Answer / aashish

Because a very high voltage is generated in the secondary due to large no of turns so if open circuited ct will get damage

Answer / raja

Understand CT is a voltage step up transformer.

HT side has higher voltage but low current.
LT side has lower voltage but high current.

So for CT of rating 500/1 Amps, 500 Amps side(Primary of CT) has very low voltage and 1 Amp side (Secondary of (CT)
has very high voltage(Insane value in KVs).
 
This high voltage is beyond the maximum impulse withstand voltage of the CT and it blasts the CT. Hence secondary is always short circuited.

Current and voltage are inversely proportional. 

Answer / vipin singhal

when secondary is open then in primary flows very large current which burn the winding of the core.

Answer / gautam

 if we open circuit the secondary terminal current across this terminal falls to zero. as VI is constant for a Xmer voltage across secondary side increase drasticly,which results electrical breakdown across secondary.

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