CT or Current Transformer used in series of the circuit to measure current which is then used to measurement and protection system.
Current Transformer with a double (or triple) secondary –
Current transformers secondary connection-
The CT secondary is used at low impedance (virtually in short circuit). The secondary circuit should never be left open, since this would mean connecting across an infinite impedance. Under these conditions, hazardous voltages for personnel and equipment may exist across the terminals.
Current transformers Terminal marking
CT connection is made to the terminals identified according to the IEC; 1.P1 and P2 on the MV side; 2. S1 and S2 on the corresponding secondary. In the case of a double output, the first output is identified by 1S1 and 1S2, the second by 2S1 and 2S2.
Why secondary circuit of current transformer should not open:
To open the secondary circuit of current transformer while its energized is a dangerous act.
If the secondary circuit open circuited then there will be zero current in the secondary circuit, hence reduced back emf. This back emf restrains the primary flux to exceed. Without back emf the primary flux increases and core get saturated. This increased flux develops high voltage across secondary that is dangerous enough to caught a lethal shock.