CT short time thermal current 1

[bashar2008]

hi all
i have seen a low tension CT of a ration 400/5 and read that its Ith=20KA
and i wondered how can they specify that short time current level,
as i know it is specified for primary current which it is not built in ct
i mention that i know if the current in primary side risen up it is not the same case for secondary to be response to, as the ct core will be saturated
the ct is window type low voltage 400/5,class 1, 10va
i will be so grateful if any one can help to explain how to specify Ith for LV CTs?
thank you in advance

 

[scottf]

If it is a window-type CT, 20 kA/1sec is the primary thermal current rating, but it is controlled by the secondary thermal current capability. Therefore, 20 kA/1sec would be 50 x Inom. Frankly, I'm surprised it's that low, if it's a window-type CT.

Note that the short-time thermal current rating has nothing to do with core saturation, which is also a function of the burden applied to the CT. The short-time thermal current rating is simply how much current the CT can withstand for 1 second without over-heating, with "over-heating" being defined in the applicable standard.

 

[bashar2008]

is that means that secondary current has calculated to be able to attain 50 x In=250 Amp. for 1 sec?
thank you sir for your kind attention

 

[scottf]

correct