Hi dear;

When the output current  transformer is open voltage of winding increase and flash over.

Best regard

salehi 

When 'short', nothing will happen (only rated current 00/05 Amp, will flow), generally, secondary keeps shorted when not in use. When open, the (independent)primary ampere turn of bar primary, which depends on load current(voltage drop of primary steps up), and will generate very high voltage(working flux density is low, in lower linear region of B-H curve) in malty turn secondary, and will damage the insulation of secondary winding,  with flash over, with higher load current of primary.

As Bhupendra Desai said, short ckting the secondary terminals is normal in CTs. When they are open and the primary is subjected to rated current or atleast 30 percent of the rated current, you could notice sparking across the terminals. If you increase the current through the primary terminals beyond this, the secondary core insulation could start burning. The reason has been quoted by Bhupendra.