Why secondary windings of current transformers are shorted?
Current transformers (CTs) -together with voltage (or potential) transformers- are 'instrument transformers'. CTs are used to enable large currents flowing in high-voltage systems to be measured, while electrically-isolating the ammeter (termed a 'burden') from the h.v. system. They are also used to monitor h.v. currents for high-voltage protection systems.
disconnecting its burden, the secondary of a CT
be short-circuited, and remain short-circuited until
the burden has been replaced. This is because a dangerously-high voltage will appear across the open terminals of any energised CT.