 12,5 kA/1s or below or 20 kA/3s
09-06-2014 11:09 AM
Top #4
dharmawan, what you indicated are thermal withstand ratings.
If these are for breakers, then, the Fault breaking capacity also will be 20kA and 12.5kA respectively (generally speaking). Now, whether you use 12.5kA breaker or 20kA breaker cannot be decided without understanding the fault levels in your system.
If your fault levels are below 10kA, of course, 20kA is better than 12.5kA as it is more capable breaker and can be used even after your fault levels go up in future.
09-06-2014 02:06 PM
Top #5
20kA 3 seconds would mean that the equipment can cope with a fault current of 20kA for 3 seconds without any damage. In what context are these figures being quoted? Where have you got the figures from?
09-06-2014 04:52 PM
Top #6
I am very much interested in the answer to Stuart question
09-06-2014 07:22 PM
Top #7
As Stuart says if the switchgear is rated at 20Ka for 3 seconds without any damage. Then its simple mathematics I²t (current squared multiplied by time) to calculate what the withstand Ka value is at 1 second.
20Ka @ 3 seconds equates to 34.64Ka at 1 second, 54.77Ka at 0.4 seconds and 77.46Ka at 0.2 seconds.
Some switchgear will be quoted with a maximum fault level rating in MVA.
e.g. if your switchgear is rated at 20Ka for 3 seconds and is 11Kv then the MVA fault rating is 381MVA.
You must know the maximum fault level at each relevant point within your own network/system before you can install switchgear or cables and remember motors and generators will contribute to the local DNO maximum fault level.
I hope this helps?
09-06-2014 09:30 PM
Top #8
20KA is the amount of short circuit current that the breaker can withstand for 3 seconds without any damage
12.5KA is the amount of short circuit current that the breaker can withstand for 1 sec without any damage.
09-06-2014 11:10 PM
Top #9
Dears ;

I would like highlight to the following points :

1- The total tripping time of a MV Circuit Breaker with a timing delay is " 500 - 600 ms " ( 0.5 - 0.6 sec ), and it will be certainly less without timing delay, so, in the most if case, 1 sec is enough.

2- When we define the value of " Isc " for " 1 or 3 sec " of a Circuit Breaker, the Bus Bar installed in the panel of that Circuit Breaker should absolutely withstand the same value of " Isc " or more during the same time.

3- To define the " Isc " value of a Circuit Breaker, we should know the value of " the maximum Three Phases Short Circuit Current " at the installation's point, that can know by calculation depending on the specifications of the components like : Network Short-Circuit Power, Operating Voltage, Cables, Transformers, Generators, Electric Motor, ... ", then " Isc " should be equal or bigger than " maximum Three Phases Short Circuit Current ".

4- Another important value should be know to define and chose the MV Circuit Breaker that is " the Peak value of the maximum Short Circuit Current - Idyn ", that can be calculated after knowing " Isc " as follow :
- According to " IEC 60 056 " : Idyn = 2.5 x Isc ( for 50 Hz ), or Idyn = 2.6 x Isc ( for 60 Hz )
- According to " ANSI " : Idyn = 2.7 x Isc