The current transformer is working normally, and the secondary circuit is approaching a stop state. At this point, the secondary electricity generates a secondary winding magnetic potential F2, and the secondary winding magnetic potential F1 has a demagnetization effect. Therefore, the synthesized magnetic potential F0=F1-F2 is not large, and the synthesized magnetic flux φ 0 is also not large. The value of the induced electromotive force E2 in the secondary winding does not exceed several tens of volts at most. Therefore, in order to reduce the size and cost of the current transformer, the cross-section of the transformer core is designed based on the fact that the combined magnetic flux of the current transformer is very small under normal operating conditions.

If the current transformer in use experiences a secondary circuit open circuit, the magnetic potential F2 of the secondary winding is equal to zero, while the magnetic potential F1 of the primary winding remains unchanged and is entirely used for excitation. The composite magnetic potential F0=F1, at which point F0 increases many times compared to the normal composite magnetic potential (F1-F2), causing the magnetic flux in the iron core to rapidly increase and reach saturation.

Due to the saturation of the iron core, the magnetic flux waveform becomes a flat topped wave. As the induced electromotive force is proportional to the rate of change of the magnetic flux d φ/dt, a high induced electromotive force e2 will be induced in the secondary winding at this time. When the secondary winding is open circuited, the induced electromotive force e2 of the secondary winding is a pointed non sinusoidal wave with a peak value of several thousand volts, which is extremely dangerous for workers, secondary equipment, and insulation of secondary cables. Another impact is that the sharp increase in magnetic flux inside the iron core causes an increase in iron core loss, leading to severe heating and causing the current transformer to burn out. The third impact is due to excessive residual magnetism in the iron core, which increases the error of the current transformer.

The principle of current transformer is based on electromagnetic induction. A current transformer is composed of a closed iron core and winding. Its primary winding turns are connected in series in the line where the current needs to be measured, so it often has all the current flowing through the line. The secondary winding turns are relatively large and connected in series in the measuring instrument and protection circuit. When the current transformer is working, its secondary circuit is always closed, so the impedance of the measuring instrument and protection circuit series coil is very small, and the working state of the current transformer is close to a short circuit. A current transformer is used to convert high primary current into low secondary current, and the secondary side must not be open circuited.

A current transformer is a special type of transformer where the primary coil has very few turns (usually only 1 turn for low voltage), while the secondary coil has many turns (such as a 1000/5 transformer, where the secondary coil is 200 times larger than the primary coil). When there is an open circuit on both sides, the voltage on the secondary side will rise to many times the voltage drop of the primary coil (200 times for a 1000/5 transformer), which will affect the normal operation of the secondary circuit and endanger personal safety.

Electrician's tip: Open circuit is not allowed on the secondary side of current transformers!

Current transformerIf there is a secondary open circuit, high voltage will be generated. It is very dangerous for both people and equipment. A current transformer is equivalent to a step-up transformer with few primary turns and many secondary turns. So installing a current transformer requires a secondary reliable grounding. This can prevent personal safety accidents, remember!