Why the secondary winding of a current transformer cannot be left open

The current transformer works normally in short-circuit mode and does not allow idle operation. When working with current transformers, it is necessary to ensure that the secondary winding of the current transformer does not remain open when the primary is connected.

The secondary winding of the current transformer should not be left open if the measured current flows through the primary winding, for the following reasons.

When the secondary circuit opens, which can be, for example, when the ammeter is turned off, the magnetic flux counter F2 disappears, therefore a large alternating flux F1 begins to pass through the core, which induces a large EMF in the secondary winding of the current transformer (up to a thousand volts) , since the secondary winding has a large number of turns. The presence of such a large EMF is undesirable, as it is dangerous to the operating personnel and may lead to the destruction of the insulation of the secondary winding of the current transformer.

Connection diagram of a transformer of a current device

When a large flux F1 appears in the core, large eddy currents, the core begins to heat up strongly and with prolonged heating the insulation of the two windings of the transformer may fail. Therefore, we must remember that if it is necessary to turn off the measuring devices, you must first short-circuit the secondary or primary winding of the transformer.

Some current transformers have special devices for this purpose (sockets with plugs, jumpers, etc.). If there are no such devices, then you need to make them yourself.