Measuring transformers
The short circuit on terminals or the connection to earth can be achieved by the double fast-on or connecting two wires on the same terminal.
Protection transformers
The C.T. when used as a current generator for protection relays has electrical characteristics which differ from those of the measuring transformer.
In fact, the measuring C.T. is expected to give a saturation of the magnetic circuit with 5P in primary currents, while in the case of protective C.T., it is necessary for the secondary current value to follow the increase in the primary current up to 10-15-20In, so as to guarantee the intervention of the relay in the case of unforeseen breakdown current.
It is important not to load the C.T. with a P performance which is greater than that indicated, so as not to modify the saturation value of the C.T.
P=R∗I2
P = Load on the C.T.
R = Resistance of the relay and resistance of the cables
I = Nominal secondary current of the C.T.
