Why the current transformer behaves like this?
Never open the secondary side of a Current Transformer when main current is flowing.

A CT (Current Transformer) is a step-up transformer.  Step-up transformer means voltage is stepped up and current is stepped down.



Assuming 400A/5A CT on 440V, 400A 3 phase line:

1 turn on primary vs a number on secondary.  400A/5A means 80 turns on secondary (400A/1A means 400 turns).

$$I_2 = I_1 \ \frac {N_1}{N_2} = 400A \times \frac {1\ turn}{80\ turns} = 5A$$

If the secondary is opened while current is flowing in primary, the primary winding current becomes magnetizing current.  

This large magnetizing current creates a large flux, which when coupled with the large turns ratio can create dangerously high voltages, which can break down insulation, damage connected switchgear or create a shock hazard.

$$V_2 = V_1 \ \frac {N_2}{N_1} = 440V \times \frac {80\ turns}{1\ turn} = 35.2kV$$

The magnetic core of the CT will probably saturate before this voltage is reached, but high voltages greater than design voltages will result.

If the primary is connected, then Secondary side must be shorted if no load or ammeter is connected.